WO2006032110A1

WO2006032110A1 - A selectable multiwavelength laser

Info

Publication number: WO2006032110A1
Application number: PCT/AU2005/001470
Authority: WO
Inventors: James Austin Piper; Helen Margaret Pask
Original assignee: Macquarie University
Priority date: 2004-09-23
Filing date: 2005-09-23
Publication date: 2006-03-30

Abstract

A laser system capable of selecting between at least two different wavelengths of output laser light. The system (100) comprises a resonator cavity (105) comprising at least two reflectors (110, 115), and a laser material (125) located in the cavity. A pump source (120) is located outside of the cavity for pumping the laser material (125) with a pump beam to generate a cavity laser beam. A Raman-active medium (135) is also located in the resonator cavity (105) for shifting the wavelength of the cavity laser beam such that at least two different frequencies are generated. A frequency selector is provided for selecting a single frequency of output laser light by either deselecting all but one of the at least two different frequencies or by selecting one of the at least two different frequencies, and an output coupler is used for outputting the single wavelength of output laser light.

Description

A Selectable Multiwavelength Laser Technical Field

The present invention relates to a selectable multiwavelength laser system, a method for selectably providing an output laser beam from a laser system at different wavelengths and methods of using laser light. Background of the Invention

Solid-state Raman lasers are a practical and efficient approach to optical frequency down conversion, offering high (up to 70 to 80%) conversion efficiencies with respect to the pump power, excellent beam quality and ease of alignment. In recent years use of crystals for stimulated Raman scattering (SRS) has been gaining interest because, in comparison with high-pressure gaseous and dye (liquid) Raman lasers, crystalline Raman lasers offer better gain, better thermal and mechanical properties, and the ability to operate at high pulse repetition frequency. Further they are compatible with compact all solid-state laser technology.

Solid-state lasers are commonly used in the ophthalmologic^ and dermatological fields. For these applications there is commonly a need to have available a range of different wavelengths.

United States Patent no. 4,165,469 (Ammann, 1979) revealed a solid-state laser capable of providing different frequencies of laser output light. The laser of the invention is limited to the use of lithium iodate crystal, which performs the functions both of Raman-shifting and of frequency doubling to generate a plurality of possible output frequencies based on the frequency-doubled first, second or higher order Stokes stimulated Raman scattering in the lithium iodate crystal. This limitation is a significant disadvantage, as the laser of the invention is limited to the output frequencies obtainable using lithium iodate, since it is rare to find crystals capable of performing both of these functions together. A further disadvantage is that, since the lithium iodate crystal serves two discrete functions, it is not possible to optimise the position of that crystal independently for two functions. Yet a further disadvantage is that lithium iodate has limited utility in high power applications, since it has a relatively low damage threshold. Yet another disadvantage is that, since the laser of the prior art requires a reasonably long Raman crystal (in order to obtain sufficient gain), and this Raman crystal needs to be rotated in order to achieve phase matching, there will be substantial beam displacement due to refraction when the crystal is off normal incidence. This means that the cavity of the prior art will need to be significantly realigned for each wavelength, which thus greatly reduces the utility of the laser of the prior art. There is therefore a need for a versatile laser system capable of selecting between two or more different wavelengths of output laser light, and which is capable of providing adequate output power for therapeutic use. There is a further need for such a system to be designed so that it is straightforward to manufacture, can be incorporated into a practical device without undue difficulty, s and so that the step of selecting a wavelength is easily carried out by the operator or end-user of the system. Object of the Invention

It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages. It is another object to at least partially satisfy at least one of the o abovementioned needs or provide an alternative to the prior art. It is a further object to provide a selectable multiwavelength laser system. Summary of the Invention

In a broad aspect of the invention there is provided a laser system capable of selecting between at least two different wavelengths of output laser light, said system comprising: s a) a resonator cavity comprising at least two reflectors; b) a laser material located in the resonator cavity; c) a pump source located outside of the cavity for pumping the laser material with a pump beam to generate a cavity laser beam; d) a Raman-active medium located in the resonator cavity for shifting the wavelength of the o cavity laser beam such that at least two different frequencies are generated; e) a frequency selector for selecting a single frequency of output laser light by either deselecting all but one of the at least two different frequencies or by selecting one of the at least two different frequencies; and f) an output coupler for outputting the single frequency of output laser light. 5

The resonator cavity may be capable of having at least two different frequencies of laser light resonating therein. The resonator cavity may be a high-Q resonator, and may be high Q for each wavelength of laser light that resonates in the resonator cavity. One of the reflectors may be partially reflective, or may be reflective towards some wavelengths and transmissive towards other o wavelengths, and may be transmissive towards the wavelength of output laser light. One of the reflectors may function as the output coupler. The resonator cavity may comprise two, three or more than three reflectors. The system may comprise two different Raman-active media for shifting the wavelength of the cavity laser beam such that at least two different frequencies are generated. Each of the two different Raman-active media may generate one or more different Stokes frequencies. The system may also comprise a Q-switch, which is capable of converting continuous laser light into pulsed laser light, which may be pulsed high power laser light In a first aspect of the invention there is provided a laser system capable of selecting between at least two different wavelengths of output laser light, said system comprising: a) at least one resonator cavity comprising at least two reflectors; b) a laser material located in the cavity; c) a pump source located outside of the cavity for pumping the laser material with a pump beam to generate a cavity laser beam; d) a Raman-active medium located in the resonator cavity for shifting the wavelength of the cavity laser beam; e) a seeding device to seed the Raman-active medium with a seed beam in order to cause to produce predominantly a single wavelength of laser light with a Raman-shifted wavelength corresponding to the wavelength of the seed beam; and f) an output coupler to output a single wavelength of output laser beam from the resonator cavity.

The resonator cavity may comprise two, three or more than three reflectors. At least one of the reflectors may be partially reflective, or may be reflective towards some wavelengths and transmissive towards other wavelengths, and may be transmissive towards the wavelength of output laser light. At least one of the reflectors may be at least partially reflective towards the wavelengths of laser light that resonate within the resonator cavity, and at least one of the reflectors may be at least partially transmissive towards a wavelength of output laser light.

At least one of the reflectors may function as the output coupler, or there may be a separate output coupler. In the case that there is a separate output coupler, none of the reflectors may be transmissive towards the wavelength of output laser light, or one or more reflectors may be transmissive towards the wavelength of output laser light.

The laser system may additionally comprise a non-linear medium such as a frequency doubler or a sum frequency generator, a difference frequency generator, or other parametric frequency converter disposed in the cavity for converting the wavelength of a laser beam selected from the group consisting of a laser beam produced by the laser material and a laser beam which has been wavelength shifted by the Raman-active medium. The non-linear medium may be located after the Raman active medium or it may be located in some other position. The output laser beam may be selected from the group consisting of the Raman-shifted wavelength and a laser beam that has been converted by the non-linear medium. ^•

One or more of the laser material, the Raman-active medium and the non-linear medium may be solid. The pump source may be a diode laser and the Raman-active medium may be s capable of end pumping or side pumping the laser material. The output coupler may comprise at least one of the at least two reflectors which define the resonator cavity, and may be at least partially transmissive for the wavelength of the output laser beam, and may be at least partially reflective, or highly reflective, at the wavelength generated by the laser material. The output coupler may comprise a polarizing beam splitter. The system may also comprise a Q-switch, which is Q capable of converting continuous laser light into pulsed laser light, which may be pulsed high power laser light. The operation of the laser is not strongly dependent on the location of the Q-switch. The seeding device may comprise a seed beam generator, such as a laser, a diode laser, a light emitting diode (LED), a monochromatic light source in combination with a filter or grating, a polychromatic light source in combination with a filter or grating, or some other suitable seeding s device. The wavelength of the seed beam may be such that the seed beam is capable of seeding the Raman-active medium in order to cause the Raman-active medium to provide primarily a single selected wavelength of laser light, said selected wavelength being the same as the wavelength of the seed beam. Where the Raman laser is pulsed or otherwise modulated the temporal output of the seed beam may be synchronized with the temporal output of the output laser beam. The 0 temporal output of the seed beam may be the same as the temporal output of the output laser beam.

The seed beam may be polarized so that the Raman-active medium is caused to provide primarily a single selected polarization of laser light. In a second aspect of the invention there is provided a laser system capable of selecting between 5 at least two different wavelengths of output laser light, said system comprising: a) a resonator cavity comprising at least two reflectors; b) a laser material located in the cavity; c) a pump source located outside of the cavity for pumping the laser material with a pump beam to generate a cavity laser beam; o d) a birefringent Raman-active medium located in the resonator cavity for shifting the wavelength of the cavity laser beam; e) a selector for selecting a single wavelength of Raman-shifted laser light; and f) an output coupler to output a single wavelength of output laser beam from the resonator cavity.

The resonator cavity may comprise two, three or more than three reflectors. One of the reflectors may be partially reflective, or may be reflective towards some wavelengths and

5 transmissive towards other wavelengths, and may be transmissive towards the wavelength of output laser light. One of the reflectors may function as the output coupler. The birefringent Raman-active medium may be capable of generating two different polarisations of laser light spatially separated from each other, each having a different Stokes wavelength. The selector may be capable of selecting the single wavelength of Raman-shifted laser light by realigning one of the Q reflectors and/or the Raman-active medium.

The laser system may additionally comprise a non-linear medium disposed in the cavity for converting the wavelength of a laser beam selected from the group consisting of a laser beam produced by the laser material and a laser beam which has been wavelength shifted by the Raman-active medium. The non-linear medium may be located after the Raman active medium or s the non-linear medium may be located in some other position in the cavity. The non-linear medium may be a second harmonic generator, sum frequency generator, difference frequency generator, optical parametric oscillator or may be some other type of non-linear medium. The single wavelength of output laser light may comprise the single wavelength of Raman-shifted laser light selected by the selector or may comprise a laser beam that has been converted by the non-linear Q medium.

The frequency converting by the non-linear medium may comprise frequency doubling, sum frequency generation, difference frequency generation, or other parametric non-linear frequency conversion. Thus the non-linear medium may be capable of selectively converting a single wavelength by frequency doubling (second harmonic generation: SHG), or a pair of 5 wavelengths by sum frequency generation (SFG) or difference frequency generation (DFG).

The laser may be any multiwavelength laser, and may be a Raman laser, and may be a solid state laser. The laser system may comprise a cavity, or resonator, defined by at least two reflectors, and the non-linear medium may be in the cavity. The resonator may be a high-Q resonator, and may be high Q for each wavelength of laser light that resonates in the resonator. 0 This may be achieved by use of mirrors that are highly reflective for said wavelengths. The reflectance of the mirrors may be greater than about 95%, or greater than about 96, 97, 98, 99 or 99.5% at said wavelengths, and may be about 95, 96, 97, 98, 99 or 99.5%. In an embodiment, the single converted wavelength of laser light may not resonate in the cavity. For high efficiency, the single converted wavelength may make as few passes within the resonator as possible. Accordingly, the output coupler may be as highly transmissive as possible toward the single converted wavelength. In an embodiment, the output coupler may be greater than about 50% transmissive toward the single converted wavelength, or greater than about 60, about 70, about 80,

5 about 90, about 95 or about 99% transmissive towards the single converted wavelength, and further may be for example about 50, about 55, about 60, about 65, about 70, about 75, about 80, about 85, about 90, about 95, about 96, about 97, about 98, about 99 or about 99.5 transmissive towards the single converted wavelength.

One or more of the laser material, the Raman-active medium and the non-linear medium

IQ may be solid.

The pump source may be a diode laser, a fibre coupled diode laser or may be light from an arclamp or flashlamp or other pump source. The pump source may be capable of end pumping or side pumping the laser material. The output coupler may comprise an output reflector, and may comprise at least one of the at least two reflectors which define the resonator cavity, which may be i₅ at least partially transmissive for the wavelength that is outputted from the cavity. The output coupler may comprise a polarizing beam splitter. The system may also comprise a Q-switch, which is capable of converting continuous laser light into pulsed laser light, which may be pulsed high power laser light. The orientation of the Raman-active medium, and the curvature, orientation and position of at least one of the reflectors that define the resonator cavity may be such that only one

2Q of the Stokes wavelengths emanating from the Raman-active medium is capable of efficiently resonating within the cavity, due to the effects of birefringence on different polarisations.

In an embodiment, the selector comprises an adjustor to adjust the orientation of the Raman-active medium and/or of at least one of the reflectors that define the cavity so that only a selected wavelength is directed in such a way that allows the selected wavelength to resonate

2₅ within the cavity. The adjustor may comprise one or more orientation adjustors, for example one or more motors or piezoelectric devices coupled to the Raman-active medium and/or to of at least one of the reflectors. In a further embodiment, at least one of the reflectors is curved. In another embodiment, the at least one reflector which is adjusted is at least partially transmissive to at least one wavelength which is different to the selected wavelength. The one or more orientation

3Q adjustors may be mechanically coupled to the Raman-active medium and/or of at least one of the reflectors.

In another embodiment of this aspect, the Raman-active medium may comprise a separate Raman- active element and birefringent element, whereby the Raman-active element is capable of generating Raman-shifted wavelengths of laser light, and the birefringent element is capable of separating the Raman-shifted wavelengths according to their polarisation. Alternatively, the Raman-active medium may also have birefringent properties together with Raman-active properties. In a third aspect of the invention there is provided a laser system capable of selecting between at least two different wavelengths of output laser light, said system comprising: a) at least one resonator cavity comprising at least two reflectors; b) a laser material located in the at least one resonator cavity; c) a pump source located outside of the or each resonator cavity for pumping the host laser crystal with a pump beam to generate a cavity laser beam; d) a Raman-active medium located in the or each resonator cavity for shifting the wavelength of the cavity laser beam; e) a polarisation selector; and f) an output coupler to output a single wavelength of output laser light from the resonator cavity.

The at least one resonator cavity may comprise two, three or more than three reflectors. One or more of the reflectors may be partially reflective, or may be reflective towards some wavelengths and transmissive towards other wavelengths, and may be transmissive towards the wavelength of output laser light. One of the reflectors may function as the output coupler and may be curved.

The laser system may additionally comprise a non-linear medium disposed in the at least one cavity for converting the wavelength of a laser beam selected from the group consisting of a laser beam produced by the laser material and a laser beam which has been wavelength shifted by the Raman-active medium. The non-linear medium may be located after the Raman active medium or it may be located in some other position. The non-linear medium may be a second harmonic generator or may be some other type of non-linear medium. The single wavelength of output laser light may comprise a laser beam that has been selected by the polarization selector or may comprise a laser beam that has been converted^' by the non-linear medium.

One or more of the laser material, the Raman-active medium and the non-linear medium may be solid. The Raman-active medium may be capable of producing different Stokes wavelengths in response to different polarisations of incident laser light. The different Stokes wavelengths may have the same polarization or they may have different polarizations. The pump source may be a diode laser, a fibre coupled, diode laser or may be light from an arclamp or flashlamp or other pump source. The pump source may be capable of end pumping or side pumping the laser material. The output coupler may comprise at least one of the at least two reflectors which define the resonator cavity, which may be at least partially transmissive for the wavelength that is outputted from the cavity. The output coupler may comprise a polarizing beam splitter.

The polarisation selector may be a mechanically rotatable selector or may be a Faraday rotator or an electro-optic rotator whereby selecting the polarisation is accomplished electronically. The polarisation selector may be located before the laser material or after the laser material, and may be located before the Raman-active medium or after the Raman-active medium. The polarization selector may also be located between the Raman-active medium and the output coupler, or between the pump source and the laser material, or between the laser material and the Raman-active medium. The polarisation selector may be transmissive for both polarisations of laser light in such a way that the transmitted intensity of a selected polarisation is greater than the transmitted intensity of a deselected polarisation. In this way, the efficiency of frequency shifting by the Raman-active medium is greater for the selected polarisation, leading to a more efficient conversion of input power to the selected polarisation. The system may also comprise a Q-switch, which is capable of converting continuous laser light into pulsed laser light, which may be pulsed high power laser light. '

In a fourth aspect of the invention there is provided a laser system capable of selecting between at least two different wavelengths of output laser light, said system comprising: a) at least one resonator cavity comprising at least two reflectors; b) a laser material located in the at least one resonator cavity; c) a pump source located outside of the at least one resonator cavity for pumping the laser material with a pump beam to generate a cavity laser beam; d) a Raman-active medium located in the at least one resonator cavity for shifting the wavelength of the cavity laser beam; e) a wavelength tunable element for selecting a single wavelength of laser beam; and f) an output coupler to output a single wavelength of output laser beam from the at least one resonator cavity. The at least one resonator cavity may comprise two, three or more than three reflectors. In another embodiment, there may be one, two or three resonator cavities. One of the reflectors may be partially reflective, or may be reflective towards- some wavelengths and transmissive towards other wavelengths, and may be transmissive towards the wavelength of output laser light. One of the reflectors may function as the output coupler. The laser system may additionally comprise a non¬ linear medium disposed in the cavity for converting the wavelength of a laser beam selected from the group consisting of a laser beam produced by the laser material and a laser beam which has been wavelength shifted by the Raman-active medium. The non-linear medium may be located

5 between the Raman active medium and the output coupler or it may be located in some other position in the resonator cavity. The single wavelength of output laser beam may comprise a laser beam that has been selected by the wavelength tunable element or it may comprise a laser beam that has been converted by the non-linear medium. The wavelength tunable element may be for example an optical filter, a prism, a grating, an etalon, o an interference filter or some other element for selecting the wavelength of laser light to be outputted from the resonator cavity. In this aspect of the invention, the output coupler may comprise one of the reflectors that define the resonator cavity. The output coupler may be selectively transmissive for particular wavelength ranges. For example the output coupler may only transmit the two second Stokes wavelengths, and may reflect the fundamental and first Stokes wavelengths. s The wavelength tunable element, or some other element, may then be used to select between the two second Stokes wavelengths. The system may also comprise a Q-switch, which is capable of. converting continuous laser light into pulsed laser light, which may be pulsed high power laser light. In a fifth aspect of the invention there is provided a method for providing an output laser beam from a laser system, said laser beam having a wavelength which may be selected from two or more o wavelengths, said method comprising: a) generating a laser beam within a resonator cavity by pumping a laser material located in the cavity with a pump beam from a pump source located outside the cavity; b) shifting the wavelength of at least a portion of the laser beam by passing the laser beam through the Raman-active medium such that at least two different frequencies are generated; ₅ c) selecting a single wavelength of output laser light by either deselecting all but one of the at least two different frequencies or by selecting one of the at least two different frequencies; and d) outputting from the laser system the single wavelength of output laser light.

Step a) may comprise focusing the pump beam on the laser material. The method may additionally comprise the step of passing the output laser beam from the Raman-active medium o through a non-linear medium, thereby wavelength converting said output laser beam. The wavelength converting may be frequency doubling, or sum frequency generating, or difference frequency generating or some other parametric frequency conversion. The single wavelength of output laser light may be a wavelength that has been wavelength shifted by the Raman-active medium or it may be a wavelength that has been converted by the non-linear medium.

The pumping may comprise end-pumping or side-pumping. The non-linear medium may be a second harmonic generator or it may be some other type of non-linear medium. The method

5 may also comprise the step of passing a laser beam through a Q-switch in order to provide pulsed laser light, which may be pulsed high power laser light. The step of outputting may comprise passing the laser beam through a reflector which is at least partially transmissive for the wavelength of laser light that has been shifted by the non-linear medium. In this step, the reflector may be curved reflector. io In a sixth aspect of the invention there is provided a method for providing an output laser beam from a laser system, said laser beam having a wavelength which may be selected from two or more wavelengths, said method comprising: a) generating a laser beam within at least one resonator cavity by pumping a laser material located in the cavity with a pump beam from a pump source located outside the cavity; i₅ b) selecting a single wavelength of output laser light by seeding a Raman-active medium located in the resonator cavity using a seed beam with a wavelength which is capable of causing the Raman-active medium to generate predominantly a single wavelength, said single wavelength being the same as the wavelength of the seed beam; c) shifting the wavelength of at least a portion of the laser beam by passing the laser beam 20 through the Raman-active medium; and d) outputting from the laser system the single wavelength of output laser light.

Step a) may comprise focusing the pump beam on the laser material. The method may additionally comprise the step of passing the output laser beam from the Raman-active medium through a non-linear medium, thereby wavelength converting said laser beam. The wavelength

2s converting may be frequency doubling, sum frequency generating, difference frequency generating or some other parametric frequency conversion. The single wavelength of output laser light may be a wavelength that has been wavelength shifted by the Raman-active medium or may be a wavelength that has been converted by the non-linear medium.

Step b) may comprise a step of synchronisng the frequency of the seed beam with the

3o frequency of the output laser light. The step of synchronizing the frequency of the seed beam may comprise detecting the temporal frequency of the output laser light and subsequently adjusting the frequency of the seed beam so that it is synchronized with the temporal frequency of the output laser light. The step of synchronizing the frequency and phase of the seed beam may comprise detecting the temporal frequency and phase of the output laser light and subsequently adjusting the frequency and phase of the seed beam so that it is synchronized with the temporal frequency and phase of the output laser light.

The pumping may comprise end-pumping or side-pumping of the laser material. The non- linear medium may be a second harmonic generator, difference frequency generator, sum frequency generator, optical parametric oscillator or it may be some other type of non-linear medium. The method may also comprise the step of passing a laser beam through a Q-switch in order to provide pulsed laser light, which may be pulsed high power laser light. The step of outputting may comprise passing the laser beam through a reflector which is at least partially transmissive for the wavelength of laser light that has been shifted by the non-linear medium. The step of seeding may use a diode laser or it may use some other suitable device.

In a seventh aspect of the invention there is provided a method for providing an output laser beam from a laser system, said laser beam having a wavelength which may be selected from two or more wavelengths, said method comprising: a) generating a laser beam within at least one resonator cavity by pumping a laser material located in the at least one resonator cavity with a pump beam from a pump source located outside the or each cavity; b) passing the laser beam through a birefringent Raman-active medium located in the at least one resonator cavity to provide at least two different wavelengths of Raman-shifted laser light which are spatially separated from each other; c) selecting a single wavelength of output laser light by orienting at least one of the Raman- active medium and a reflector so that one of the at least two different wavelengths of Raman- shifted laser light is capable of resonating in the at least one resonator cavity more efficiently than the other different wavelength(s) of Raman-shifted laser light ; and d) outputting from the laser system the single wavelength of output laser light.

Step a) may comprise focusing the pump beam on the laser material. The method may additionally comprise the step of passing one or more output laser beams from the Raman-active medium through a non-linear medium (for example a frequency doubler or a sum frequency generator), thereby wavelength converting said laser beam. The wavelength converting may be frequency doubling, sum frequency generating, difference frequency generating or some parametric frequency . The single wavelength of output laser light may be one of the at least two different wavelengths of Raman-shifted laser light or may be a wavelength that has been converted by the non-linear medium. The pumping may comprise end-pumping or side-pumping the laser material. The non¬ linear medium may be a second harmonic generator, difference frequency generator, sum frequency generator, optical parametric oscillator or may be some other type of non-linear medium. The method may also comprise the step of passing a laser beam through a Q-switch in order to provide pulsed laser light, which may be pulsed high power laser light. The step of outputting may comprise passing the laser beam through a reflector which is at least partially transmissive for the wavelength of laser light to be outputted by the system, and is reflective for the other wavelengths generated within the cavity.

In an eighth aspect of the invention there is provided a method for providing an output laser beam from a laser system, said laser beam having a wavelength which may be selected from two or more wavelengths, said method comprising: a) generating a laser beam within at least one resonator cavity by pumping a laser material located in the at least one resonator cavity with a pump beam from a pump source located outside the or each cavity; b) shifting the wavelength of at least a portion of the laser beam by passing the laser beam through a Raman-active medium capable of shifting different polarisations of incident light to different Stokes wavelengths; c) selecting a single wavelength of output laser light using a polarisation selector; and d) outputting the single wavelength of output laser light from the laser system. Step a) may comprise focusing the pump beam on the laser^" material. The pumping may comprise end-pumping or side-pumping the laser material. The method may comprise the step of passing a laser beam through a Q-switch in order to provide pulsed laser light, which may be pulsed high power laser light. The method may comprise supplying only a single polarisation of laser light to the Raman-active medium or may comprise selecting a single polarisation of laser light that has been wavelength-shifted by the Raman-active medium.

The method may comprise the additional step of wavelength-converting at least a portion of the laser beam output from the Raman-active medium by passing the portion of the laser beam output through a non-linear medium. The non-linear medium may be a second harmonic generator, or a sum frequency generator, a difference frequency generator parametric frequency oscillator or some other type of non-linear medium.

The step of outputting may comprise passing the laser beam through a reflector which is at least partially transmissive for the output wavelength of laser light. A polarisation selector may be located before the laser material or after the laser material, and it may be located before the Raman-active medium or after the Raman-active medium. In particular, the polarization selector may be located between the pump source and the Raman-active medium or between the Raman-active medium and the output coupler. If the system includes a non-linear medium for wavelength-shifting the laser beam output from the Raman-active medium, the polarisation selector may be located before The non-linear medium or after the non-linear medium. In particular, if the system includes a non¬ linear medium then the polarization selector may be located between the Raman-active medium and the non-linear medium or between the non-linear medium and the output coupler. The single wavelength of output laser light may be a wavelength that has been wavelength shifted by the Raman-active medium or it may be a wavelength that has been converted by the non-linear medium.

In a ninth aspect of the invention there is provided a method for providing an output laser beam from a laser system, said laser beam having a wavelength which may be selected from two or more wavelengths, said method comprising: a) generating a laser beam within a resonator cavity by pumping a laser material located in the cavity with a pump beam from a pump source located outside the cavity; b) selecting a single wavelength of output laser light by tuning a wavelength tunable element, and optionally by use of a reflector that is selectively transmissive for particular wavelength ranges; c) shifting the wavelength of at least a portion of the laser beam by passing the laser beam through the Raman-active medium; and d) outputting from the laser system the single wavelength of output laser light.

Step a) may comprise focusing the pump beam on the laser material. The method may additionally comprise the step of passing the output laser beam from the Raman-active medium through a non-linear medium, thereby wavelength converting said laser beam. The wavelength converting may be frequency doubling, sum frequency generating, difference frequency generating or other parametric frequency conversion. The single wavelength of output laser light may comprise a laser beam that has been wavelength shifted by the Raman-active medium or may comprise a wavelength that has been converted by the non-linear medium.

In a tenth aspect of the invention there is provided a method of using a laser system according to the invention for treating, detecting or diagnosing a selected area on or in a subject requiring such diagnosis or treatment, comprising illuminating the selected area with the output laser beam from the laser system of the invention. The selected area may be illuminated with a laser beam having a wavelength, and for a time and at a power level, which is appropriate and effective for the diagnosis or therapeutically effective for the treatment. The subject may be a mammal or vertebrate or other animal or insect, or fish. The method of the invention may find particular application in treating the eyes and skin of a mammal or vertebrate. The system may be a solid-state laser system. In an embodiment, the method of the ninth aspect comprises using a laser system according to the invention, wherein the system comprises a non-linear medium for frequency converting at least one frequency outputted by the Raman-active medium.

In an eleventh aspect of the invention there is provided a laser system according to the invention when used for treating, detecting or diagnosing a selected area requiring such diagnosis or treatment on or in a subject. The system may be a solid-state laser system. Detailed Description of the Invention

In the laser of the present invention, a laser beam is generated within a cavity by a laser material. The laser material may be capable of emitting, in use, a cavity laser beam, when pumped by a pump laser beam. The pump beam may be generated by a pump source, which may be selected from the group consisting of a flashlamp, an arclamp, a fibre coupled diode laser, a diode laser or some other pump source. The laser material may be end pumped or side pumped. The pump laser beam may be generated by supplying current to a diode pump laser, such that a portion of the power of the pump laser beam is absorbed by the laser material. There may be one or more collimating lenses and one or more focusing lenses, for collimating and/or focusing the pump beam, and, if present, each lens may be either intracavity or extracavity. The focusing lens may be located between the collimating lens and the laser material. The focusing lens may focus the pump beam on the laser material. The size of the focused beam on the laser material may be given by: Dmin = 2.44TA/D where Dmin is the focal spot size (ie the size of the focused beam), f is the focal length of the focusing lens, λ is the wavelength of the pump beam and D is the beam diameter of the unfocussed beam. The cavity laser beam is passed to an intracavity Raman-active medium which is capable of generating at least two laser beams of different frequency which may also differ from each other in either polarisation or location or some other property. The system is fitted with a selector which either promotes resonance of a selected wavelength of laser beam or discourages resonance within the cavity of all but one of the at least two laser beams. This may be by means of a polariser, or by means of motors which orient either a mirror or the Raman-active crystal or both so that only one beam is capable of efficiently resonating, or may be by some other method such as seeding. The selected beam will have greater intensity, and will thus be more effectively frequency shifted by the Raman-active medium. The deselected beam will correspondingly be less effectively frequency shifted. In this manner, power will be concentrated in the selected Raman frequency, said frequency being selectable by the selector. The selected frequency may be directly outputted from the laser cavity, or it may be passed to a non-linear medium, which may be a frequency doubler or a sum frequency generator or a difference frequency generator, for converting the frequency, for example to a visible laser beam, for outputting. The outputting may by means of an output reflector or of a polarizing beam splitter. The output reflector may be an output coupler, for decoupling and outputting an output beam from the cavity.

The laser system of the present invention may be a diode-pumped laser system, a flashlamp pumped laser system or may be a solid-state laser system.

In a laser cavity according to the invention, there may potentially be a plurality of different wavelengths of laser light resonating in the cavity. This may be achieved by selecting the reflectivity of the reflectors which define the cavity such that the cavity is a high Q cavity for all wavelengths that are required to resonate therein. Thus there may be one or more of a fundamental wavelength, a first Stokes wavelength and a second Stokes wavelength. Further, in cases in which the Raman active medium has two or more Raman shifts, there may be a first and a second Stokes wavelengths from each of the two or more Raman shifted beams generated by the Raman active medium. The laser cavity of the invention may also have a non-linear medium capable of frequency doubling or sum frequency generation or difference frequency generation. Thus each of the above wavelengths may be frequency doubled, or any two may be frequency summed or frequency differenced, depending on the tuning of the non-linear medium. Therefore the present invention provides means to selectively output a wide variety of different wavelengths from the cavity.

The pump beam may be a beam from a diode laser, a fibre coupled diode laser or it may be from an arclamp or flashlamp, or from some other pump source. The pumping may be end pumping or side pumping. The power of the output laser beam from the laser system may be dependent on the frequency of the pump laser beam, and the system may have means (such as a frequency controller) for altering the frequency of the pump laser beam in order to alter the power of the output laser beam. It will be understood by one skilled in the art that the frequency and wavelength of a laser beam are connected by the equation: Speed of light = wavelength ^* frequency As a consequence, when reference is made to frequency shifting, frequency converting, different frequencies, and similar terms, these are interchangeable with the corresponding terms wavelength shifting, wavelength converting, different wavelengths, and the like. Throughout the specification and claims the expression single wavelength may be used interchangeably with single frequency where frequency and wavelength are related to each other in accordance with the above equation. Further, when the terms single frequency, single wavelength, single converted wavelength are used to describe the light propagating inside or outside the resonator, what is being referred to is the central frequency of the light beam and any band associated therewith.

In constructing a laser according to the present invention, it is crucial that components of the laser are correctly positioned in order to achieve acceptable conversion efficiency to output laser power. The laser according to the present invention may be a solid state laser. Materials

The materials used for the laser material, the Raman-active medium and the non-linear medium are well known in the art. Commonly neodymium is used as the dopant in the laser material, and suitable laser media include Nd:YLF, Nd:YAG, NdΥALO, Nd: Glass, NdISB, Nd:GdVO₄ and Nd:YVO₄, although other dopant metals may be used. Other dopant metals that may be used include ytterbium, erbium and thulium, and other host materials that may be used include YAB, YCOB, KGW and KYW. Examples of suitable Raman-active media include KGW (potassium gadolinium tungstate), KYW (potassium yttrium tungstate) barium nitrate, lithium iodate, barium tungstate, lead tungstate calcium tungstate, gadolinium vanadate and yttrium vanadate. In use, the Raman crystal may be mounted on a copper heat sink via direct contact or via a thermoconductive compound. The Raman crystal may be water cooled in use. Each of the laser media produces a characteristic output frequency, and each of the Raman-active media produces at least one characteristic Raman shift (to generate at least one characteristic Stokes wavelength). Two identical raman-active crystals oriented differently to the fundamental polarisation may be present in the cavity so that 2 different Raman shifted frequencies are generated. Alternatively, two different raman-active crystals may be present in the cavity so that 2 different Raman shifted frequencies are generated. Where frequency doubling or other^'frequency conversion of the Raman shifted frequencies (wavelengths) are required two tunable non linear media may be used whereby in operation a first non linear medium is tuned to frequency convert the first Raman shifted frequency which is thereafter outputted from the cavity and a second non linear medium is detuned in respect of the second Raman shifted frequency and thereafter the first non linear medium is detuned in respect of the first Raman shifted frequency and the second non linear medium is tuned to frequency convert the second Raman shifted frequency which is thereafter outputted from the cavity.

By combining the two types of media, therefore, it is possible to achieve a range of output laser wavelengths. Even with a single combination of media, for example, Nd:YAG and KGW, several wavelengths may be selected as described in this invention. In this example, Nd:YAG produces an output at 1064nm and KGW can generate first Stokes wavelengths of 1158nm and 1176nm and second Stokes wavelengths of 1272nm and 1320nm. Thus five discrete wavelengths may be produced from this combination. A second harmonic generator may then serve as a frequency doubler to convert these wavelengths to 532nm (green), 579nm (yellow), 588nm (yellow/orange), 636nm (red) or 660nm (red). Also a sum frequency generator may be used to generate additional wavelengths by combining any two of the wavelengths. For example by summing the 1064nm and 1158nm wavelengths, a wavelength of 555nm is obtained. Thus a wide range of visible wavelengths are potentially available. Suitable second harmonic generators may for example be lithium borate or barium borate. As described in this invention, tuning the second harmonic generator may allow an operator to select one of these wavelengths as required.

Table 1 shows the Raman shifts for a range of Raman-active media, and Table 2 shows the Raman shifts and corresponding Stokes wavelengths for several Raman-active media. Table 1. Raman shifts for selected Raman-active media

Table 2 Raman shifts and corres ondin Stokes wavelen ths for selected Raman-active media

Each non-linear medium may be configured to select which wavelength will be converted by frequency doubling, sum frequency generation or difference frequency generation.

Examples of materials used for frequency doubling or sum frequency generation include crystalline LBO, BBO, KTP, CLBO, or periodically poled materials such as lithium niobate, KTP, KTA, RTA or other suitable materials. Periodically poled materials may generate frequency doubled or summed frequency outputs through quasi-phase matching. Frequency doubling is most efficient when "phase-matching" is achieved between a wavelength and its second harmonic. A way to configure a non-linear crystal relates to the way the crystal is "cut" relative to its "crystal axes". These crystal axes are a fundamental property of the type of crystal. The crystal may be manufactured with a "cut" to best provide phase-matching between a selected wavelength and its second harmonic. Fine tuning of this phase-matching may be achieved by "angle-tuning" the medium. The angle tolerance may be less than 0.1 degree, and temperature may be maintained within 0.1 degree. The angle tolerance may be less than 0.1 degree, and temperature may be maintained within 0.1 degree. The tolerance may be up to about 10 degrees of angle or of temperature, or up to about 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.5, 0.4, 0.3 or 0.2 degrees of angle or of temperature, and may be about 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 degrees of angle or of temperature. These tolerances vary depending on the nature of the crystal. Alternatively the fine tuning may be achieved by temperature tuning the medium. The non-linear crystal may be cut for Type I phase matching (o+o=e) or for Type Il phase matching

5 (o+e=e). In the case of Type I phase matching, the polarizations of the input frequencies are parallel to each other and to a defined plane of the crystal, and the polarization of the output is orthogonal to the input frequencies. In the case of Type Il phase matching, the polarizations of the input frequencies are orthogonal to each other, and one of the input frequencies is parallel to a defined plane of the crystal. In this case the output is orthogonal to the defined plane and to the

I₀ polarization of one of the input frequencies and parallel to the polarization of the other input frequency. The following reference describes type 1 and type 2 phase matching in detail and is herein incorporated by cross reference: Handbook of nonlinear optical crystals, V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan, 2nd edition, Springer Series in Optical Sciences, vol.64, New York, 1997, ISBN 3-540-61275-0 is A switchable Raman laser system according to the present invention may be constructed using separate Raman-active and non-linear media. In this manner, the properties of the individual media may be optimized independently. The inventors have found that in this manner a laser system may be constructed that provides acceptable power output despite the additional insertion loss due to the use of an additional optical component. In selectively providing an output laser beam from this

2o laser system a cavity laser beam is generated within the resonator cavity by pumping a laser material located in the cavity with a pump beam from a pump source located outside the cavity. The wavelength of at least a portion of the laser beam is then shifted by passing the laser beam through a Raman-active medium located in the resonator cavity. A single wavelength (in the case of SHG) or pair of wavelengths (in the case of SFG or DFG) of laser light to be frequency converted is (are)

25 then selected from the various wavelengths resonating in the cavity. The selected single wavelength or pair of wavelengths of laser light is (are) then frequency converted in a separate step, using a tunable non-linear medium to generate a single converted wavelength of laser beam, which may then be outputted from the laser system. The use of separate Raman-active and non¬ linear media enables the use of materials with higher damage thresholds than a single medium

30 performing both functions, and enables independent optimization of parameters such as position in the cavity for each medium. It also provides access to a wide range of output frequencies, as described below. Location of elements It is important for the efficient operation of the laser system described herein that the component parts of the system be located correctly. In particular, the non-linear medium should be located at a position in the cavity where the diameter of the beam to be wavelength converted is sufficiently small to achieve acceptable conversion efficiency. Thermal lensing arises from the

5 inelastic nature of the stimulated Raman scattering processes. Thus for every scattering event, a small amount of the fundamental photon is deposited as heat in the Raman-active medium. This leads to a non-uniform temperature profile across the Raman-active medium. Commonly, the refractive index of a laser material increases with an increase in temperature, and consequently said laser material acts as a convex lens. Commonly the refractive index of a Raman-active o medium decreases with an increase in temperature, and consequently said Raman-active medium acts as a concave (diverging) lens. The combination of these two effects may lead to a complex beam width profile along the resonator cavity. The laser system of the present invention may be operated under conditions in which thermal lensing arises. Achievement of increased output power necessitates a consideration of the resonator spatial-mode dynamics which will depend on the s thermal tensing. Thermal lensing may in some laser systems according to the invention be controlled by cooling at least one of the Raman-active medium, the laser material and the non¬ linear medium. This may be achieved using for example a cooler, a heat sink, a water cooler, a copper heat sink, a water cooled heat sink, a thermoelectric cooler, such as a Peltier device, or some other device. The thermal lens may impact on the stability characteristics of the laser system. o The laser material may have a positive thermal lensing effect and the Raman-active medium may have a negative thermal lensing effect, and the positive lensing effect may be comparable in size to the negative lensing effect. The thermal lensing effect of the components of the laser system may change with a change in pump power. The power of the thermal lens in the laser material is primarily dependant upon the output power of the pump source, the fraction of this power that is 5 absorbed in the laser material, and the size of the pump laser beam inside the laser material. The power of the thermal lens in the Raman-active medium is primarily dependant upon the power of the Raman-shifted beam, the size of the Raman-shifted beam inside the Raman-active medium, the wavelength of the Raman-shifted beam and the Raman shift.

Due to thermal lensing within the different components of the laser system, in addition to o curvature of the cavity mirrors and natural diffraction, the beam width of a laser beam within the resonator cavity of the laser system will vary along the length of the cavity as a result of heating effects within the various components. Since the efficiency of the processes occurring in the non¬ linear medium increases with an increase of the power of the incident laser beam, the location of the non-linear medium is critical to the efficient operation of the system. Furthermore, since the heating of components of the system is due to passage of a laser beam through those elements, the optimum location of the elements will vary both with time during warm-up of the system and with the power of the laser system. A laser system may be designed for a particular output power, and will be designed to operate at peak efficiency after reaching normal operating temperature.

The present inventors have discovered that the resonator stability problems associated with operation of Raman solid-state lasers can be solved by designing a solid-state Raman laser taking into account the thermal lensing power of the laser material and the Raman-active medium. Thermal lenses are formed in the laser material in the following way. In the process of generating laser output at the fundamental wavelength in the laser material, heat is deposited in the laser material and a temperature distribution is established. Through the thermo-optic coefficient, a variation in refractive index develops across the laser material, which acts to focus light passing through the material, the laser material acting as a converging lens - this is a thermal lensing effect having a focal length fι_. The magnitude of fi. decreases with increasing absorbed pump power and for maximum absorbed pump power fL=fι.(min) i.e. from initial start-up current to maximum operating current, there is a decrease in focal lengths of the laser material.

Thermal lenses are also generated in the Raman-active medium. With frequency conversion by SRS (stimulated Raman scattering) heat is generated inside the Raman-active

^' medium leading to significant lensing effects and a focal length fa. These effects arise from the inelastic nature of the nonlinear process and for every scattering event, a small fraction of the photon energy (7.9% in the case of UIO3) is deposited as heat in the Raman-active medium. The degree of heating increases with the power generated at the Stokes wavelengths, more specifically for every first or second Stokes photon generated inside the laser cavity, a small but fixed amount of heat is deposited inside the medium. The resulting temperature distribution which is affected by the thermal conductivity of the crystal and the size of the laser beam inside the resonator cavity causes a variation of refractive index across the medium. While the positive thermal lens in for example a Nd:YAG laser material scales approximately linearly with absorbed power from a diode laser, the thermal lens in for example UIO3 depends on the intracavity power density at the first- Stokes wavelength and any higher order Stokes wavelength. For UIO3, the thermo-optic coefficient (dn/dT) is -84.9x10^-6K-¹ at a wavelength of 1 micron (according to Optical Society of America Handbook of Optics, ed. Bass, 1995) (over ten times larger than in Nd.ΥAG and of opposite sign). This means that light passing through the Raman-active medium is caused to diverge as though passing through a conventional lens with focal length "-fa". Based on measurement of the thermal lens by the inventors in an arclamp-pumped UIO3 Raman laser, the size of the negative thermal lens in L1IO3 may be as short as -10cm (comparable to that in the Nd:YAG medium).

Both the thermal lenses in the laser material and in the Raman-active medium impact substantially on the stability characteristics of the resonator in a dynamic way. Suitably the position of the laser material and the Raman-active medium in the cavity and/or reflector (mirror) curvatures is such that the laser is capable of stable operation over a sufficiently-wide range of combinations for FL and FR including the special case where fι=fR=infinite (so that laser action can be initiated) and also f_R=infinite, fi>fL(min) (so that laser action desirably does not cease if SRS ceases). The laser systems of the present invention may be capable of stable operation. They may be capable of providing stable output power over at least 1 hour, or for at least 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45 or 50 hours. The stability may be such that the output power varies over the stated period by less than about 20%, or by less than about 15, 10, 9, 8, 7, 6, 5, 4, 3, 2 or 1%.

Suitably a curvature of at least one of the reflectors and/or the positions of the laser material and the Raman-active medium relative to the cavity configuration are such that the focal lengths of the laser material at pump input powers and the focal lengths of the Raman-active medium at the desired Raman laser output power range are maintained within a stable and preferably efficient operating region. In preferred embodiments this can be achieved by optimising the cavity configuration as a function of the focal lengths by in addition to positioning the laser material and Raman-active medium within the cavity and/or selecting a curvature of at least one of the reflectors, optimising one of more of: a separation between one or more of the reflectors, the laser material and the Raman- active medium; transmission characteristics of the output coupler; and the pulse repetition frequency. Additional effects such as gain focussing and self-focussing of the Raman and/or laser beams may affect the resonator stability but these are considered to be of lesser importance than the effects already discussed.

The transmission characteristics of the output coupler may be such that the output coupling at the desired wavelength(s) is between about 0.1 and about 100%, or between about 0.1 and about 80, about 0.1 and 50, 0.1 and 30, 0.1 and 20, 0.1 and 10, 0.1 and 5, 5 and 100, 10 and 100, 50 and 100, 70 and 100, 5 and 50, 5 and 20, 80 and 100, 80 and 90, 90 and 100, 90 and 95 or 10 and 50%, and may be about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, 99, 99.5 or 99.95%. The output coupler may have high transmissions at the desired wavelength(s) of from about 10 to about 99.95% which may apply to Q-switched Raman lasers, while the output coupler may have transmissions from about 0.1% which may be used for continuous wave (cw) Raman lasers. In an embodiment, the higher transmission values may be used for high power devices and applications whilst the lower transmission may be used for low power devices. The transmission characteristics of the output coupler for SHG/SFG output may be in the range of from about 50 to about 100%, or from about 60 to about 100%, or from about 70 to about 99.9%, or from about 70.to about 90%.

Thermal lensing may also be addressed by the inclusion of one or more additional components in the resonator cavity that themselves give rise to thermal lenses in such a manner as to at least partially counteract the thermal lenses of the other components. Thus for example if the non-linear medium provides a negative lens, an additional component may be located in the cavity that provides a positive lens of comparable magnitude to the negative lens. Alternatively, a means may be included to move the components of the laser system in order to compensate for the thermal lens. Thus one or more motors may be provided in order to move one or more components of the laser system to an optimum position. The motors may be controlled by a computer, which may be capable of receiving information from the cavity (e.g. temperature, intensity of laser radiation etc.), using the information for determining the optimum position of the components of the system, and providing one or more signals to the one or more motors in order to signal them to move the one or more components to the optimum position(s). The feedback system as described above may be continuous, in order to compensate for changes in the thermal lensing with temperature during operation of the laser system.

In another embodiment, the laser system may also comprise an intracavity etalon in order to prevent generation of the parasitic Nd:YAG laser wavelengths at 1060 and 1074nm. The oscillation of these parasitic wavelengths with the fundamental may be the cause of very high laser fields (spiking) which surpass the damage threshold of the laser optic materials. The etalon in one example may be inserted in a resonator portion of the laser system and is tuned to allow passage of the 1064nm radiation, but which has a free spectral range and finesse in an arrangement such that losses due to the parasitic radiation are sufficient to prevent lasing in use. The etalon may be an uncoated etalon and is thin in profile. In one^' example, the thickness of the etalon is in the range from about 60 to about 120μm, about 70 to about 110 μm and may be about 100.0μm.

In another embodiment, the output power of the laser system of the present invention may be increased above 3OW by taking the following steps to reduce the thermal lens in the arclamp pumped laser material. These steps may include using a close-coupled pump chamber design, using one or more laser rods, and filtering the arclamp emission to reduce parasitic heating of the one or more laser rods. In particular, these steps may scale average 532nm output power of the laser system of the present invention beyond 3OW. The pumped laser material may be one or more arclamp pumped YAG rods or other suitable laser rod(s). In further preferred embodiments of the invention, the laser is also optimised for given pump powers for optimum mode sizes in the laser gain material and in the Raman gain medium and if present a non-linear medium and optimum laser output power so as to obtain efficient energy extraction from the laser material as well as efficient conversion through stimulated Raman scattering (SRS) in the Raman-active medium and if present the non-linear medium whilst maintaining cavity stability and avoiding optical damage of the laser components i.e., the various components are matched on the basis of their associated mode sizes. Since SRS may not require phase matching, the conversion efficiency of the process is not limited by such factors as angular acceptance, back-conversion and walkoff. Consequently, the photon conversion efficiency may approach 100%. The optimum spot size and power density in the Raman-active medium may be a compromise between maximising the conversion efficiency and avoiding optical damage. The cavity is suitably optimised so that the relative mode size in each of the materials present in the cavity is such so as to provide efficient stable output. Suitably conversion efficiencies from fundamental laser wavelengths to Raman wavelengths of greater than 40%, more preferably greater than 50% are obtainable (for example between about 50 and about 95%, or between about 50 and 90, 50 and 80, 50 and 70, 50 and 60, 60 and 95, 70 and 95, 60 and 90 or 60 and 80%, e.g. about 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95%) and from Raman wavelengths to frequency doubled (visible) wavelengths of greater than 30%, more preferably greater than 50% (for example between about 50 and. about 95%, or between about 50 and 90, 50 and 80, 50 and 70, 50 and 60, 60 and 95, 70 and 95, 60 and 90 or 60 and 80%, e.g. about 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95%). Suitably, overall conversion efficiencies from optical pump power to visible output power of about 2, 3, 4, 5, 6, 7, 8, 9 or 10% or greater than 10% are obtainable.

In order for the Raman laser to operate with suitable optimal efficiency the key design parameters (i.e. mirror curvatures, cavity length, positioning of the various components) are suitably chosen so that the resonator mode sizes in the laser material (A), the Raman-active medium (B) and if present the non-linear medium (frequency-doubling crystal) (C) are near-optimum at a desired operating point. One can denote the beam sizes (radii) in these media as G>A, COB and ωc respectively. In cases where the laser beam is not circular, it is commonly elliptical, and the beam size may be considered along the long and short axes of the ellipse. The beam size is taken to be the distance from the beam axis to the point where the intensity of the beam falls to 1/(e²) of the intensity of the beam axis. The beam size may vary along the length of a particular component. The beam size in a particular component may be taken as the average beam size within the component (commonly used for the Raman-active medium) or as the minimum beam size within that

5 component (commonly used for the laser material and for the non-linear medium).coA is suitably mode-matched to the dimension of the pumped region of the laser material i.e., the pump spot size (ωp). ωp can vary according to the power of the pump laser source (e.g., a diode laser) and the pumping configuration. For example a laser crystal end-pumped with a low power (~ 1 W) diode laser may have a ωp of approximately 100 μm, for example from about 50 to about 200μm, or from o about 50 to about 150, from about 50 to about 120, from about 50 to about 100, from about 50 to about 70, from about 70 to about 200, from about 100 to about 200, from about 120 to about 200, from about 150 to about 200, from about 70 to about 150, from about 80 to about 130 or from about 90 to about 11 Oμm, and may have a ω_P of about 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200μm. . A laser crystal end-pumped with a 10-60 W diode laser may have s a ωp in the range from about 90 to about 700 μm, for example approximately 100 to 700, 100 to 500, 100 to 300, 150 to 650, 150 to 250, 200 to 600, 300 to 400, 250 to 350, 200 to 375, 90 to 400, 200 to 700, 400 to 700, 500 to 700, 200 to 400 or 400 to 600 μm, and may have a ωp about 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650 or 700 μm. A laser crystal side- pumped by one or more diode lasers may have a ωp in the range of about 500 to about 1500 μm. 0 A laser crystal side-pumped by one or more diode lasers may have a ωp in the range from about 500 to about 1500 μm, for example from about 500 to about 1200, from about 500 to about 1000, from about 500 to about 700, from about 700 to about 1500, from about 1000 to about 1500, from about 1200 to about 1500, from about 600 to about 1400, from about 700 to about 1300, from about 800 to about 1200 or from about 900 to about 1100μm, and may have a ωp about 500, 550, s 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350, 1400, 1450 or 1500 μm. Optimal mode-matching of ωp and GOA is a suitable pre-requisite for enabling efficient extraction of the gain in the laser material. When ωp and COA are mode matched, the pump laser beam spot overlaps with the cavity laser beam within the laser material. If GOA is too small, then (i) laser gain may not be extracted efficiently into the TEMoo resonator mode and (ii) the laser o may oscillate on higher-order modes which are generally not desirable. If OOA is too large, then diffraction losses can occur in the resonator due to aberrations in the thermal lens associated with the laser crystal. This effect is undesirable and deleterious for pumping powers approximately > 3 W. -^- may be in the range 0.45 to 1.55, 0.5 to 1.5, 0.6 to 1.4, 0.7 to 1.3, or 0.75 to 1.25 or 0.7 to ω_?

1.25 or 0.75 to 1.3 or 0.8 to 1.2 or 0.9 to 1.1 or 0.95 to 1.05. ^- may be about 1.01. 1.02, 1.03,

1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.12, 1.14, 1.16, 1.18, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 0.99, 0.98, 0.97, 0.96, 0.95, 0.94, 0.93, 0.92, 0.91, 0.9, 0.88, 0.86, 0.84, 0.82, 0.8, 0.75, 0.7, 0.65, 0.6, 0.55, 0.5 or 0.45, or may be equal to or about 1. COA may be greater than or equal to ωp. The pump spot size may overlap completely with the cavity laser beam within the laser material. When the pump spot size is mode matched to the mode of the cavity laser beam in the laser material in the resonator, the excitation of the fundamental Gaussian (TEMoo) mode may be the main mode in the resonator cavity, or there may be higher-order transverse modes present, COA may be in the range of about 70 to 850 μm, for example about 100 to 850, 250 to 850, 400 to 850, 550 to 850, 70 to 500, 70 to 300, 70 to 150, 100 to 600, 200 to 500, 100 to 300, 300 to 500, 500 to 700 or 700 to 850 μm, and may be for example about 70, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800 or 850 μm. OOB may be optimised for high conversion through the SRS process, while at the same time optical damage to the Raman media is suitably avoided. The optimum value for OOB varies from crystal to crystal because (i) different Raman-active crystals have different Raman gains and different thresholds for optical damage. If COB is too large, then the conversion efficiency of the SRS process will be lower than optimum. If COB is too small, then (i) the optical power density in the Raman-active medium can approach the threshold for optical damage in that crystal and (ii) the thermal lens associated with the Raman-active medium may become more aberrated, resulting in increased resonator losses (due to diffraction). Typical values for COB are in the range of about 90 - 600 μm, and may be in the range of about 100 to 600, 200 to 600, 300 to 400, 250 to 350, 200 to 375, 90 to 400, 100 to 300, 400 to 600, 200 to 400 or 400 to 600 μm, and may be about 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550 or 600μm. ωc is suitably optimised for efficient frequency conversion through the frequency doubling or sum frequency generation processes. The optimum value for ωc varies according to the type of crystal used. Different crystals have different non-linear coefficients, walk-off angles and damage thresholds. If ωc is too large, then (i) conversion efficiency will be lower than optimum; (ii) the optical field at the Stokes frequency can be "under-coupled" - in this case, unwanted non-linear processes such as higher-order Stokes generation and self-focussing can occur. If ωc is too small then (i) optical damage can occur to the crystal, (ii) the effective length of the non-linear interaction can become too short due to "walk-off" effects and (iii) the optical field at the Stokes wavelength can be "over-coupled" which can result in reduced conversion efficiency of the SRS process. Typical values for D₀ are in the range of about 90 - 600 μm, and may be in the range of about 100 to 600, 200 to 600, 300 to 400, 250 to 350, 200 to 375, 90 to 400, 100 to 300, 400 to 600, 200 to 400 or 400 to 600 μm, and may be about 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550 or 600μm. This discussion assumes that the mode size in A, B and C is the same for optical fields at different wavelengths. In practice OOA, COB and ωc may be slightly different (by < 10%) owing to effects such as gain-guiding and self-focussing.-

Suitably the mode size (beam size) in the laser material is approximately equal to the pump spot size. Suitably in a Raman-active medium such as a KGW, BaNCte or UIO3 crystal the spot size COB is optimised for stable operation and efficient conversion such that COB is similar to or smaller than the beam size COA in the laser material. Suitably the beam size ωc in the non-linear medium if present is similar to or smaller than the beam size COB in the Raman-active medium. A preferred situation therefore is when COA > COB > ωc. Stable arrangements may be achieved in which COA > COB and/or COB > ωc and/or COA > ωc, and/or ωc >OOB. The mode size (or spot size, cos) of the seed beam may also be mode matched or about mode-matched to the mode size (or spot size) of the beam waist in the Raman-active medium. The ratio of the mode size of the beam waist in the Raman-active medium to the mode size of the seed beam in the Raman-active medium may be between about 0.5 and 2, or between about 0.75 and 1.5, 0.8 and 1.25 or 0.9 and 1.1, and may be about 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2. In one form of the invention cos « COB. The seed beam may not be mode matched or may not be about mode-matched to the mode size (or spot size) of the beam waist in the Raman-active medium.

In coupled cavity resonators in which there are separate cavities for the cavity laser beam and the Raman-shifted laser beam, there is an additional mode matching requirement, that the cavity laser beam and the Raman-shifted laser beam have similar mode sizes in the Raman-active medium. The ratio of the mode size of the Raman-shifted beam in the Raman-active medium to the mode size of the cavity laser beam in the Raman-active medium may be between about 0.5 and 2, or between about 0.75 and 1.5, 0.8 and 1.25 or 0.9 and 1.1, and may be about 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2. In preferred embodiments of the invention, the thermal lens focal lengths for the laser material at the laser input powers and the thermal lens focal lengths of the Raman-active medium at the laser output powers are determined and the position of the laser material and the Raman- active medium in the cavity are selected to ensure that during operation of the laser the resonator is stable. Suitably the thermal lenses for the laser material can be calculated and then confirmed by cavity stability measurement. Alternatively the thermal lenses can be determined by standard measurement techniques such as lateral shearing interferometry measurements which can also provide information on any aberrations. A suitable interferometric technique is described in M. Revermann, H.M. Pask, J.L. Blows, T. Omatsu 'Thermal lensing measurements in an intracavity UIO3 Laser", ASSL Conference Proceedings February 2000; in Jl. Blows, J.M. Dawes and T. Omatsu, 'Thermal lensing measurement in line-focus end-pumped neodymium yttrium aluminium garnet using holographic lateral shearing interferometry", J. Applied Physics, Vol. 83, No. 6, March 1998; and in H.M Pask, Jl. Blows, J.A. Piper, M. Revermann, T. Omatsu, 'Thermal lensing in a barium nitrate Raman laser", ASSL Conference Proceedings February 2001.

Suitably at least the position of the laser material and the Raman-active medium in the cavity are selected such that the combination of the thermal lens powers for the laser and Raman media fall within a stable operating region of a stability plot.

A stability plot of a simple two mirror cavity is a plot of the parameters gi on the y-axis and g2 on the x-axis of a graph. These parameters can be represented by the equations:

g₂ = 1-L/R₂ (2) wherein L is the distance between the two mirrors, Ri is the radius of curvature of one of the two mirrors and R2 is the radius of curvature of the other. It has been determined that, for a resonator cavity to be stable,

0 ≤ gi x g₂≤ 1 (3)

If either one of gi and g₂ is negative and the other one is positive, their product is negative and the resonator cavity will be unstable. If both are positive or if both are negative and if their product is less than 1 , then the resonator cavity will be stable. If the thermal lensing effect of the laser material is positive and the thermal lensing effect of the Raman-active medium is negative and if they are of similar magnitude under operating conditions, they can counteract one another to ensure that the resonator cavity remains stable under operating conditions.

In order to ensure that the cavity remains stable at elevated temperatures of the laser material and Raman-active medium, the laser material and the Raman-active medium advantageously have thermal lensing effects of opposite sign, and the length of the resonator cavity and the relative positions of the laser material and the Raman-active medium relative to the mirrors defining the resonator cavity are selected such that the laser modes do not expand to the extent that the radiation suffers large losses. Thus the position of the Raman active medium relative to the positions of the laser material and the at least two reflectors, the length of the cavity, the curvature of at least one of the reflectors that define the cavity, as well as the combination of the focal lengths of the thermal lenses formed in the laser material and the Raman active medium during operation of the laser may be such that the laser resonator (cavity) remains optically stable when the current to the pump laser is increased from zero to a desired operating laser power. The desired operating power may be such that the output power is greater than 1 W.

A suitable stability plot for a two-mirror resonator can be determined as follows. The ray transfer matrix (M) is calculated for a transit of the optical resonator. The elements of this matrix

A B M = enable an equivalent (two-mirror) resonator to be defined with equivalent g- C D parameters gi^*= A, g2^* = D and L^* = B. The optical system in the resonator cavity may be described by an ABCD matrix which is the product of one or more ABCD matrices, each of which corresponds to an optical element through which light passes. The ABCD law enables one to calculate the change in a Gaussian laser beam as the beam passes through a particular element. The determinant of the matrix M should be unity for a stable arrangement of the resonator cavity, i.e. AD-BC=I . The stability regime for the resonator cavity is where the cavity laser beam obeys the inequality

I S I < 1 , where S=0.5^*(A-D). The predominant mode of the cavity laser beam may be a Gaussian beam. A Gaussian beam is one in which the. cross-sectional power profile of the beam has a Gaussian distribution. The q parameter of a Gaussian laser beam at a particular position in a resonator needs to satisfy the ABCD law: q=(Aq+B)/(Cq+D). The solutions to this are given by:

The allowed solution should have a negative imaginary component. The q parameter incorporates the mode size and the beam curvature, and is described in detail in the B.E.A. Saleh and M.C. Teich, Fundamentals of Photonics, John Wiley and Sons, New York, 1991, the contents of which are incorporated herein by cross-reference. The mode size of the cavity laser beam may be determined along the resonator cavity from the q parameter.

In particular, for a system having a lens of focal length f (i.e. refractive power 1/f) located a distance di from a first mirror having radius of curvature Ri, and a distance 02 from a second mirror having radius of curvature R2, the elements of the matrix M are:

A = gi^* _,

30

B = L^*

C = (gi^** g2^*-1)/L\ where L = di+d₂- DWd₂

D = g₂ ^* where gf = gi - D^*dj(1 -di/Ri) ; i, j = 1 , 2; i ≠ j Texts describing this method are N. Hodgson and A. Weber, Optical Resonators",

Springer- Verlag London Limited, 1997 and W. Koechner, "Solid-state Laser Engineering", Springer- Verlag, 1992.

The dynamic nature of the Raman laser resonator as the diode current is increased can be simulated by calculating gi^* and g₂ ^* for suitable combinations of the thermal lenses in the laser and Raman-active crystals. When plotted on a stability plot, a curve can be defined. In a well- designed resonator, this curve will lie in a stable region of the stability plot (ie O ≤ gi^** g₂ ^*≤ 1) from the point where laser action is initiated to the point corresponding to the desired operating power.

In preferred embodiments of the invention, a computer model is used to determine suitable cavity configurations for a particular power regime using different combinations of media. In such an embodiment the thermal lensing power for a variety of Raman media crystals can be measured over a wide parameter space of Raman laser output powers and mode sizes and thermally modelled. A standard resonator design program using 2-mirror configurations to more complex folded resonators can then be used to determine the fundamental and Raman laser mode sizes as a function of pump power enabling stable resonators to be designed to produce output ^' powers in specified regions from mWs to multiwatt outputs. The output power may be varied by varying the frequency of the pump laser beam.

In the present context, mode matching is the process of matching the pump laser beam waist in the laser material with the beam waist of the cavity laser beam in the laser material. In order to perform mode matching of the pump laser beam with the cavity laser beam, the ABCD law may be used to determine the mode size of the cavity laser beam in the laser material and the pump laser beam may be focussed onto or into the laser material such that the mode size of the pump laser beam matches or about matches the mode size of the cavity laser beam. An example of mode matching the pump laser beam with the cavity laser beam is provided in PCT/AU01 /00906, the contents of which are incorporated herein by cross-reference. Mode matching may be required in order to achieve optimal power from the laser system.

The laser material can be pumped/stimulated by a pulsed or continuous arclamp, flashlamp or diode (semiconductor) laser using a side-pumped, single end-pumped or double end- pumped geometry. End pumping of the laser crystal is a very efficient approach to generating Raman laser output or its second harmonic. For example the inventors have demonstrated optical to optical conversion efficiencies as high as about 10% for end pumping with an 18W output from a fibre-coupled diode laser to produce 1.7W frequency-doubled KGW Raman laser output at 579nm. Compared to side-pumped laser crystals, end-pumped laser crystals generally have high gain and give rise to short Q-switched pulses, and the pump spot size in the laser crystal can be adjusted to match the resonator mode size. However end-pumped laser crystals can also give rise to strong (and abberated) thermal lensing, and this ultimately limits the scalability of end-pumped Raman lasers.

Side-pumping of the laser crystal may not result in such high optical-optical conversion efficiency, but it is a cheaper approach, is more easily scalable and enables greater flexibility in where the resonator components can be placed.

The laser beam may be Q-switched in order to obtain sufficiently high peak powers for efficient frequency conversion. However laser systems may be constructed according to the present invention with no Q-switch. For example, when using a flashlamp pump source, it may be unnecessary to use a Q switch. The power of the laser beam at each element of the laser system should however be below the damage threshold of that element. Thus the energy of the laser beam in the laser material should be below the damage threshold for that particular laser material, the energy of the laser beam in the Raman active medium should be below the damage threshold for that particular Raman active medium and the energy of the laser beam in the non-linear medium (if present) should be below the damage threshold for that particular non-linear medium. The damage threshold of a particular element will depend, inter alia, on the nature of that element. The peak power of a laser pulse generated by a Q-switch may be calculated by dividing the energy by the pulse width. Thus for example if the laser pulse energy is 200μJ and the pulse width of the Q- switched laser beam is 10ns, then the laser power will be 200μ J/1 Ons, ie 2OkW. The power density of the laser beam at any particular location may be calculated by dividing the power of the laser beam at that location by the mode size (area) at that location. The power density of the laser beam at each element of the system may be below the damage threshold for that particular element, that is the power densities for the laser material, the Raman active medium and, if present, the non¬ linear medium, should be below their respective damage thresholds. Thus for example for a UIO3 crystal with a IOOMWcnr² damage threshold, the above Q-switched laser beam with 2OkW peak power should have a mode size of greater than 80μm. This will be the minimum mode size that may be used without damage to that element. Since the repetition rate of the Q-switch affects the power deposition in the elements of the laser system, it will affect the heating and hence the thermal lensing of those elements. Most importantly, and usefully in the design of the laser system, the choice of repetition rate affects the peak power of the cavity laser beam and therefore the conversion efficiency into the Raman laser beam. Because the power of the thermal lens in the Raman-active medium is dependent on the power of the Raman laser beam, the repetition rate may be used to vary the thermal lens power in the Raman-active crystal, without significantly changing the thermal lens power in the laser crystal. The repetition rate should therefore be chosen such that the system is stable and so that the damage thresholds of the elements are not exceeded. The repetition rate may be between about 1Hz and 5OkHz, and may be between about 1Hz and 1OkHz or about 1Hz and 1kHz or about 1 and 100Hz or about 1 and 10Hz or about 100Hz and 50kHz or about 1 and 5OkHz or about 10 and 5OkHz or about 20 and 5OkHz or about 1 and 15kHz or about 15 and 5OkHz or about 10 and 3OkHz or about 5 and 1OkHz or about 5 and 15kHz or about 5 and 2OkHz or about 5 and 25kHz or about 7.5 and 10kHz or about 7.5 and 15kHz or about 7.5 and 2OkHz or about 7.5 and 25kHz or about 7.5 and 3OkHz or about 10 and 15kHz or about 10 and 2OkHz or about 10 and 25kHz, and may be about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100,150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850 or 900Hz or about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45 or 5OkHz. The pulse duration of the Q-switched laser beam may be in the range of about 1 to 100ns, or about 1 to 50ns, or about 1 to 20ns or about 1 to 10ns or about 5 to 80ns or about 5 to 75ns or about 10 to 50ns or about 10 to 75ns or about 20 to 75ns or about 5 to 100ns or about 10 to 100ns or about 20 to 100ns or about 50 to 100ns or about 5 to 50ns or about 10 to 50ns, and may be about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100ns. In general, a laser system according to the present invention may have a pulse duration that may range from picoseconds, for mode-locked systems, to nanoseconds, for example for Q-switched systems, to hundreds of microseconds or several ms for example for pulse pumped systems. The system may in some circumstances be continuous wave (CW) systems. Thus the pulse duration (for pulsed systems) may therefore range between about 1 ps to about 1ms and may be between about 1ps and 1, 1ps and 1ns, 1ns and 1ms, 1μs and 1ms or 1ns and 1μs, and may be for example about 1, 5, 10, 50, 100 or 500ps, about 1, 5, 10, 50, 100 or 500ns, about 1, 5, 10, 50, 100 or 500μs or about 1ms. The resonator cavity may have a folded, bent or linear configuration or other suitable configuration. The resonator cavity may comprise a coupled cavity resonator. The resonator cavity may be for example a bent cavity, a coupled cavity, Z-cavity, and L-cavity. The position of the laser material and Raman-active medium in a chosen configuration are suitably chosen to provide cavity stability for a wide range of combinations of 1 and ΪR.

The laser material suitably generates laser beams at a fundamental wavelength (1064nm for Nd:YAG) when stimulated by pump light of an appropriate wavelength, and the fundamental laser beam then propagates inside the laser resonator. Suitably the laser material is formed by one of the following crystals: Nd:YAG, NdiYLFJ Nd:glass, Ti-sapphire, Erbium:glass, Ruby, ErbiumΥAG, ErbiumΥAB, Nd:YAIO₃₎ Yb:YAI0₃, Nd:SFAP, Yb:YAG, Yb:YAB, Cobalt:MgF₂, Yb:YV0₄, Nd:YAB, Nd:YVO₄, Nd:YALO, Yb:YLF, Nd:YC0B, Nd:GdCOB, Yb.ΥCOB, Yb:GdCOB or other suitable laser material. The laser material may be broadband AR-coated for the 1-1.35 micron region to minimise resonator losses. Optionally the laser material is wavelength tunable and capable of generating high power output which can be mode-locked.

The Raman-active medium suitably enables the fundamental radiation to be converted to first (or higher) Stokes wavelength through the nonlinear process Stimulated Raman Scattering (SRS). Depending on application, the Raman-active medium suitably converts the fundamental wavelength to the first Stokes wavelength, to the second Stokes wavelength or to a higher Stokes wavelength. The Raman-active medium may be broadband AR-coated for the 1-1.35 micron region to minimise resonator losses. The Raman-active medium is suitably chosen on the basis of high transmission at the fundamental and Stokes wavelengths, useful Raman shift, fairly high Raman cross-section,^' high damage threshold and availability in lengths exceeding 1cm and chosen such that the Raman gain is adequate. The Raman-active medium may be a crystal, and may be a single crystal. The length of the crystal may be between 0.5 and 9cm long, and may be 1-7cm long. For example, the length of the crystal may be from about 1 to about 7 cm, 0.5 to 7cm, 0.5 to 5cm, 0.5 to 3cm, 1 to 9cm, 3 to 9cm, 5 to 9cm, 7 to 9cm, 2 to 7 cm, 3 to 6cm, or 4 to 6cm, for example, about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, about 6, 6.5, 7, 7.5, 8, 8.5 or 9cm long. A typical dimension of the crystal is 0.5x0.5xy cm. where y is crystal length and is in the range 1-7cm or 0.5 to 9cm, for example 1 to 7cm. Longer crystal lengths may be used where higher output powers are desired since the Raman gain coefficient is proportional to the length of the Raman gain medium. Alternatively a longer path through the Raman-active medium can be achieved using a multipass of zigzag geometry such as described in Byer US Patent no. 5,673,281. Suitably the Raman-active medium is a single crystal of KGW, UIO3, Ba(NU3)2 or other suitable Raman active material such as KDP (potassium dihydrogen phosphate), KD^*P (deuterated), lithium niobate, and various tungstates (KYW, CaWO₄) and molybdate or vanadate crystals. Other suitable Raman active crystals are described in the CRC Handbook of Laser or the text "Quantum Electronics" by Pantell and Puthoff. KGW, UIO3 and Ba(NOe)₂ are preferred. KGW is a biaxial crystal with a high damage threshold, and is capable of providing Raman shifts of 768 and 901cm-¹. Ba(NO3)2 is an isotropic crystal with a high gain coefficient (11cm/GW with 1064nm pump) leading to low threshold operation and can provide a Raman shift of 1048.6cm-¹. LJIO3 is a polar uniaxial crystal with a complex Raman spectrum which depends on the crystal cut and orientation with respect to the pump propagation direction and polarisation vectors and can provide Raman shifts of between 745 cm-¹ and 848cm-¹ (which are useful when targeting wavelengths for specific applications for example 578nm which is useful for medical applications including ophthalmology and dermatology) but has a lower damage threshold (about lOOMWcnr²) compared with Ba(NOs)₂ (about 400MWcπr ²). KGW has a far higher damage threshold of about lOGWcnr². KGW, Ba(NO₃)2 and LiIO₃ all have good slope efficiencies (determined by the ratio of Stokes to fundamental photon energies) with optical to optical conversion efficiencies of 70-80% being reported for all three. The laser system is preferably operated such that optical damage of the Raman active medium is avoided.

The following laser material/Raman-active medium combinations are particularly desirable: Nd:YAG/LilO₃, Nd:YAG/Ba(NO₃)₂, Nd:YAG/KGW, Nd:GdVO₄/LilO₃₎ Nd:GdVO₄/Ba(NO₃)2, Nd:GdVO₄/KGW, Nd:YVO₄/LilO₃, NdIYVO₄ZBa(NOs)₂, Nd:YV0₄/KGW and Nd:YLF/CaWO₄. In one embodiment the laser may further include a non-linear medium for frequency doubling the Raman laser beam to produce an output at its second harmonic or for sum or difference frequency mixing of the fundamental and Raman laser frequencies, the non-linear medium being present in the resonator cavity or external to the resonator cavity.

Optionally a solid non-linear medium is used for frequency doubling the Raman laser beam to produce an output at its second harmonic or other subfrequency or different frequency wavelength. The solid non-linear medium can be located in the cavity (intra cavity doubled - doubling crystal located inside the resonator) or external to the cavity (extra cavity doubled - doubling crystal located outside of the laser resonator). Suitably a folded resonator is used. Suitable solid non-linear mediums include a second harmonic generator (SHG), a sum frequency generator (SFG)' a difference frequency generator (DFG), or other parametric frequency generator. As examples of non-linear medium mention can be made of LBO, KTP, BBO, LUO3, KDP, KD^*P, KBO, KTA, ADP, LN (lithium niobate) or periodically-poled LN or combinations thereof (e.g. to generate green and yellow lasers simultaneously). Suitably a LBO, BBO or KTP crystal is used. The light can be frequency doubled or frequency summed by angle-tuning and/or controlling the temperature of the solid non-linear medium. In preferred embodiments of the invention, the light is frequency doubled so as to generate yellow light. Typical variations in the visible wavelength with a LBO crystal cut for type 1 non-critical phase-matching with temperature tuning to approximately 149⁰C, 40°C or 0°C include 532nm (green), 578-580nm (yellow) and 632-636nm (red). By such frequency doubling it may possible to generate wavelengths in the yellow or orange spectral region suitable for dermatological, ophthalmic and visual display applications, and by means of other processes such as sum frequency generation, still further wavelengths may be generated. The resonator design may be such that the size of the laser beam in the doubling medium is sufficiently small to allow efficient conversion and high output powers but large enough to avoid optical damage. Suitably the solid non-linear medium is AR-coated to minimise losses in the 1-1.2 micron region and in the visible where possible. A suitable AR coated LBO crystal for intracavity use is 4x4x1 Omm and for extracavity use is 4x4x1 Omm although other sizes can be used.

For extracavity doubling, the Raman laser output is focussed by lenses into the crystal for maximum conversion.

Preferably the resonator cavity is defined by at least two reflectors which can be two mirrors at least one of which is preferably curved to provide a stable output laser beam (the other mirror may be flat). Other suitable reflectors that can be used in the present invention include prisms or gratings. More preferably at least two curved mirrors are used, although it is possible to use more than two mirrors, different sets of mirrors reflecting the propagating laser beam and the propagating Raman-shifted beam such as in a bow-tie resonator. When a solid non-linear medium is used, another mirror may be present such as in a dichroic cavity. Suitable reflectors defining the resonator cavity are well known in the art and can be coated to enable operation at lower Raman thresholds for the first Stokes order thereby helping to suppress higher-order Stokes generation and self-focussing. The mirrors may also be coated to have high transmission at the output wavelengths of interest. Reflectors can be provided with special dielectric coating for any desired frequency. The mirrors can provide for the laser output to be coupled out of the cavity such as by use of a broadband dichroic mirror transmissive at the frequency of the output beam but suitably highly reflective at other frequencies so as to cause build-up of the power intensities of the beams in the cavity. Alternatively a polarisation beam, splitter can be used to outcouple the laser output. The radius of curvature and separation between the reflectors (cavity length) and transmission characteristics of the outcoupling mirror are suitably chosen to provide cavity stability for a sufficiently wide range of combinations of f_. and fR. The radius of curvature of the reflectors are appropriately selected on the basis of the Raman-active and laser crystal used (for some Raman- active crystals +ve effective lens powers of the reflector are desirable and for others -ve effective lens powers of the reflectors are desirable). Suitably the mirrors are chosen so as to be greater than 99% reflective at the laser wavelengths. The output mirror may be chosen (to optimise the first Stokes output) to be 10 to 90% reflective at the Raman wavelength with the other mirror being greater than 99% reflective at the Raman wavelengths. The laser resonator cavity is suitably a stable resonator which supports the TEMoo mode. For the intracavity-doubled laser, all

5 mirrors/reflectors are suitably chosen to be >99% reflective at the fundamental wavelength and the Raman wavelength. The frequency-doubled laser beam is suitably coupled out of the resonator through a dichroic mirror - i.e., a mirror which has high transmission at the frequency-doubled wavelength but high reflectivity at the fundamental and Raman wavelengths. Preferably the resonator has three or more mirrors/reflectors and is configured so that the frequency-doubled or o frequency summed beams which are generated in both directions in the non-linear medium can be extracted efficiently in a single beam. In such a configuration, the end mirror closest to the non¬ linear medium will have high reflectivity at the frequency-doubled wavelength. The reflectors and/or mirrors may, independently, be flat or may be non-flat. They may have a radius of curvature from about 5 to about 100cm or more, depending on the factors described above and the particular s application. The radius of curvature of each mirror and/or reflector may range from about 10 to about 100, from about 20 to about 100, from about 50 to about 100, from about 5 to about 50, from about 5 to about 20, from about 5 to about 10, from about 10 to about 50, from about 10 to about 30, from about 15 to about 25 or from about 18 to about 22cm, and may be about 5, 10, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 or 100 cm, or may be some Q other value.

Suitably the transmission characteristics, radius of curvatures and separation of the reflectors are tailored to achieve efficient and stable operation of the Raman laser and when a solid non-linear medium is used, to generate output at the visible wavelengths by frequency doubling or sum frequency generation in the non-linear medium. Suitably the curvature of the reflectors and ₅ cavity length are optimised to obtain the desired mode diameter such that near-optimum beam sizes are achieved simultaneously in the laser material, the Raman-active medium and when present, the solid non-linear medium such that changes in the focal lengths of the laser material and the Raman-active medium as a result of thermal effects in the laser material and the Raman- active medium during operation of the laser do not cause the laser modes to expand to the extent o that the radiation suffers large losses. The laser material, Raman-active medium and when present, the non-linear medium can be positioned in the cavity as discrete elements. Alternatively one or more of the components can be non-discrete, one component performing the dual function of both the laser material and the non-linear medium (such as self-frequency doubling or self doubling materials such as Yb:YAB and Nd:YCOB) or performing the dual function of the laser material and the Raman-active medium (Nd:KGW) or the dual function of the Raman-active medium and the non-linear medium (such as by use of a non-centrosymmetric crystal such as LiIO₃).

5 The pulse repetition frequency of the output can be varied by using a Q-switch such as an active Q-switch or a passive Q-switch. An acousto-optic Q-switch, an electro-optic Q-switch or passive Q-switches (CnYAG) can be used. Alternatively a cavity dumping configuration or other suitable means can be adopted (see 'The Laser Guidebook" by Jeff Hecht, 2^nd Edition, McGraw-Hill 1992, the whole content of which is incorporated by cross reference). The Q-switch causes o fundamental laser output to occur in a pulsed format with high peak powers as required to achieve efficient Raman conversion. The Q-switch may be broadband AR-coated for the 1-1.2 micron region to minimise resonator losses. The selection and alignment of the Q-switch is tailored to achieve a high-Q resonator for the fundamental. The pulse frequency is suitably chosen to provide cavity stability for a wide range of combinations of 1 and fR~. Selection of the pulse repetition s frequency affects the conversion efficiency to the Stokes wavelength, and therefore the Stokes output power and therefore also the thermal lens in the Raman-active medium. It is a useful parameter to vary because (to first-order) it affects the thermal lens in the Raman-active crystal while having very little effect on the thermal lens in the laser crystal. The pulse repetition frequency may be selected so that the beam size in the laser material is greater than the beam size in the o Raman-active medium.

Varying the prf has two effects:

(i) because the Raman gain varies in proportion to the peak power at the fundamental wavelength, an increase of the prf may result in a decrease of the average output power at the Stokes wavelength (or its second harmonic); ₅ (ii) varying the prf has very little effect on the thermal lens power associated with the laser crystal (that power is determined by the power density of the diode laser pump light). However as stated in (i) above, varying the prf does impact on the output power at the Stokes wavelength. Because the thermal lens power in the Raman-active crystal is proportional to power density at the Stokes frequency, a variation in prf also changes the thermal lens power in the Raman-active o crystal (independently of the thermal lens power in the laser crystal). This can be useful in optimising the region of the stability where the system is desired to operate, and also the "transit" through the stability diagram. At least one polariser may be included in the cavity and may be one or two plates of glass at Brewsters angle and/or a cube or other polariser. Such polarisers cause the fundamental to lase on only one linear polarisation. Some polarisation discrimination can also be introduced through the use of mirrors at non-normal incidence. Reflectors

The transmission properties of the dielectric coatings on the cavity reflectors may be optimized to suit the output wavelength(s) of the laser system. Thus for example when the system comprises a non-linear medium for converting the frequency of the laser beam outputting from the Raman-active medium, the reflector may be transmissive for the converted frequency and reflective for all other frequencies generated in the cavity. Thus for example one reflector of the cavity may be optimised to transmit the pump beam frequency and reflect other frequencies that resonate in the cavity. Another reflector, the output reflector, may be optimised to be transmissive at the frequencies that may be outputted from the cavity (for example green, red and yellow) and reflective at other frequencies that may resonate in the cavity (for example the laser material frequency and the Stokes wavelength(s)). Alternatively the output laser beam may be coupled out of the cavity using a polarization selector. For example if a Type I phase matched crystal is used, the input frequencies are polarized parallel to each other and the output frequency is polarized orthogonally to the input frequencies. A polarization selector may thus be used to couple only the orthogonal output frequency out of the cavity, while reflecting the input frequencies to resonate in the cavity.

Resonator Configuration

The methods described for wavelength selection may be applied to Raman lasers constructed using a variety of resonator designs, including but not limited to coupled cavity resonators, shared intracavity resonators and self-Raman resonators. Q-switching

The Q-switch may be any of the following types: acousto-optic, electro-optic or passive. The operation of the laser is not strongly dependent on the location of the Q-switch. Wavelength selection

The present invention envisages a variety of methods in which to select the predominant Raman-shifted frequency that will resonate within the cavity. These methods include:

Seeding: Seeding may be used when the Raman-active crystal has a spontaneous Raman spectrum which includes 2 or more sufficiently strong peaks, corresponding to two or more Raman shifts. A Raman-active medium that is capable of producing more than one Stokes wavelength may be seeded by irradiating it with a seed beam of the desired wavelength, said wavelength being one of the Stokes wavelengths of the Raman-active medium. This causes the Raman-active medium to convert most or all of the photons reaching it from the laser material to the wavelength of the seed beam. Therefore the wavelength of the output laser light beam may be selected by selecting an

5 appropriate wavelength of seed beam. For example, to produce a yellow output laser light beam at

579nm from a system comprising a Nd:YAG laser material with a KGW Raman-active medium, a

' seed beam at 1158nm applied to the KGW crystal would cause it to direct a laser beam of 1158nm to the non-linear medium. If, for example, the non-linear medium is a frequency-doubling crystal, this would generate the selected yellow output laser light beam at 579nm. Alternatively, if a seed o beam at 1176nm were applied to the KGW crystal, it would cause it to direct a laser beam of 1176nm to the non-linear medium. Frequency doubling by the non-linear medium would then generate an output laser light beam at 588nm.

The seed laser may be a low power diode laser or it may be an LED or it may be some other type of seed laser. Low powered diode lasers are readily available at the desired wavelengths. LEDs s have the advantage of very low cost. The seed laser may be any type of laser which operates at a suitable wavelength to spectrally overlap with trie desired Stokes wavelength. The seed laser may be another Raman laser, most likely a low power device. Seeding may be continuous wave or modulated. Preferably the seed laser would operate continuously, and in the event that the seed laser produced pulsed or modulated output it would be necessary to synchronise the seed laser o with the Q-switch (if present) of the wavelength selectable Raman laser or with the output laser beam of the Raman . laser. The temporal frequency and phase of the seed beam may be synchronized with the frequency and phase of the output laser beam. The frequency and phase of the seed beam may be the same as the frequency and phase of the output laser beam.The power of the seed beam should be sufficient to cause one Raman transition to reach threshold and ₅ significantly deplete the fundamental field in order to prevent the other Raman transition from reaching threshold. The power of the seed beam;cavity laser beam may be in the range of 1 :100 to 1 : 1000 or 1 :200 to 1 :600 or 1 :300 to 1 :500 or 1 :370 to 1 :470 or other suitable ratio. The seed power may be between 1μW and 1OmW, or between 10μW and ^"ImW or between 100μW and 500μW, and may be about 1, 2, 5, 10, 20, 50, 100, 200, 300, 400, 500, 600, 700, 800 or 900μW or about 1, o 2, 3, 4, 5, 6, 7, 8, 9 or 1OmW, or it may be below 1 μ W or it may be above 1OmW. The angle of incidence of the seed beam is not critical, although higher seed powers may be required if the seed power is injected off-axis. The seed beam in the Raman medium may be mode matched to the beam waist in the Raman crystal. The seed beam in the Raman medium may not be mode matched to the beam waist in the Raman crystal. The seed beam in the Raman medium may overlap spatially with the beam waist in the Raman crystal. The seeding may involve seeding with a polarized beam. This causes the Raman-active medium to convert most or all of the photons reaching it from the laser material to the polarization of the seed beam. Therefore the polarization of the output laser light beam may be selected by selecting an appropriate polarization of seed beam. Since a particular polarization is associated with a particular frequency, this will in turn select the particular frequency of output laser light.

Birefringence: A birefringent Raman-active crystal may produce different Stokes wavelengths which are shifted spatially relative to each other. If the reflector that defines the output end of the laser system is curved, that reflector may be oriented so that only one Stokes wavelength is capable of resonating within the resonator cavity. By altering the orientation of that reflector and/or of the Raman-active crystal, a particular wavelength of output laser light may be selected. In the case where the fundamental beam is unpolarised, a preferred method may be to leave the Raman- active crystal fixed and move the mirrors to choose the particular Stokes frequency. In the case where the fundamental beam is polarised, it may be preferred to rotate the Raman-active crystal and adjust the mirror in order to optimise the resonance of the desired wavelength of laser light. Polarisation selection: A Raman-active crystal may be capable of producing a different Stokes wavelength in response to different polarisations of incident laser light. If the incident laser light is unpolarised, such a crystal would produce two separate wavelengths of output laser light, each polarised orthogonally to each other. However if the incident laser light is polarised, then only one Stokes wavelength is produced, and consequently only one frequency of output laser light beam is generated from the system. The polarizer, which polarises the light resonating within the resonator cavity, may be a mechanically rotatable polariser, or it may be a Faraday rotator or an electro-optic rotator whereby selecting the polarisation is accomplished electronically. By rotating the polarisation of the polariser, a wavelength of output laser light from the laser system may be selected.

Direct wavelength selection: A wavelength selector for directly selecting the wavelength to be outputted may be incorporated in the cavity of the invention. The wavelength selector may be for example an optical filter, a prism, a grating, an etalon, an interference filter or some other element for selecting the wavelength of laser light to be outputted from the resonator cavity. In this case, the selected wavelength will be outputted from the cavity, and those wavelengths not selected will be suppressed or continue to resonate within the cavity until, through the various wavelength conversion process occurring in the cavity, they are converted to the selected wavelength and outputted from the cavity. The output coupler may comprise one of the reflectors that define the cavity, and may be selectively transmissive for particular wavelength ranges. For example the output coupler may only transmit the two second Stokes wavelengths, and may reflect the fundamental and first Stokes wavelengths. The wavelength tunable element, or some other element, may then be used to select between the two second Stokes wavelengths. In this manner, the output coupler maybe a component of the selector which is used to select the wavelength to be outputted from the cavity.

Methods for tuning nonlinear media (if present) The laser of the present invention may have a non-linear medium for frequency converting the output laser beam from the Raman-active medium. It may be desirable to tune the non-linear medium in order to enable it to convert a particular frequency of laser light. Suitable methods to tune the non-ϋnear medium to a particular frequency include:

Angle tuning: a birefringent crystal may be responsive to different wavelengths of incident light at different angles. Thus if an incident beam comprises more than one wavelength of light, the wavelength that is shifted by the crystal may be selected by rotating the crystal. For example, an Nd:YAG laser material may produce a beam at 1064nm wavelength. If this is directed to a KGW Raman-active crystal, the KGW crystal may produce both 1158nm and 1272nm Stokes wavelengths. If this output beam is directed to a second harmonic generator crystal, either the 1158nm input wavelength or the 1272nm wavelength may be selectively converted to its second harmonic. Consequently rotating the crystal can select, between output wavelengths of either 579nm or 636nm (being the second harmonics of 1158 and 1272nm respectively). A disadvantage of this method is that the beam waist for an 1158nm beam will be in a different position to that for a 1272nm beam due to thermal lensing effects. Thus the position of the crystal may be chosen to be a compromise between the two optimum positions, or to favour the beam whose output power is the most critical for the end application. Alternatively, a device may be provided to move the crystal to the optimum position when selecting a particular wavelength of output laser light. A further disadvantage of this method is that the reflectors may need to be retuned when switching between wavelengths of output laser light. Temperature tuning: It is possible to tune the wavelength to which a non-linear medium will respond by altering the temperature of the non-linear medium. Thus, in the example of a Nd:YAG laser material with a KGW Raman-active medium, a non-linear medium may receive an input beam comprising 1064, 1158 and 1272nm wavelengths. At 15O⁰C, the crystal may be responsive to 1064nm laser light to produce a green output beam at 532nm, at 4O⁰C it may be responsive to 1158nm laser light to produce a yellow output beam at 579nm, and at O⁰C it may be responsive to 1272nm laser light to produce a red output beam at 636nm. A disadvantage of this method is that the thermal mass of non-linear medium causes the changing between different output laser light wavelengths to be slower than for other methods. Switching times when using temperature tuning

5 in the laser system of the present invention may be around 1 minute, and may be less than about 2 minutes, or less than about 1.5, 1 or 0.5 minutes, and may be about 30, 40, 50, 60, 70, 80, 90, 100, 110 or 120 seconds. In addition, there may be practical problems with resistance of materials of construction to high temperatures, and with condensation caused by sub-ambient temperatures. As with angle-tuning, the location of the non-linear medium must be a compromise between the o optimum positions for the different wavelengths. An advantage of temperature tuning is the fact that, when changing wavelengths, it is not necessary to realign the non-linear medium, and the medium may be aligned norma) to the axis of the cavity, where insertion losses may be low. In a variation of temperature tuning, the non-linear medium comprises a single non-linear medium, in which the non-linear medium is cut such that it may be temperature tuned to convert a first s frequency of incident laser light passing through the medium at a first angle, or a second frequency of incident laser light passing through the medium at a second angle (non-linear bounce geometry). The tuner may comprise a temperature tuner for temperature-tuning the non-linear medium in order to select which frequency of incident laser light is converted. Thus for example, the non-linear medium may comprise a crystal cut so that it is capable of doubling the fundamental frequency at o the unreflected (incident) angle when held at a first temperature, and is capable of doubling the first Stokes frequency at the reflected angle when held at a second temperature. In this case, when the crystal is maintained at the first temperature, the fundamental will be doubled to form a visible frequency which may be outputted from the cavity. When the crystal is maintained at the second temperature, the fundamental frequency will resonate in the cavity and be converted by the Raman- s active medium into the first Stokes wavelength, which may then be doubled by the crystal to generate a visible output frequency. This variation is similar to the multiple crystal method described below, whereby the multiple crystals are represented by two different propagation angles within the same crystal. The bounce angle may be between about 1 to about 90⁹, between about 1 to about 60⁹, between o about 1 to about 50^s, between about 1 to about 45⁹, between about 1 to about 40^s, between about 1 to about 35^s, between about 1 to about 30^Q, between about 1 to about 25^s, between about 1 to about 20⁹, 1 to 15^Q, 1 to 10⁹, 5 to 45⁹, 20 to 45^s, 30 to 45^Q, 2 to 10^s, 3 to 8^s, 4 to 6⁹, and may be about 1^s, 2⁹, 3^s, 4⁹, 5, ⁹6^s, 7^s, 8^e, 9^s, 10⁹, 15⁹, 20^s, 25⁹ 30^s, 35⁹ 40⁹ or 45°. In this case the bounce angle is defined as the angle between the incident beam and the surface from which it reflects. The angle between the incident beam and the angle of the crystal may be less than about 10°, or less than about 8⁹, 6^Q, 4^e or 2°, and may be about 0, 1⁹, 2⁹, 3^e, 4^Q, 5⁹, 6^e, T- ,8⁹ 9⁹ or 10°. The angle by which the beam is deflected by reflection within the crystal may be from between about 1 to about and 90°, or between about 1 and 60, 1 and 40, 1 and 20, 1 to 60, 1 to and 10, 10. and 90, 40, 1 to 20, 1 to 10, 10 to and 90, 40 to 90, 60 to and 90, 4 to and 20, 6 to and 16, 8 to and 14 or 8 to and 12°, and may be about 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 30, 40, 50, 60, 70, 80 or 90°. The temperature difference between the two temperatures to which the crystal may be tuned may be less than about 5O⁰C, or less than about 40⁰C, 3O⁰C, 20⁰C, 10⁰C or 5⁰C , and may be between about 5 to 5O⁰C, 10 to 50⁰C, 20 to 5O⁰C, 5 to 40⁰C, 5 to 30⁰C, 30 to 50⁰C, 5 to 40⁰C, 5 to 20⁰C, 5 to 10°C, 10 to 40⁰C, 10 to 20⁰C, and 20 to 5O⁰C, 20 to 4O⁰C₁ 15 to 3O⁰C or 15 to 2O⁰C or 15 to 25 ⁰C, and may be about 5⁰C, 10⁰C, 15⁰C, 20⁰C, 25⁰C, 30⁰C, 35⁰C, 40⁰C, 45⁰C or 5O⁰C . For example the crystal may be set to phase match yellow at 45⁰C (from frequency doubling the first Stokes wavelength) and green at 25°C(form frequency doubling the fundamental). In essence, in non-linear crystal bounce geometry, the grazing incidence bounce geometry allows multiple crystal axes to be accessed (much like^' a dual crystal arrangement) in a single crystal. The benefits of this geometry are essentially the same as for a dual crystal arrangement, except the bounce geometry has the added advantage of having two less optical interfaces, which substantially reduces insertion losses. Insertion losses have been outlined as a significant issue in a multiple crystal system.

Non-linear bounce geometry provides the following benefits and alternatives: The bounce angle may be chosen to bisect a wide variety of angle differentials. The cut of the crystal may affect the magnitude of the temperature difference between the two temperatures to which the crystal may be tuned. It will be understood that for a particular crystal axis, the wavelength of output will be a function of temperature. By choice of appropriate crystal cut, the curves of wavelength vs temperature for the different crystal axes may overlap. As a result, it may be possible to obtain two' different frequencies of converted (e.g. visible) laser light at a single temperature, and consequently the laser system may be capable of producing multiple output frequencies simultaneously. Hence there is disclosed:

(A) A single crystal of a non linear medium in which the propagating laser beams in the crystal are totally internally reflected at an internal surface in the crystal, said crystal being cut with respect to the crystallographic axes of the crystal whereby: (i) said crystal is capable of frequency converting a first laser light beam comprising a first frequency propagating through the crystal when said crystal is at a first temperature up to a temperature less than said second temperature, said frequency converting of said first laser light beam at said first temperature up to a temperature less than said second temperature, the second temperature being higher than the first temperature, not being frequency converted subsequent to undergoing total internal reflection in the material; and wherein

(ii) said crystal is capable of frequency converting a second laser light beam comprising a second frequency when reflected in said crystal at a totally internally reflected angle when said crystal is held at a temperature higher than the first temperature up to at least a second temperature said frequency converting of said second laser light beam at said temperature higher than the first temperature up to at least a second temperature not being frequency converted prior to undergoing total internal reflection in the crystal.

(B) A single crystal of a non linear medium in which the propagating laser beams in the crystal are totally internally reflected at an internal surface in the crystal, said crystal being cut with respect to the crystallographic axes of the crystal whereby:

(i) said crystal is capable of frequency converting a first laser light beam comprising a first frequency propagating through the crystal when said crystal is at a first temperature up to a temperature less than said second temperature, said frequency converting of said first laser light beam at said first temperature up to a temperature less than said second temperature, the second temperature being higher than the first temperature, not being frequency converted prior to undergoing total internal reflection in the material; and wherein

(ii) said crystal is capable of frequency converting a second laser light beam comprising a second frequency when reflected in said crystal at a totally internally reflected angle when said crystal is held at a temperature higher than the first temperature up to at least a second temperature said frequency converting of said second laser light beam at said temperature higher than the first temperature up to at least a second temperature not being frequency converted subsequent to undergoing total internal reflection in the crystal.

The first temperature and the second temperature may be such that said first laser beam is not frequency converted at said second temperature and said second laser beam is not frequency converted at said first temperature.

The first temperature is different from the second temperature and is dependent on the temperature dependence of frequency converting characteristics of the non linear material and the crystal cut. The frequency converting may comprise frequency doubling (SHG)₁ frequency summing, frequency differencing or some other non linear frequency converting.

(C) A method of frequency converting a first laser light beam comprising a first frequency and a second laser light beam comprising a second frequency comprising passing the first laser light

5 beam through a single crystal of a non linear medium such that the first laser beam is totally internally reflected at an internal surface of the crystal said crystal being at a temperature in the range of from a first temperature up to but less than a second- temperature, said second temperature being higher than said first temperature, to frequency convert said first laser light beam and passing the second laser light beam through the crystal such that the first laser beam is totally

I₀ internally reflected at an internal surface of the crystal said crystal being at a temperature higher than said first temperature up to a second temperature to frequency convert said second light beam said single crystal being cut with respect to the crystallographic axes of the crystal whereby: (i) said crystal is capable of frequency converting a first laser light beam comprising a first frequency propagating through the crystal when said crystal is at a first temperature, said frequency is converting of said first laser light beam at said first temperature not being frequency converted subsequent to undergoing total internal reflection in the material, and wherein: (ii) said crystal is capable of frequency converting a second laser light beam comprising a second frequency when reflected in said crystal at a totally internally reflected angle when said crystal is held at a second temperature said frequency converting of said second laser light beam at

2o said second temperature not being frequency converted prior to undergoing total internal reflection in the crystal.

(D) A method of frequency converting a first laser light beam comprising a first frequency and a second laser light beam comprising a second frequency comprising passing the first laser light beam through a single crystal of a non linear medium such that the first laser beam is totally

25 internally reflected at an internal surface of the crystal said crystal being at a temperature in the range of from a first temperature up to but less than a second temperature, said second temperature being higher than said first temperature, to frequency convert said first laser light beam and passing the second laser light beam through the crystal such that the first laser beam is totally internally reflected at an internal surface of the crystal said crystal being at a temperature higher

30 than said first temperature up to a second temperature to frequency convert said second light beam said single crystal being cut with respect to the crystallographic axes of the crystal whereby: (i) said crystal is capable of frequency converting a first laser light beam comprising a first frequency propagating through the crystal when said crystal is at a first temperature, said frequency converting of said first laser light beam at said first temperature not being frequency converted prior to undergoing total internal reflection in the material, and wherein:

(ii) said crystal is capable of frequency converting a second laser light beam comprising a second frequency when reflected in said crystal at a totally internally reflected angle when said crystal is held at a second temperature said frequency converting of said second laser light beam at said second temperature not being frequency converted subsequent to undergoing total internal reflection in the crystal.

The frequency converting may comprise frequency doubling (SHG) frequency summing, frequency differencing or some other non linear frequency converting. SHG

(E) A non linear medium in which the propagating laser beams are totally internally reflected within the crystal, comprising: a single crystal of a non linear material which is cut with respect to the crystallography axes of the crystal whereby: (i) said crystal is capable of frequency converting a first laser light beam comprising a first frequency propagating through the material when said crystal is at a first temperature, said first laser light beam not being converted subsequent to undergoing total internal reflection and wherein: (ii) said non linear medium is capable of frequency converting a second laser light beam comprising a second frequency after being reflected in said crystal at a totally internally reflected angle when said crystal is held at a second temperature said first laser light beam not being converted prior to undergoing total internal reflection and wherein the first temperature is different from the second temperature.

The first and second temperatures are dependent on the non linear material and the cut. The frequency converting may comprise frequency doubling or some other non linear frequency converting. SFG/DFG

(F) A non linear medium in which the propagating laser beams are totally internally reflected within the crystal, comprising: a single crystal of a non linear material which is cut with respect to the crystallographic axes of the crystal whereby:

(i) said crystal is capable of frequency converting a first pair of propagating laser light beams comprising two frequencies propagating through the material when said crystal is at a first temperature, the said laser light beams not being converted subsequent to undergoing total internal reflection and wherein:

(ii) said non linear medium is capable of frequency converting a second, different pair of propagating laser light beams comprising two frequencies propagating through the material after being reflected in said crystal at a totally internally reflected angle when said crystal is held at a second temperature said second pair of laser light beams not being converted prior to undergoing total internal reflection and wherein the first temperature is different from the second temperature . and is dependent on the non linear material and the cut. The frequency converting may comprise sum frequency mixing or difference frequency mixing or some other non linear frequency converting.

The SFG/DFG case can be generalised to include the SHG case by making the pair of laser beams have the identical frequencies.

The resonating light beams may be reflected off an interior face of a non-linear medium, such that a single non-linear media exhibits two simultaneous angular phase-matching conditions for a given said temperature for the incident and reflected beams. The non-linear medium may be selected from the group consisting of a temperature tunable non-linear medium, an angle tunable non-linear medium. The non-linear medium may selectively frequency convert at least one of the laser light beams to a frequency altered laser light beam comprising a single converted wavelength said frequency converting not comprising a Raman frequency shift by temperature tuning the incident beam and detuning the reflected beam simultaneously inside the non-linear media whereby the reflected beam inside the non-linear medium does not frequency convert at least one of the laser light beams. The non-linear medium may selectively frequency convert at least one of the laser light beams to a frequency altered laser light beam comprising a single converted wavelength said frequency converting not comprising a Raman frequency shift by temperature tuning the reflected beam and detuning the incident beam simultaneously inside the non-linear media whereby the incident beam inside the non-linear medium does not frequency convert at least one of the laser light beams. The non-linear medium may simultaneously selectively frequency convert a) at least one of the laser light beams to a frequency altered laser light beam comprising a single converted wavelength said frequency converting not comprising a Raman frequency shift by temperature tuning the incident beam inside the non-linear medium and b) a second frequency altered laser light beam comprising a single converted wavelength said frequency converting not comprising a Raman frequency shift by temperature tuning the reflected beam inside the non-linear medium. The said second frequency conversion may involve both one of the original light beams and the frequency altered laser beam from the incident beam conversion inside the non-linear medium. Multiple crystals: A method to overcome at least some of the disadvantages of angle-tuning and of temperature-tuning comprises the use of a plurality of individual non-linear media. Each of the individual non-linear media may be composed of the same material as each of the others, or they may be composed of different materials or some may be composed of the same material and others may be composed of a different material. In this method, an individual non-linear medium may be located at or near the beam waist of each wavelength of output from the Raman-active medium. In the example of a Nd:YAG laser material with a KGW Raman-active medium, this output comprises two Stokes wavelengths (1158nm and 1272nm) as well as the laser material wavelength (1064nm). Since non-linear media need to be maintained at the correct temperature to be active (as described above), it is possible to detune an individual non-linear medium by altering its temperature. The change in temperature required to do so is quite small, and consequently the problems described above, associated with large temperature changes, may be avoided. The temperature change to detune a crystal may be less than about 30 Celsius degrees. It may be between about 0.1 and 30 Celsius degrees, or between about 0.5 and 20 or between about 0.5 and 10 or between about 1 and 30 or between about 10 and 30 or between about 1 and 10 or between about 2 and 10 or between about 5 and 10 Celsius degrees, and may be about 0.1, 0.2, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 25 or 30 Celsius degrees or it may be greater than about 30 Celsius degrees. Thus a wavelength of output laser light may be selected by tuning only the individual non-linear medium that corresponds to that wavelength. For example a yellow output at 579nm may be selected by maintaining the individual non-linear medium responsive to 1158nm at the correct temperature for tuning, and optionally detuning the other individual non-linear media. For another example a green output at 532nm may be selected by maintaining the individual non¬ linear medium responsive to 1064nm (the laser material wavelength) at the correct temperature for tuning, and detuning the other individual non-linear media.

Alternatively the two or more non-linear media may be cut such that they are sequentially tuned to double or frequency sum different wavelengths whilst the temperature of the non-linear media is varied, that temperature being the same temperature for both or all of the non-linear media. In a further alternative two non-linear media may be aligned to convert the Raman frequencies of different polarizations. For example one non-linear medium may be aligned for frequency doubling the first Stokes wavelength of KGW at 1158nm to generate a wavelength of 579nm, or for sum frequency generation with the fundamental frequency to generate a wavelength of 555nm, at one polarization. A second non-linear medium may be aligned for frequency doubling or frequency summing of the other first Stokes wavelength at 1176nm to generate 588nm (frequency doubling) or 559nm (frequency sum with the fundamental at 1064nm) at the orthogonal polarization. In another alternative, three separate non-linear media may be used: one may be aligned for frequency doubling the fundamental (1064nm) to generate a beam at 532nm (green); another may be aligned for frequency doubling the first Stokes wavelength of 1158nm to generate a beam at 579nm (yellow) and the third may be aligned for frequency doubling the alternative Stokes wavelength of 1176nm to generate a beam at 588nm (orange), being polarized orthogonally to the beam at 579nm.

The non-linear media may be positioned to optimize the output at important or desirable wavelengths. The effect of their relative positioning may be due to the transmission of their antireflection coatings and to the average beam size in each crystal. For example to design a laser which enabled an operator to select green or yellow output, and where as much yellow as possible was desired, the nonlinear medium which produced yellow output may be positioned where the beam width was smallest and so that the yellow photons were subject to low as possible transmission losses. A folded resonator may be appropriate, and the yellow doubling crystal may be placed next to the folding mirror. Introduction of additional optical elements into the cavity may give rise to optical losses (insertion losses) due to the introduction of additional interfaces. This may be at least partially combated by bonding the plurality of individual non-linear media, e.g. by diffusion bonding. At least two of the non-linear media may be bonded together. Since the individual non-linear media may be chemically similar or identical, although with different cuts, such bonding may be effective and may reduce the number of interfaces compared to a situation in which they were not bonded. Bonded non-linear crystal geometry

Bonded crystal geometry is very similar to the dual crystal arrangement, except that the crystals are bonded (e.g. diffusion bonded) together. This eliminates two optical interfaces, thereby reducing insertion losses. Diffusion bonding information may be obtained at, for example http://www.onvxoptics.com. the contents of which are incorporated herein by cross reference. This. arrangement has the multiple non-linear interactions advantages of multiple crystals without the added insertion losses. Additionally it is easier to align the components of the system than when using bounce geometry described above. This can thus be an extension of the multiple crystal geometry where the crystals are bonded together. The use of bonded non-linear crystal geometry offers the following flexibility and advantages:

• There may be any number of crystals

• The crystals may all be the same material, such as LBO, or they may be different materials.

Quasi-phasematching

There is a class of SHG/SFG materials, such as periodically-poled lithium niobate (PPLN), that use quasi-phasematching rather than the birefringence properties of the medium to achieve efficient conversion. Quasi-phasematching relies on the use of a periodic structure which forms a grating within the crystal, with alternating crystal domain direction (and hence sign of the nonlinear coefficient) so that the phase mismatch introduced in each domain is compensated in the next domain. As well as angle and temperature tuning, quasi-phasematched materials may also be tuned by altering the period of the grating. This may be achieved by using a medium with multiple gratings or a medium with a fan-shaped grating structure, and then tuning by translating the medium laterally to the laser beam in the plane of the grating. Thus in this case the wavelength may be selected by translating the laser beam laterally to the laser beam so that the laser beam is exposed to a grating structure in the non-linear medium corresponding to the desired wavelength of output laser light. In this case the tuner may comprise a mechanical translator, for translating the non-linear medium laterally to the laser beam. The wavelength shifted laser light beam generated by the non-linear medium may then be outputted from the cavity using the output coupler. Use of the laser

The invention includes a use or method of using laser light for treating, detecting or diagnosing a selected area requiring such diagnosis or treatment on or in a subject comprising illuminating the selected area with the output laser beam of the invention. The invention may also comprise use of an aim beam in order to aim the output laser beam towards the selected area. The aim beam may have a wavelength in the visible range. Accordingly, the laser system may also comprise a source of the aim beam, which may be a diode laser, a laser, an LED or some other suitable source of monochromatic light. A mirror, which may be a dichroic mirror, may also be provided in order to direct the aim beam in the same direction as the output laser beam. The selected area may be illuminated with a laser beam having a wavelength for a time and at a power level which is appropriate and effective for the diagnosis or therapeutically effective for the treatment. The subject may be a mammal or vertebrate or other animal or insect, or fish. The subject- may be a mammal or vertebrate which is a bovine, human, ovine, equine, caprine, leporine, feline or canine vertebrate. Advantageously the vertebrate is a bovine, human, ovine, equine, caprine, leporine, domestic fowl, feline or canine vertebrate. The use or method of the invention finds particular application in treating the eyes and skin of a mammal or vertebrate. A yellow/green laser beam produced by the system or method of the invention has the advantage of having selectable wavelengths of 532 and 579nm which are particularly advantageous in treating, detecting or diagnosing certain disorders especially certain disorders in ophthalmology and dermatology.

The use or method of the invention also finds particular application in treating soft tissue and in particular, the laser treatment of the prostate. A yellow/green laser beam produced by the system or includes a method of the invention has the advantage of having selectable wavelengths of 532 and 579nm which are particularly advantageous in treating, detecting or diagnosing benign prostatic hyperplasia (BPH). In another embodiment of the use or method of the invention, theusing laser output is fibre delivered and allows switchability between green and yellow light which may be beneficial in the treatment of soft tissue and in particular the prostate. The laser of the present invention may also be used in connection with holograms, in diagnostic applications (for example in displays, fluorescence detection, cell separation, cell counting, imaging applications), military systems (e.g. for military countermeasures, underwater systems, communication, illumination, ranging, depth sounding, mapping contours such as a sea floor), ophthalmology, urology, surgery (e.g. vascular surgery) for purposes including cutting, coagulation, vaporization, destruction of tissue etc., stimulation, photodynamic therapy etc., gas detection, treatment of skin disorders e.g. psoriasis. It may be used in dermatological applications such as treatment of spider veins, or treatment of acne, skin rejuvenation or treatment of hypopigmentation due to sun damage. The laser may be used in combination with other therapies, for example treatment with drugs, creams, lotions, ointments etc. (e.g. steroids), optically clearing agents, other device based therapies etc.

The invention includes a use or method for displaying laser light on a selected area comprising illuminating the selected area with the output laser beam of the invention. The invention may also comprise use of an aim beam in order to aim the output laser beam towards the selected area. The aim beam may have a wavelength in the visible range. Accordingly, the laser system may also comprise a source of the aim beam, which may be a diode laser, an LED or some other suitable source. A mirror, which may be a dichroic mirror, may also be provided in order to direct the aim beam in the same direction as the output laser beam. It is well-known that visible light, in particular green/yellow and red light can be used to target a variety of chromophores present in human or animal tissue. These chromophores include melanin, haemoglobin, collagen-related constituents and also porphyrin, which is present for example at bacteria sites associated with acne. As a consequence, green, yellow and red light can be used to treat a wide variety of medical conditions and to perform a variety of cosmetic procedures. Many of these treatments involve eye and skin, and examples include retinal procedures, treatment of vascular and pigmented lesions, collagen rejuvenation, wound and scar healing and acne treatment. In addition to the natural chromophores listed above, special dyes may be incorporated into body tissues, which react with certain components of body tissue when activated by particular wavelengths of light. This process is called photodynamic therapy, and is being used increasingly to treat a range of medical disorders ranging from cancer to skin and eye disorders.

In using a laser to provide any of the treatments above, there is an optimum wavelength of the laser light which provides the best clinical effectiveness with fewest side effects. This optimum wavelength depends on the condition being treated, the chromophore being targeted and the characteristics of the surrounding tissues (eg. skin type).

The laser systems described in this specification offer a particular advantage to clinicians, in that several wavelengths can be output from a single solid-state laser device. The ability to switch between wavelengths is an important benefit to clinicians (for example doctors, dermatologists, ophthalmologists, cosmetic physicians) because it enables them to treat patients with a wider range of skin types and a wider range of medical or cosmetic complaints. The laser described herein has the ability to be made compact and portable.

To achieve a similar range of wavelengths using conventional laser sources, a clinician would need to use multiple laser sources, which is a costly and space-consuming option. The invention may also comprise use of an aim beam in order to aim the output laser beam towards the selected area. The aim beam may have a wavelength in the visible range. Accordingly, the laser system may also comprise a source of the aim beam, which may be a diode laser, an LED or some other suitable source. A mirror, which may be a dichroic mirror, may also be provided in order to direct the aim beam in the same direction as the output laser beam. The selected area may be illuminated with a laser beam having a wavelength for a time and at a power level which is appropriate and effective for the diagnosis or therapeutically effective for the treatment. The subject may be a mammal or vertebrate or other animal or insect, or fish. The subject may be a mammal or vertebrate which is a bovine, human, ovine, equine, caprine, leporine, feline or canine vertebrate. Advantageously the vertebrate is a bovine, human, ovine, equine, caprine, leporine, domestic fowl, feline or canine vertebrate. The method of the invention finds particular application in treating the eyes and skin of a mammal or vertebrate. A yellow/green laser beam produced by the system or method of the invention has the advantage of having selectable wavelengths of 532 and 579nm which are particularly advantageous in treating, detecting or diagnosing certain disorders especially certain disorders in ophthalmology and dermatology.

The invention includes a method of using laser light for displaying laser light on a selected area comprising illuminating the selected area with the output laser beam of the invention. As a consequence, green, yellow and red light can be used to treat a wide variety of medical conditions and to perform a variety of cosmetic procedures. Many of these treatments involve eye and skin, and examples include retinal procedures, treatment of vascular and pigmented lesions, collagen rejuvenation, wound and scar healing and acne treatment.

In addition to the natural chromophores listed above, special dyes may be incorporated into body tissues, which react with certain components of body tissue when activated by particular wavelengths of light. This process is called photodynamic therapy, and is being used increasingly to treat a range of medical disorders ranging from cancer to skin and eye disorders.

To achieve a similar range of wavelengths using conventional laser sources, a clinician would need to use multiple laser sources, which is a costly and space-consuming option. The table below summarises the applications to which the laser of the present invention may be applied, together with the wavelengths suitable for those a lications.

The symbol (?) in the above table indicates that the indication is likely but not certain. For tattoo removal applications it is preferable that the laser system be Q switched. Likewise a number of pigmented lesion applications may require a Q switched laser. The present invention provides a laser system and/or methods to treat any of the above conditions by using a single wavelength or multiple wavelengths in the order and spaced by time that is matched to a patient's clinical status. Alternatively, multiple wavelengths may be applied to a patient concurrently e.g. as the IR and visible lasers may come from separate rods it is possible to apply IR and visible together or spaced by a time factor selected by the clinician from a range offered by the apparatus. Thus it may be possible to house more than one, e.g.2, 3 or more than 3, laser systems according to the present invention in the one housing or box in order to provide the concurrent multiple wavelengths. Using the technology described in this specification, a laser system may also be constructed that provides more than one, e.g. 2, 3 or more than 3, visible output frequency simultaneously. The invention may also comprise a software control system. Such a system may comprise a computer or other data processing/control device, having software therein. The software control system may be connected by for example electrical connections, to one or more components of the laser system, for example the pump beam source, the tuner etc. The software system, if present, may allow the laser system of the invention to be configured for a particular treatment condition. The software system may have a user interface, for example a keyboard, control panel and/or visual display unit (e.g. monitor). The user interface may allow a user to select a particular condition to be treated, whereby the software contains parameters (times of irradiation, intensity, wavelength, pulse width, pulse number etc.) appropriate for that condition, thus allowing automatic or semiautomatic control of the laser system by the software control system to provide the appropriate treatment to a patient. Alternatively or additionally, the user interface may enable a user to program the software control system with the parameters (times of irradiation, intensity, wavelength, pulse width, pulse number etc.) appropriate for a particular condition so that those parameters may be subsequently selected for treatment of a patient.

In an embodiment of the software control system of the invention, there may be provided electrical connections between the components of the software control system and the frequency selector for selecting a single frequency of output laser light of the invention. In an embodiment when the frequency selector is an adjustor which moves the Raman-active medium and/or the at least one reflector in the resonator cavity, a user may interface with the adjustor by way of a keyboard, control panel, and/or a visual display unit (monitor). The software control system may also use software to automate the software control system. The software control systems of the invention may allow the laser system to be configured for a particular treatment condition, by having a control panel selected with the optimal wavelengths and temporal characteristics (i.e. pulse width, pulse number and the like). For example, selection by a user of an acne program may provide parameters appropriate for treatment in one or separate treatment session with yellow to target bacteria followed (or concurrent or spaced by a selectable time interval) by infrared (most likely 1320 nm to target sebaceous gland), and in another treatment for severe cases apply photodynamic therapy using red wavelength. Similarly, venous lesions can present in different sizes of blood vessels and be located at different depths (e.g. within one lesion such as a port wine stain, or multiple lesions such as spider veins and varicose veins on legs). Similarly to the acne example, a vascular lesion "treatment program" may be constructed to include green, yellow and infrared wavelengths. It should be also noted that photodynamic therapy may be used alone or in conjunction with other laser treatments using photoactive bio-molecules (such as conventional pharmaceuticals and/or biotech products or combinations with photoactive groups activated at one or more wavelength, e.g. 5-aminolaevulinic acid (ALA)) can be used in conjunction with a number of treatments, such as acne, skin rejuvenation, psoriasis. The chromophores for different bio-molecules vary and there may be absorption peaks at one or more wavelengths. The laser system of the present invention has the advantage that one device is capable of activating multiple chromophores of the same bio-molecule, thereby allowing the selection of the wavelength that is best absorbed by a patient (for example, based on their skin type) or alternately the laser system may be used by activating a "cocktail" of multiple bio-molecules having different chromophores.

Brief Description of the Drawings

A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a diagrammatic representation of a laser system according to the present invention wherein wavelength selection is by means of seeding of a Raman-active crystal;

Figure 2 is a diagrammatic representation of a laser system according to the present invention wherein the Raman-active crystal is birefringent and a curved mirror is used; Figure 3 is a diagrammatic representation of a laser system according to the present invention wherein wavelength selection is by means of a polarisation selector;

Figure 4 is a diagrammatic representation of a laser system according to the present invention wherein wavelength selection is by means of a polarisation selector, additionally comprising two non-linear crystals for frequency doubling the output laser beam from the Raman- active crystal;

Figure 5 is a diagrammatic representation of a laser system according to the present invention wherein wavelength selection is by means of a polarisation selector, and wherein the system comprises two coupled resonator cavities in a linear configuration;

Figure 6 is a diagrammatic representation of a laser system according to the present invention wherein wavelength selection is by means of a polarisation selector, and wherein the system comprises two coupled resonator cavities in a folded configuration;

Figure 7 is a diagrammatic representation of a laser system according to the present invention having a wavelength tunable element for wavelength selection;

Figure 8 is a diagrammatic representation of a laser system according to the present invention corresponding to the example;

Figure 9 is a diagrammatic representation of another embodiment of a laser system according to the present invention which is similar to Figure 1 ;

Figure 10 is a diagrammatic representation of another embodiment of a laser system according to the present invention which is similar to Figure 2; Figure 11 is a diagrammatic representation of another embodiment of a laser system according to the present invention which is similar to Figure 3;

Figure 12 is a diagrammatic representation of another embodiment of a laser system according to the present invention which is similar to Figure 5;

Figure 13 is a diagrammatic representation of another embodiment of a laser system according to the present invention which is similar to Figure 6;

Figure 14 is a diagrammatic representation of another embodiment of a laser system according to the present invention which is similar to Figure 7; Figure 15 is a diagrammatic representation of another embodiment of a laser system according to the present invention;

Figures 16 and 16a are graphical representations showing laser pulse lengths obtained from the laser system as described and shown in Figure 15;

5 Figures 17 and 17a are graphical representations showing a comparison of Nd:YAG and Raman laser lasing characteristics using a flash!amp/Nd:YAG combination as described and shown in Figure 15;

Figure 18 is a diagrammatic representation of another embodiment of a laser system according to the present invention; and o Figure 19 is a diagrammatic representation of yet another embodiment of a laser system according to the present invention. Detailed Description of the Preferred Embodiments

Referring to Fig. 1, a laser system 100 comprises a resonator cavity 105 defined by reflectors 110 and 115. The reflector 110 is highly reflective at all wavelengths generated within the s resonator cavity 105, although the reflector 110 should be capable of allowing a pump beam from a pump source 120 to pass to a laser material 125. The laser system 100 may have at least one collimating lens 127 and at least one focusing lens 128, located between the pump source 120 and the laser material 125, for collimating and focusing the pump beam. The pump source 120 may be for example a diode laser, and is capable of exciting the laser material 125. The laser material 125 o may be for example Nd:YAG, which is capable of generating laser light at 1064nm. A Q-switch 130 is capable of converting the output from the laser material 125 into pulsed high power laser light capable of interacting with a Raman-active crystal 135 to generate one or more Stokes wavelengths of laser light. The Raman-active crystal 135 may for example be KGW, which can produce a first Stokes wavelength of 1158nm and another first Stokes wavelength of 1176nm when 5 excited by a 1064nm laser. A Light emitting diode (LED), laser or laser diode (or other suitable source of monochromatic light such as laser light, for example) 150 is capable of seeding the Raman-active crystal 135 with one of the wavelengths corresponding to one of the first Stokes wavelengths of the Raman-active crystal 135 (ie 1158 or 1176nm). A second light emitting diode (LED), laser or laser diode (or other suitable source of monochromatic light such as laser light, for o example) 150a is capable of seeding the Raman-active crystal 135 with the other one of the wavelengths corresponding to one of the first Stokes wavelengths of the Raman-active crystal 135 (ie 1176 or 1158nm). The seed beam may also be mode matched to the mode size of the beam waist (not shown) in the Raman-active medium 135. A beam splitter 190 is located adjacent to the output coupler 115 which directs a portion of the Raman-shifted laser beam to a first detection means 160 by way of line 175. The first detection means 160 is located in the laser system 100 in order to detect the frequency of the beam output from the Raman-active crystal 135 corresponding to one of the first Stokes wavelengths of the Raman-active crystal 135 (ie 1158 or 1176ήm). The detection means 160 provides an input to a CPU 170 by way of line 165. The CPU 170 then, in use, can adjust the frequency of the seed beam from the LED, laser or laser diode (or other suitable source of monochromatic light such as laser light, for example) 150.

The beam splitter 190 also directs a portion of the Raman-shifted laser beam to a second detection means 160a by way of line 175a. The second detection means 160a is located in the laser system 100 in order to detect the frequency of the beam output from the Raman-active crystal 135 corresponding to the other one of the first Stokes wavelengths of the Raman-active crystal 135 (ie 1158 or 1176nm). The second detection means 160a provides an input to a second CPU 170a by way of line 165a. The second CPU 170a then, in use, can adjust the frequency of the seed beam from the second LED, laser or laser diode (or other suitable source of monochromatic light such as laser light, for example) 150a. The reflector 115 is highly reflective at the wavelength of the laser material 125 (in this example 1064nm) and is at least partly transmissive at the frequency Raman-shifted wavelengths (1158nm and 1176nm). The laser system 100 may comprise at least one polariser (not shown) which may be included in the resonator cavity 105. The polariser may be one, two or more plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals- or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 105 where there is no significant visible field.

In operation, the pump source 120 generates a pump beam which excites the laser material 125. The pump beam may be focused on and/or in the laser material 125 using the collimating lens 127 and the focusing lens 128 in order to mode match the pump beam and the cavity laser beam within the laser material 125. In the example that the laser material 125 is Nd:YAG, this generates a laser beam at 1064nm, which is converted by the Q-switch 130 to a high power pulsed laser beam. In order to select an output of 1158nm, the LED, laser or laser diode 150 directs a seed beam at 1158nm towards the Raman-active crystal 135. The seed beam may be mode matched with the beam waist (not shown) in the Raman-active crystal 135 or the seed beam may overlap spatially with the beam waist (not shown) in the Raman-active crystal 135. This causes the Raman-active crystal 135 to generate a laser beam at 1158nm. The reflector 115 reflects the unshifted beam at 1064nm and permits (by transmission) at least part of the Raman-shifted laser beam at 1158nm to exit the resonator cavity 105. Those wavelengths that are reflected by the reflector 115 continue to resonate within the resonator cavity 105 until they are converted to the desired wavelength for output, or else leak away elsewhere. The Raman-shifted laser beam upon exit from the cavity 105 then is partly split by the beam splitter 190 which directs a portion of the Raman-shifted laser beam through line 175 to the first detection means 160. The first detection means 160 detects the frequency and phase of the Raman shifted laser beam which exits the resonator 105 and sends a signal with this information through line 165 to the CPU 170. The CPU 170 then directs the LED or laser diode 150 to synchronise the frequency and phase of the seed beam at 1158nm with the frequency and phase of the Raman shifted beam which exits the resonator 105. Once the frequency and phase of the seed beam at 1158nm has been synchronized with the frequency and phase of the Raman shifted beam which exits the resonator 105, the Raman-active crystal 135 produces a Raman-shifted beam at 1158nm only which exits resonator 105. In order to change the output laser to 1176nm, the LED 150 is changed to direct a beam of wavelength 1176nm towards the Raman-active- crystal 135 or alternatively, the LED, laser or laser diode 150 is switched off and the second LED, laser or laser diode 150a is switched on thereby directing a seed beam of wavelength 1176nm towards the Raman-active crystal 135. The seed beam may be mode matched to the beam waist (not shown) in the Raman-active crystal 135.

This causes the Raman-active crystal 135 to generate a laser beam at 1176nm. The reflector 115 reflects the unshifted beam at 1064nm and permits at least part of the Raman-shifted laser beam at 1176nm to exit the resonator cavity. The Raman-shifted laser beam upon exit from the cavity 105 then is partly split by the beam splitter 190 which directs a portion of the Raman- shifted laser beam through line 175a to the second detection means 160a. The second detection means 160a detects the frequency and phase of the Raman shifted laser beam which exits the resonator 105 and sends a signal with this information through line 165a to the second CPU 170a. The second CPU 170a then directs the second LED, laser or laser diode 150a to synchronise the frequency and phase of the seed beam at 1176nm with the frequency and phase of the Raman shifted beam which exits the resonator 105. Once the frequency and phase of the seed beam at 1176nm has been synchronised with the frequency and phase of the Raman shifted beam which exits the resonator 105, the Raman-active crystal 135 produces a Raman-shifted beam at 1176nm only which exits resonator 105. Alternatively, such synchronisation may be achieved electronically eg by using a signal generator to provide dual triggers to the Q-switch and the seed laser electronics or alternatively the "synch output" from one unit can be used to trigger the other. Alternatively continuous LED, laser or laser diodes 150 and 150a may be continuous wave in which case detectors 160 and 160a and CPUs 170 and 170a are not required.

Referring to Fig. 2, a laser system 200 comprises a resonator cavity 205 defined by reflectors 210 and 215. The reflector 210 is highly reflective at all wavelengths generated within the resonator cavity, although the reflector 210 should be capable of allowing a pump beam from a pump source 220 to pass to a laser material 225. The reflector 215 is a curved reflector as can be seen in Fig.2. The laser system 200 may have at least one collimating lens and at least one focusing lens (not shown), located between the pump source 220 and the laser material 225, for collimating and focusing the pump beam. The pump source 220 may be for example a diode laser, and is capable of exciting the laser material 225. The laser material 225 may be for example Nd:YAG, which is capable of generating laser light at 1064nm. A Q-switch 230 is capable of converting the output from the laser material 225 into pulsed high power laser light capable of interacting with a birefringent Raman-active crystal 235 to generate two first Stokes wavelengths of laser light which are shifted spatially relative to each other. A tuning device 240 is capable of tuning the birefringent Raman-active crystal 235 to selectively direct one or other of the Stokes wavelengths of laser light emanating from the birefringent Raman-active crystal 235 in a direction that it can resonate within the resonator cavity 205. The curved reflector 215 is highly reflective at the wavelength of the laser material 225 and at least partly transmissive at the Stokes wavelengths of the birefringent Raman-active crystal 235. The tuning device 250 is provided in order to tune the curved reflector 215 in order that it is angled such that it can reflect the selected Stokes wavelength in such a direction that it can resonate within the resonator cavity 205.

The tuning device 240 may be a motor or a piezoelectric or other device coupled to the crystal 235 which alters the orientation of the Raman-active crystal 235 medium so that one of the at least two different wavelengths of Raman-shifted laser light is capable of resonating in the cavity more efficiently than the other different wavelength(s) of Raman-shifted laser light. The tuning device 250 may be a motor or a piezoelectric or other device coupled to the curved reflector 215 which alters the orientation of the curved reflector 215 so that one of the at least two different wavelengths of Raman-shifted laser light is capable of resonating in the cavity more efficiently than the other different wavelength(s) of Raman-shifted laser light. The tuning device 240 may be mechanically coupled to the Raman-active crystal 235. Further, the tuning device 250 may be mechanically coupled to the curved reflector 215.

The laser system 200 may comprise at least one polariser (not shown) which may be included in the resonator cavity 205. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 205 where there is no significant visible field. s In operation, the pump source 220 generates a pump beam which excites the laser material 225. The pump beam may be focused on and/or in the laser material 225 using the collimating and focusing lenses in order to mode match the pump beam and the cavity laser beam within the laser material 225. In the example that the laser material 225 is Nd:YAG, this generates a laser beam at 1064nm, which is converted by a Q-switch 230 to a high power pulsed laser beam. o The excitation of the birefringent Raman-active crystal 235 by the high power pulsed laser beam leads to generation of two first Stokes-shifted beams, which, in the example that the laser material 225 is Nd:YAG, are at 1158nm, and at 1176nm, which are shifted spatially relative to each other. Using either device 240 to tune the birefringent Raman-active crystal 235 or device 250 to tune the reflector 215, or both, the laser system 200 is tuned so that only one of the two Stokes-shifted s beams is capable of resonating within the resonator cavity 205. The other of the two Stokes-shifted beams is not capable of resonating within the resonator cavity 205 and is absorbed or exits the resonator cavity without resonating. The curved reflector 215 partially reflects the selected Stokes- shifted beam, enabling the Stokes-shifted beam to resonate within the resonator cavity 205, and at least partially transmits the the Stokes-shifted beam, allowing the Stokes-shifted beam to exit the o resonator cavity 205. As mentioned above, the wavelength of the output laser beam may be selected by tuning either the birefringent Raman-active crystal 235 or the curved reflector 215 in order to select which Stokes-shifted beam may resonate and exit through the curved reflector 215.

Referring to Fig. 3, a laser system 300 comprises a resonator cavity 305 defined by reflectors 310 and 315. The reflector 310 is highly reflective at all wavelengths generated within the 5 resonator cavity 305, although the reflector 310 should be capable of allowing a pump beam from the pump source 320 to pass to the laser material 325. The laser system 300 may have at least one collimating lens and at least one focusing lens (not shown), located between the pump source 320 and the laser material 325, for collimating and focusing the pump beam. The pump source 320 may be for example a diode laser, and is capable of exciting the laser material 325. The laser 0 material 325 may be for example Nd:YAG, which is capable of generating laser light at 1064nm. A Q-switch 330 is capable of converting the output from laser material 325 into pulsed high power laser light capable of interacting with a Raman-active crystal 335. The Raman-active crystal 335 is capable of producing different Stokes-shifted wavelengths in response to different polarisations of input laser light. A polarisation selector 333 is for example a Faraday selector whereby the polarisation of the selector is selected electronically. The reflector 315 is highly reflective at the wavelength of the laser material 325, and may have a transmission of 10 to 50% at the Stokes- shifted wavelengths of the Raman-active crystal 335, although the transmission may be below 10% s or it may be above 50%.

The laser system 300 may comprise at least one polariser (not shown) which may be included in the resonator cavity 305. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The o polariser may be positioned in a location in the resonator cavity 305 where there is no significant visible field.

In operation, the pump source 320 generates a pump beam which excites the laser material 325. The pump beam may be focused on and/or in the laser material 325 using the collimating and focusing lenses (not shown) in order to mode match the pump beam and the cavity s laser beam within the laser material 325. In the example that the laser material 325 is Nd:YAG, this generates a laser beam at 1064nm, which is converted by the Q-switch 330 to a high power pulsed laser beam. The polarisation selector 333 is then tuned to select a particular polarisation of laser light, and that selected polarisation excites the Raman-active crystal 335 to generate a single Stokes-shifted wavelength, which in the case that the laser material material 325 is Nd:YAG and 0 crystal 335 is KGW, is either 1158 or 1176nm. The selected wavelength can then exit the laser system 300 through the reflector 315, and the portion of the laser beam from the laser material 325 that exited from the Raman-active crystal 335 is reflected and resonates within the resonator cavity 305. The wavelength of the laser light produced by the laser system 300 is selected by selecting the polarisation of laser light that excites the Raman-active crystal 335, which is accomplished 5 using the polarization selector 333.

Referring to Fig. 4, a laser system 400 comprises a resonator cavity 405 defined by reflectors 410, 415 and 418. The reflector 410 is highly reflective at all wavelengths generated within the resonator cavity 405, although the reflector 410 should be capable of allowing a pump beam from a pump source 420 to pass to a laser material 425. The laser system 400 may have at 0 least one collimating lens and at least one focusing lens (not shown), located between the pump source 420 and the laser material 425, for collimating and focusing the pump beam. The pump source 420 may be for example a diode laser, and is capable of exciting the laser material 425. The laser material 425 may be for example Nd.ΥAG, which is capable of generating laser light at 1064nm. A Q-switch 430 is capable of converting the output from the laser material 425 into pulsed high power laser light capable of interacting with the Raman-active crystal 435. The Raman-active crystal 435 is capable of producing different Stokes-shifted wavelengths in response to different polarisations of input laser light. A polarisation selector 433 is for example a Faraday selector

5 whereby the polarisation of the selector is selected electronically. The reflector 418 is highly reflective at the wavelength of the laser material 425 and at the Raman-shifted wavelengths, and is highly transmissive at the frequency doubled wavelengths. For example, if the laser material 425 is Nd:YAG and the Raman-active crystal 435 is KGW, then the reflector 418 is highly reflective at 1064, 1158 and 1176nm, and highly transmissive at 532, 576 and 588nm. The reflector 415 is io highly reflective at all wavelengths generated within the resonator cavity 405, in the example above these being 1064, 1158, 1176, 532, 576 and 588nm. Frequency doubling crystals 440 and 445 are capable of doubling the two Raman-shifted frequencies of laser light from the Raman-active crystal 435. Each of the crystals 440 and 445 is optimised to a different Raman-shifted frequency.

The laser system 400 may comprise at least one polariser (not shown) which may be i₅ included in the resonator cavity 405. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 405 where there is no significant visible field.

2o In operation, the pump source 420 generates a pump beam which excites the laser material 425. The pump beam may be focused on and/or in the laser material 425 using the collimating and focusing lenses in order to mode match the pump beam and the cavity laser beam within the laser material 425. In the example that the laser material 425 is Nd:YAG, this generates a laser beam at 1064nm, which is converted by the Q-switch 430 to a high power pulsed laser

2₅ beam. The polarisation selector 433 is then tuned to select a particular polarisation of laser light, and that selected polarisation excites the Raman-active crystal 435 to generate a single Stokes- shifted wavelength, which in the case that the laser material 425 is Nd:YAG and the Raman-active crystal 435 is KGW, is either 1158 or 1176nm. The reflector 418 directs the resulting wavelengths of laser light towards frequency doubling crystals 440 and 445. The selected wavelength of the

30 Raman-shifted laser light is then doubled by the frequency doubling crystal that is optimised for that particular wavelength. After reflection from the reflector 415, the selected frequency doubled laser beam can exit the laser system 400 through the reflector 418, whereas other wavelengths are reflected by the reflector 418 towards reflector the 410, and continue to resonate within the resonator cavity 405. The wavelength of the laser light produced by the laser system 400 is selected by selecting the polarisation of laser light that excites Raman-active crystal 435, which is accomplished using the polarisation selector 433. An advantage of using the polarisation selector 433 to select wavelengths is that temperature tuning of the frequency doubling crystals is not necessary, and consequently wavelength switching is more rapid.

Referring to Fig.5, a laser system 500 comprises a resonator cavity 505, which is defined by reflectors 510 and 515, and which comprises a secondary cavity 507, defined by reflectors 540 and 515. The reflector 510 is highly reflective at all wavelengths generated within the resonator cavity 505, although the reflector 510 should be capable of allowing a pump beam from the pump source 520 to pass to the laser material 525. The laser system 500 may have at least one collimating lens and at least one focusing lens (not shown), located between the pump source 520 and the laser material 525, for collimating and focusing the pump beam. The pump source 520 may be for example a diode laser, and is capable of exciting the laser material 525. The laser material 525 may be for example Nd:YAG, which is capable of generating laser light at 1064nm. A Q-switch 530 is capable of converting the output from the laser material 525 into pulsed high power laser light capable of interacting with a Raman-active crystal 535. The Raman-active crystal 535 is capable of producing different Stokes-shifted wavelengths in response to different polarisations of input laser light. The polarisation selector 533 is for example a Faraday selector whereby the polarisation of the selector is selected electronically. The reflector 540 is highly transmissive at the wavelength of the laser material 525 (in the example of a Nd:YAG material, this is 1064nm), and is highly reflective at the Raman-shifted wavelengths of the Raman-active crystal 535. The reflector 515 is highly reflective at the wavelength of the laser material 525, and highly transmissive at the Raman-shifted wavelengths. The laser system 500 may be considered to comprise a coupled cavity, defined by reflectors 510, 540 and 515, having cavity 505 coupled with secondary cavity 507.

The laser system 500 may comprise at least one polariser (not shown) which may be included in the resonator cavity 505. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 505 where there is no significant visible field.

In operation, the pump source 520 generates a pump beam which excites the laser material 525. The pump beam may be focused on and/or in the laser material 525 using the collimating and focusing lenses in order to mode match the pump beam and the cavity laser beam within the laser material 525. In the example that the laser material 525 is Nd:YAG, this generates a laser beam at 1064nm, which is converted by the Q-switch 530 to a high power pulsed laser beam. The polarisation selector 533 is then tuned to select a particular polarisation of laser light, and that selected polarisation passes into the secondary cavity 507 through the reflector 540 and excites the Raman-active crystal 535 to generate a single Stokes-shifted wavelength, which in the case that the laser material 525 is Nd:YAG and the Raman-active crystal 535 is KGW, is either 1158 or 1176nm. The single Stokes-shifted wavelength of laser light can at least partially exit the cavity 505 through the reflector 515, and unshifted laser light is reflected and resonates within the cavity 505. Any of the Stokes-shifted wavelength that is reflected by the reflector 515 will resonate within the cavity 507 and ultimately exit through the reflector 515.

Referring to Fig. 6, a laser system 600 comprises a resonator cavity 605 defined by reflectors 610, 615 and 640, and comprising two coupled resonator cavities 606, defined by reflectors 610, 650 and 640, and 607, defined by reflectors 640 and 615. The reflector 610 is highly reflective at the wavelength of the laser material 625, and is capable of allowing a pump beam from the pump source 620 to pass to the laser material 625. The laser system 600 may have at least one collimating lens and at least one focusing lens (not shown), located between the pump source 620 and the laser material 625, for collimating and focusing the pump beam. The pump source 620 may be for example a diode laser, and is capable of exciting the laser material 625. The laser material 625 may be for example Nd:YAG, which is capable of generating laser light at 1064nm. The Q-switch 630 is capable of converting the output from the laser material 625 into pulsed high power laser light capable of interacting with the Raman-active crystal 635. The Raman-active crystal 635 is capable of producing different Stokes-shifted wavelengths in response to different polarisations of input laser light. The polarisation selector 633 is for example a Faraday selector whereby the polarisation of the selector is selected electronically. The reflector 640 is highly reflective at all wavelengths generated within the cavity 605. The reflector 650 is highly reflective at the wavelength of the laser material 625, and highly transmissive at the Stokes-shifted wavelengths. The reflector 615 is partially reflective (for example 10 to 50%) for the Stokes-shifted wavelengths. The laser system 600 may comprise at least one polariser (not shown) which may be included in the resonator cavity 605. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 605 where there is no significant visible field.

In operation, the pump source 620 generates a pump beam which excites the laser material 625. The pump beam may be focused on and/or in the laser material 625 using the collimating and focusing lenses in order to mode match the pump beam and the cavity laser beam within the laser material 625. In the example that the laser material 625 is Nd.ΥAG, this generates a laser beam at 1064nm, which is converted by the Q-switch 630 to a high power pulsed laser beam. That beam reflects off the reflector 650 towards the reflector 640 and resonates within the cavity 606. In passing through the Raman active crystal 635, a portion of that beam at 1064nm is Raman-shifted to generate two laser beams of different wavelengths and polarised orthogonally. In the case that the laser material 625 is Nd:YAG and the Raman active crystal 635 is KGW, these wavelengths are 1158 or 1176nm. These wavelengths are reflected by the reflector 640 towards the reflector 615. However the presence of the polarisation selector 633 prevents one of the Stokes-shifted wavelengths from resonating. The other, selected, wavelength resonates within the cavity 607 and exits the cavity 606 through the reflector 615. The selection of the wavelength outputted by the laser system 600 is accomplished by selection of the polarisation of the selector 633.

Referring to Fig. 7, a laser system 700 comprises a resonator cavity 705 defined by reflectors 710 and 715. The reflector 710 is highly reflective at all wavelengths generated within the resonator cavity 705, although the reflector 710 should be capable of allowing a pump beam from a pump source 720 to pass to the laser material 725. The pump source 720 may be for example a diode laser, and is capable of exciting the laser material 725. The laser system 700 may have at least one collimating lens and at least one focusing lens (not shown), located between the pump source 720 and the laser material 725, for collimating and focusing the pump beam. The laser material 725 may be for example Nd:YAG, which is capable of generating laser light at 1064nm. The Q-switch 730 is capable of converting the output from the laser material 725 into pulsed high power laser light capable of interacting with the Raman-active crystal 735 to generate one or more Stokes wavelengths of laser light. The Raman-active crystal 735 may for example be KGW, which can produce first Stokes wavelengths of 1158nm and 1176nm and second Stokes wavelengths of 1272nm and 1320nm when excited by a 1064nm laser. A wavelength tunable element 740 selected from an etalon, an optical filter, a prism, a grating, an etalon, or an interference filter is capable of selectively permitting laser beams of particular wavelengths to resonate within the resonator cavity 705. The reflector 715 is highly reflective at wavelengths that are not selected for possible outputting. For example, the reflector 715 may be highly reflecting at the fundamental wavelength (1064nm) and at the first Stokes wavelengths (1158nm and 1176nm) and at least partly transmissive at the second Stokes wavelengths (1272nm and 1320nm).

The laser system 700 may comprise at least one polariser (not shown) which may be included in the resonator cavity 705. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 705 where there is no significant visible field. In operation, the pump source 720 generates a pump beam which excites the laser material 725. The pump beam may be focused on and/or in the laser material 725 using the collimating and focusing lenses in order to mode match the pump beam and the cavity laser beam within the laser material 725. In the example that the laser material 725 is Nd:YAG, this generates a laser beam at 1064nm, which is converted by the Q-switch 730 to a high power pulsed laser beam. This causes the Raman-active crystal 735 to generate laser beams at 1158nm, 1176nm, 1272nm and 1320nm. The wavelength tunable element 740 may be configured to prevent, for example, the beam at 1320nm from resonating. The reflector 715 reflects the unshifted beam at 1064nm, and the first Stokes wavelengths of 1158nm and 1176nm, and permits at least part of the second Stokes wavelength at 1272nm to exit the resonator cavity 705. Alternatively the wavelength tunable element 740 may be configured to prevent the other second Stokes wavelength (1272nm) from resonating and allow the beam at 1320nm to pass to the reflector 715 and thereby exit the resonator cavity 705. Those wavelengths that are reflected by the reflector 715 continue to resonate within the resonator cavity 705 until they are converted to the desired wavelength for output, or else leak away elsewhere. Example

Figure 8 shows an experimental setup for a laser system that has been made according to the present invention. A cavity 810 is defined by a reflector 815 and a reflecting surface 820 of a laser material 825. The reflector 815 is a 20cm concave radius of curvature output coupler which is about 10-20% transmissive at 1150-1180nm and highly reflective at 1064nm. The laser material 825 is a Nd:YAG cylinder of length 5mm and an outside diameter of 5mm. The reflecting surface 820 is highly reflective at 1064 to 1180nm and highly transmissive at 808nm. Arrow 830 represents a pump beam at 808nm, which is directed to the laser material 825, casting a spot size of about 0.3mm radius thereon. A Raman active medium 835 is a birefringent KGW crystal cut for propagation along the crystal b axis and has dimensions 5mm^*5mm*50mm. A Q-switch 840 is located between the Raman active medium 835 and the reflector 815, and is a NEOS Model 33027 with a repetition rate of 16kHz. The distance between the laser material 825 and the Raman active medium 835 is 5mm, between the Raman active medium 835 and the Q-switch 840 is 8mm, and

5 between the Q-switch 840 and the reflector 815 is 10mm. Arrow 850 represents the laser output of the system.

The laser system 800 may comprise at least one polariser (not shown) which may be included in the resonator cavity 810. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The o polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the resonator cavity 810 where there is no significant visible field.

In operation, the pump beam 830 causes the laser material 825 to generate a laser beam at 1064nm within the cavity 810. This is converted to a high power pulsed beam by the Q-switch 840, s sufficient to excite the Raman active medium 835 to generate Stokes wavelengths 1158 and 1176nm. Due to the birefringence of the Raman active medium 835, these are offset relative to each other.

An adjustor (not shown) orients the curved reflector 815 so that the 1158nm beam is capable of resonating, whilst the other beam at 1176nm is disfavoured and can not resonate, so o that only the 1158nm beam is capable of exiting the cavity 810 as beam 850. Conversely, the adjustor (not shown) orients the reflector 815 so that only the Raman-shifted beam of 1176nm can resonate which disfavours the beam at 1158nm so that only the Raman-shifted beam at 1176nm exits the cavity 810 as beam 850. The adjustor (not shown) may be a motor or a piezoelectric or other device coupled to the reflector 815 which alters the orientation of the reflector 815 so that one ₅ of the two different wavelengths of Raman-shifted laser light is capable of resonating in the cavity 810 more efficiently than the other different wavelength of Raman-shifted laser light. Further, the adjustor (not shown) may be mechanically coupled to the curved reflector 815. For example, the adjustor (not shown) may be a standard mirror mount which enables independent angular adjustment in the horizontal and vertical planes. o Alternatively the orientation of the Raman active medium 835 may be adjusted by an adjustor (not shown) in order to select between the two first Stokes wavelengths as outputs from the system. By adjusting the output coupler, the output can be "switched" between 1.6W at 1158nm and 1.25W at 1176nm. The adjustor (not shown) may be a motor or a piezoelectric or a standard mirror mount which enables independent angular adjustment in the horizontal and vertical planes or other device coupled to the birefringent KGW crystal 835 which alters the orientation of the Raman-active crystal 835 so that one of the two different wavelengths of Raman-shifted laser light is capable of resonating in the cavity 810 more efficiently than the other different wavelength of Raman-shifted laser light. The adjuster (not^' shown) may be mechanically coupled to the Raman-active birefringent crystal 835.

The mode sizes at the faces of the components of the laser system of Fig. 8 during operation were determined to be as follows, based on a standard diode pump power of 18W: reflecting surface 820: ^■ 210 microns laser material 825 face away from surface 820: 205 microns

Raman active medium 835 on face nearest laser material 825: 200 microns

Raman active medium 835 on face nearest Q-switch 840: 190 microns

Q-switch 840 on face nearest Raman active medium 835: 150 microns Q-switch 840 on face nearest reflector 815: 140 microns reflector 815: 130 microns

The laser system 800 may also include a non-linear medium (not shown) to, for example, frequency double the desired wavelength of input laser light in a manner as will be described below with respect to Figures 9 to 13.

Referring to Fig. 9, there is shown another embodiment of the laser system 100 according to the invention which is almost identical to the laser system 100 shown in Fig.1 but which also comprises a non-linear medium 140 and an optional tuning means 145. Thus, the reference numerals described above for Fig.1 also apply equally to Fig.9. The non-linear medium 140 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to the first Stokes wavelength (1158nm) at about 4O⁰C to generate an output laser beam at 579nm (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 145 may be a heating/cooling unit or device which adjusts the temperature of the non-linear medium 140. Alternatively, the tuning means 145 may be a device to adjust the angle of the non-linear medium whereby tuning would be accomplished by orienting the non-linear medium 140 at the precise angle at which the non-linear medium 140 (for example a non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light. The remaining reference numerals depicted in Fig.9 are as described in Fig.1. In operation, the cavity operates as described with reference to Fig. 1 except that the non-linear medium 140 is tuned so that it is capable of frequency doubling one of the selected wavelengths of

5 the Raman laser light at 1158nm or 1176nm or the cavity laser beam. Alternatively, the non-linear medium 140 is tuned so that it is capable of sum frequency generation or difference frequency generation of the selected wavelength of Raman laser light at 1158nm_or 1176nm together with the. cavity laser beam. Thus, for example, Raman laser light at 1158nm entering non-linear medium 140 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or o frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector 115. Therefore the laser system 100 provides suitable means to selectively output a wide variety of different wavelengths from the cavity 105.

At least one polariser (not shown) may be included in the cavity of the laser system 100 as shown in Fig.9 and may be one or two plates of glass at Brewsters angle and/or a cube, angle s rod(s)/crystal(s) or other polariser. Such polarisers cause the fundamental to lase on only one linear polarisation. It is preferable to place the polarizer in a location where there is no significant visible field.

Referring to Fig. 10, there is shown another embodiment of the laser system 200 of the invention which is almost identical to the laser system 200 shown in Fig.2 but which also comprises o a non-linear medium 240 and an optional tuning means 245. Thus, the reference numerals described above for Fig.2 also apply equally to Fig.10.

The non-linear medium 240 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to the first Stokes wavelength (1158nm) at about^' 4O⁰C to generate an output laser beam at 579nm s (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 245 may be a heating/cooling unit or device which adjusts the temperature of the non-linear medium 240. Alternatively, the tuning means 245 may be a device to adjust the angle of the non-linear medium 240 whereby tuning would be accomplished by orienting the non-linear medium 240 at the precise angle, at which the non-linear medium 240 (for example a 0 non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light. The remaining reference numerals depicted in Fig.10 are as described in Fig.2. In operation, the cavity operates as described with reference to Fig. 2 except that the non-linear medium 240 is tuned so that it is capable of frequency doubling one of the selected wavelengths of the Raman laser light at 1158nm or 1176nm or the cavity laser beam. Alternatively, the non-linear medium 240 is tuned so that it is capable of sum frequency generation or difference frequency generation of the selected wavelength of Raman laser light at 1158nm or 1176nm together with the cavity laser beam. Thus, for example, Raman laser light at 1158nm entering the non-linear medium

5 240 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector 215. Therefore the laser system 200 provides suitable means to selectively output a wide variety of different wavelengths from the cavity.205. Referring to Fig. 11 , there is shown another embodiment of the laser system 300 of the invention o which is almost identical to the laser system 300 shown in Fig.3 but which also comprises a non¬ linear medium 340 and an optional tuning means 345. Thus, the reference numerals described above for Fig.3 also apply equally to Fig.11.

The non-linear medium 340 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to s the first Stokes wavelength (1158nm) at about 4O⁰C to generate an output laser beam at 579nm (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 345 may be a heating/cooling unit or device which adjusts the temperature of the non-linear medium 340. Alternatively, the tuning means 345 may be a device to adjust the angle of the non-linear medium whereby tuning would be accomplished by orienting the o non-linear medium 340 at the precise angle at which the non-linear medium 340 (for example a non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light. The remaining reference numerals depicted in Fig.11 are as described in Fig.3. In operation, the cavity operates as described with reference to Fig. 3 except that the non-linear medium 340 is tuned so that it is capable of frequency doubling one of the selected wavelengths of s the Raman laser light at 1158nm or 1176nm or the cavity laser beam. Alternatively, the non-linear medium 340 is tuned so that it is capable of sum frequency generation or difference frequency generation of the selected wavelength of Raman laser light at 1158nm or 1176nm together with the cavity laser beam. Thus, for example, Raman laser light at 1158nm entering the non-linear medium 340 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or 0 frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector 315. Therefore the laser system 300 provides suitable means to selectively output a wide variety of different wavelengths from the cavity 305. Referring to Fig. 12, there is shown another embodiment of the laser system 500 of the invention which is almost identical to the laser system 500 shown in Fig.5 but which also comprises a non¬ linear medium 540 and an optional tuning means 545. ^• Thus, the reference numerals described above for Fig.5 also apply equally to Fig.12. The non-linear medium 540 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to the first Stokes wavelength (1158nm) at about 4O⁰C to generate an output laser beam at 579nm (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 545 may be a heating/cooling unit or device which adjusts the temperature of the non-linear medium 540. Alternatively, the tuning means 545 may be a device to adjust the angle of the non-linear medium whereby tuning would be accomplished by orienting the non-linear medium 540 at the precise angle at which the non-linear medium 540 (for example a non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light. The remaining reference numerals depicted in Fig.12 are as described in Fig.5. In operation, the cavity operates as described with reference to Fig. 5 except that the non-linear medium 540 is tuned so that it is capable of frequency doubling one of the selected wavelengths of the Raman laser light at 1158nm or 1176nm or the cavity laser beam. Alternatively, the non-linear medium 540 is tuned so that it is capable of sum frequency generation or difference frequency generation of the selected wavelength of Raman laser light at 1158nm or 1176nm together with the cavity laser beam. Thus, for example, Raman laser light at 1158nm entering the non-linear medium 540 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector 515. Therefore the laser system 500 provides suitable means to selectively output a wide variety of different wavelengths from the cavity 505. Referring to Fig. 13, there is shown another embodiment of the laser system 600 of the invention which is almost identical to the laser system 600 shown in Fig.6 but which also comprises a non¬ linear medium 645 and an optional tuning means 650. Thus, the reference numerals described above for Fig.6 also apply equally to Fig.13. The non-linear medium 645 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to the first Stokes wavelength (1158nm) at about 40⁰C to generate an output laser beam at 579nm (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 650 may be a heating/cooling unit or device which adjusts the temperature of the non-linear medium 645. Alternatively, the tuning means 650 may be a device to adjust the angle of the non-linear medium whereby tuning would be accomplished by orienting the non-linear medium 645 at the precise angle at which the non-linear medium 645 (for example a non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light. The s remaining reference numerals depicted in Fig.13 are as described in Fig.6.

In operation, the cavity operates as described with reference to Fig. 6 except that the non-linear medium 645 is tuned so that it Is capable of frequency doubling one of the selected wavelengths of the Raman laser light at 1158nm or 1176nm or_.the cavity laser beam. Alternatively, the non-linear medium 645 is tuned so that it is capable of sum frequency generation or difference frequency o generation of the selected wavelength of Raman laser light at 1158nm or 1176nm together with the cavity laser beam. Thus, for example, Raman laser light at 1158nm entering the non-linear medium 645 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector 615. Therefore the laser system 600 provides suitable means to selectively output a wide s variety of different wavelengths from the cavity 605.

Referring to Fig. 14, there is shown another embodiment of the laser system 700 which is almost identical to the laser system 700 shown in Fig.7 but which also comprises a non-linear medium 745 and an optional tuning means 750. Thus, the reference numerals described above for Fig.7 also apply equally to Fig.14. o The non-linear medium 745 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to the first Stokes wavelength (1158nm) at about 4O⁰C to generate an output laser beam at 579nm (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 750 may be a heating/cooling unit or device which adjusts the 5 temperature of the non-linear medium 745. Alternatively, the tuning means 750 may be a device to adjust the angle of the non-linear medium whereby tuning would be accomplished by orienting the non-linear medium 745 at the precise angle at which the non-linear medium 745 (for example a non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light. The remaining reference numerals depicted in Fig.14 are as described in Fig.7. 0 In operation, the cavity operates as described with reference to Fig. 6 except that the non-linear medium 745 is tuned so that it is capable of frequency doubling one of the selected wavelengths of the Raman laser light at 1158nm or 1176nm or the cavity laser beam. Alternatively, the non-linear medium 745 is tuned so that it is capable of sum frequency generation or difference frequency generation of the selected wavelength of Raman laser light at 1158nm or 1176nm together with the cavity laser beam. Thus, for example, Raman laser light at 1158nm entering the non-linear medium 745 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector 715. Therefore the laser system 700 provides suitable means to selectively output a wide variety of different wavelengths from the cavity 705. Raman laser without Q-switch or Long Pulse Raman Lasing

Referring to Fig. 15, a cavity 900 is defined by concave mirrors 905 (high reflectivity at 1064 to 1320nm, radius of curvature 2m) and 910 (high reflectivity at 1064nm, 6% transmissive at 1176nm, radius of curvature 75mm). The mirror 910, being partially transmissive at 1176nm, is also capable of functioning as an output coupler 910. A plano-convex lens 915 is located within the cavity 900, and is a 200mm focal length lens. A laser material 920 is a 4mm diameter, 75mm length cylindrical Nd:YAG rod, located between the mirror 905 and the plano-convex lens 915. A flashlamp 925 is located outside the cavity 900, and is capable of side-pumping laser material 920. The pulse duration is of the order of about 0.5ms. A Raman-active medium 930 is located in the cavity 900 between the lens 915 and the mirror/output coupler 910. A non-linear medium 940 and an optional tuning means 945 is also located between the Raman-active medium 930 and the concave mirror 910 and in this example is a KGW crystal. A light emitting diode or laser diode (or other suitable source of monochromatic light such as laser light, for example) 950 is also located in the laser system 900 so as to be capable of seeding the Raman-active medium 930 with wavelengths corresponding to the first Stokes wavelengths of the Raman-active crystal 930 (ie 1158 or 1176nm). A second light emitting diode (LED) or laser diode (or other suitable source of monochromatic light such as laser light, for example) 950a is capable of seeding the Raman-active crystal 930 with the other of the wavelengths corresponding to one of the first Stokes wavelengths of the Raman-active crystal 930 (ie 1176 or 1158nm).

The non-linear medium 940 may be for example an LBO crystal which would be tuned to the unshifted beam (1064nm) at about 15O⁰C to generate an output laser beam at 532nm (green), to the first Stokes wavelength (1158nm) at about 4O⁰C to generate an output laser beam at 579nm (yellow), and to the second Stokes wavelength (1272nm) at about O⁰C to generate a laser beam at 636nm (red). The tuning means 945 may be a heating/cooling unit or device which adjusts the temperature of the non-linear medium 940. Alternatively, the tuning means 945 may be a device to adjust the angle of the non-linear medium 940 whereby tuning would be accomplished by orienting the non-linear medium 940 at the precise angle at which the non-linear medium 940 (for example a non-linear crystal) is capable of frequency doubling the desired wavelength of input laser light.

In operation, the flashlamp 925 provides incoherent pump radiation to side pump the laser material 920. This causes the laser material 920 to generate a fundamental laser beam at 1064nm in cavity 900, which resonates between the mirrors 905 and 910 respectively. As this fundamental laser beam passes through the Raman-active medium 930, the fundamental laser beam is Stokes shifted to produce a laser beam at 1176nm, which resonates in the cavity 900. The non-linear medium 940 is tuned by the tuning means 945 so that the non-linear medium 940 is capable of frequency doubling one of the selected wavelengths of the Raman laser light at 1158nm or 1176nm or the cavity laser beam. Alternatively, the non-linear medium 940 is tuned so that it is capable of sum frequency generation or difference frequency generation of the selected wavelength of Raman laser light at 1158nm or 1176nm together with the cavity laser beam. Thus, for example, Raman laser light at 1158nm entering the non-linear medium 940 may be frequency doubled to 579nm, or frequency summed with the cavity laser beam, or frequency differenced with the cavity laser beam and thereafter outputted from the cavity via reflector/output coupler 910. Therefore the laser system of Figure 15 provides suitable means to selectively output a wide variety of different wavelengths from the cavity 900.

In operation of the LED or laser diode 950 and 950a,and in order to select an output of 579nm, the LED or laser diode 950 directs a seed beam at 1158nm towards the Raman-active crystal 930. The seed beam may be mode matched to the beam waist in the Raman-active crystal 930. The seed beam may be mode matched and synchronized to the beam waist in the Raman- active crystal 930 to generate a shifted laser beam at 1158nm. The shifted laser beam at 1158nm then is frequency converted by the tunable non-linear medium 940 as described previously by frequency doubling, sum frequency generation, difference frequency generation or some other parametric frequency generation. In an example where the non-linear medium 940 is tuned to frequency double the shifted laser beam the output laser beam will be 579nm provided the output coupler 910 allows transmission at this wavelength and is reflective at 1064 to 1158nm. Further, the shifted laser beam reflector 910 reflects the unshifted beam at 1064nm and permits (by transmission) at least part of the visible wavelength outputut at 579nm to exit the resonator cavity 900. Those wavelengths that are reflected by the reflector 910 continue to resonate within the resonator cavity 900 until they are converted to the desired wavelength for output through reflector 910, or else leak away elsewhere. In order to change the output laser to 588nm, the LED 150 is changed to direct a beam of wavelength 1176nm towards the Raman-active crystal 930 or alternatively, LED or laser diode 150 is switched off and LED or laser diode 150a is switched on thereby directing a seed beam of wavelength 1176nm towards the Raman-active crystal 930. The seed beam may be mode matched and synchronized to the beam waist in the Raman-active crystal 930. This causes the Raman- active crystal 930 to generate a laser beam at 1176nm. The shifted laser beam at 1176nm then is frequency converted by the tunable non-linear medium 940 as described previously by frequency doubling, sum frequency generation, difference frequency generation or some other parametric frequency generation. In an example where the non-linear medium 940 is tuned to frequency double the shifted laser beam the output laser beam will be 588nm provided the output coupler 910 allows transmission at this wavelength and is highly reflective at 1064 to 1158nm. Further, the shifted laser beam reflector 910 reflects the unshifted beam at 1064nm and permits (by transmission) at least part of the visible wavelength output at 588nm to exit the resonator cavity 900 through output coupler/reflector 91O.The reflector 910 reflects the unshifted beam at 1064nm to 1176nm. The frequency of the seed beam may be synchronized with the frequency of the beam at 1176nm which exits the resonator cavity 900. The frequency of the seed beam may be the same as the frequency of the beam at 1176nm which exits the resonator cavity 900. The laser of this example may be suitable for scaling to high pulse energies. Using the cavity 900 as shown in Figure 15, the inventors have obtained Raman lasing at 1176nm with up to 50 mJ of 1^st Stokes output (with 850V on the flashlamp). The pulse length of the 1176nm output was. -64 μs (FWHM) from a fundamental pulse of -140 μs (FWHM), as shown in Figure 16. Using the cavity 900 as shown in Figure 15, the inventors have obtained Raman lasing at 1176nm with up to 46m J of 1^st Stokes output (with 75OmV on the flashlamp). The pulse length of the 1176nm output was -180 μs (total base length) as shown in Figure 16a. The same Nd:YAG laser rod with a simple hemispherical cavity (high reflector with a radius of curvature of 5m and a flat output coupler with 10% reflectivity at 1064nm) was run to obtain information regarding the optimal lasing characteristics for this Nd:YAG-flashlamp combination, thereby enabling determination of the efficiency of the Raman conversion. The maximum 1064nm output obtained from this laser was 15OmJ (with 850V on the flashlamp). Thus the Raman conversion efficiency is shown in Figure 17 to be about 4.6%. However, Figure 17a shows the Raman and Nd:YAG lasing characteristics is up to about 31% using the cavity 900 of Figure 15. It should be noted that the graphical representations as shown in Figure 16, Figure 16a, Figure 17 and Figure 17a were achieved with the non-linear medium 940 and the tuning means 945 removed from the cavity 900. It is expected that when non-linear medium 940 is a frequency doubling crystal the visible output can be expected to be about 50% of the infrared output.

The tunable non-linear medium 940 shown in Figure 15 may comprise an additional tunable non-linear medium and may comprise two or more tunable non linear medium, such as tunable frequency doublers for example, disposed therein to frequency convert the Raman shifted beam.

The cavity 900 as shown in Figure 15 represents a design for scaling to high pulse energy output and achieving visible switchable output.

In order to select an output of 579nm, the LED or laser diode 950 directs a seed beam at 1158nm towards the Raman-active crystal 930. The seed beam may be mode matched and may be synchronised to the Raman shifted beam in the Raman-active crystal 930. This causes the Raman-active crystal 930 to generate a laser beaπrat 1158nm. The reflector 905 reflects the unshifted beam at 1064nm and permits at least.part of the Raman-shifted laser beam at 1158nm to exit the resonator cavity 900. Those wavelengths that are reflected by the reflector 910 continue to resonate within the resonator cavity 900 until they are converted to the desired wavelength for output from the. cavity 900, or else leak away elsewhere. In order to change the output laser to 1158nm, the LED 950 is changed to direct a beam of wavelength 1158nm towards the Raman- active crystal 930. This causes the Raman-active crystal 930 to generate a laser beam at 1158nm. The output coupler/reflector 910 reflects the unshifted beam at 1064nm and permits at least part of the Raman-shifted laser beam at 1158nm to exit the resonator cavity 900.

The cavity 900 shown in Figure 15 may comprise at least one polariser (not shown) which may be included in the cavity 900. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the cavity 900 where there is no significant visible field. The following resonator designs, shown in Figs. 18 and 19, are intended to achieve switchable visible output. As for the laser of Fig. 15, the lasers of Figs. 18 and 19 are not Q-switched and the flashlamp pulse has a duration the order of 0.5ms (base duration) and a frequency of up to a few Hz e.g. in the range of 0.1-25Hz or 0.1 to 20hz, or 0.1 to 17.5Hz, or 0.1 to 15Hz, or 1 to 15Hz, or 5 to 15Hz, or 5 to 10Hz, or 1 to 5 Hz, or 1 to 2Hz. In a specific example, the frequency was 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 Hz.

With reference to Fig. 18, there is shown a bent cavity 1000 defined by mirrors 1005 and 1010. The mirrors 1005 and 1010 are highly reflective at 1064-1272nm and at 532- 636nm,respectively. A dichroic mirror 1015 is highly reflective at 1064-1272nm and partially transmissive in the visible range (particularly at 532-636nm). The dichroic mirror 1015 is therefore capable of functioning as an output coupler for visible wavelengths generated in the cavity 1000. A laser material 1020 is provided between mirrors 1010 and 1015, and may be as described above for Figure 15. A Flashlamp 1025 is provided extracavity for side pumping the laser material 1020, as described for Figure 15. The lens 1030 and the Raman-active medium 1035 are located in the cavity 1000 between the mirrors 1005 and 1015, and are as described for Figure 15. A non-linear medium 1040 is located in the cavity 1000 between the Raman-active medium 1035 and the lens 1030. A light emitting diode (LED) 1050 is also located in the cavity 1000 so as to be capable of seeding the Raman-active medium 1035 with wavelengths corresponding to the first Stokes wavelengths of the Raman-active crystal 1035 (ie 1158 or 1176nm). The non-linear medium 1040 may be capable of SHG or SFG. Although not shown in Fig. 18 for reasons of simplicity, the non¬ linear medium 1040 may be provided with means for tuning according to any one of the possibilities described in the present specification. Thus, for example, the non-linear medium 1040 may be provided with an angle tuner, or a temperature tuner, or may comprise a plurality of individual non¬ linear media and may be provided with a plurality of temperature and/or angle tuners.

Alternatively the non-linear medium 1040 may comprise a periodic structure which forms a grating within the medium, and the means for tuning may be a motor or similar means for translating the non-linear medium 1040 relative to the laser beam resonating in cavity 1000. Thus pump radiation from flashlamp 1025 causes the laser material 1020 to produce a fundamental laser beam in the cavity 1000. At least two different Stokes wavelengths are capable of being produced from the fundamental beam by the Raman-active material 1035. The fundamental and Stokes wavelengths resonate in the cavity 1000. When the non-linear medium 1040 is tuned to shift the selected wavelength of laser light, a visible beam is produced by the non-linear medium 1040, which is coupled out of the cavity 1000 by mirror 1015, acting as an output coupler.

In order to select an output of 579nm, the LED 1050 directs a beam at 1158nm towards the Raman-active crystal 1035. This causes the Raman-active crystal 1035 to generate a laser beam at 1158nm. The reflector 1005 reflects the unshifted beam at 1064nm and the Raman shifted wavelength of 1158nm. The non-linear medium 1040 is a frequency doubling crystal which converts the Raman shifted wavelength to 579nm. The converted visible wavelength is permitted to exit the resonator cavity 1000 by way of dichroic mirror 1015. The Raman-shifted laser beam at 1158nm and unshifted wavelength of 1064nm are reflected by the reflectors 1005 and 1010 and continue to resonate within the resonator cavity 1000 until they are converted to the desired wavelength for output, or else leak away elsewhere. In order to change the output laser to 588nm, the LED 1050 is changed to direct a beam of wavelength 1176nm towards the Raman-active crystal 1035. This causes the Raman-active crystal 1035 to generate a laser beam at 1176nm which is converted by the non-linear medium 1040 by frequency doubling to 588nm. The output coupler/reflector 1015 reflects the unshifted beam at 1064nm and Raman-shifted wavelength and permits at least part of the frequency doubled wavelength of 588nm to exit the resonator cavity 1000.

The laser shown in Figure 18 may comprise at least one polariser (not shown) which may be included in the bent cavity 1000. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the cavity 1000 where there is no significant visible field.

The laser system of Figure 19 operates similarly to that shown in Figure 18. Thus in

Figure 19, a Z-cavity 1100 is defined by mirrors.1105, 1110 and 1115, which are convex mirrors highly reflective at the fundamental wavelength, the Stokes shifted wavelength and at the output wavelengths of the Z cavity 1100 (in the case of a KGW Raman-active medium, this would be 1064-1272nm and 532-636nm). There is also shown a mirror 1120, which is highly reflective at the fundamental wavelength and the Stokes shifted wavelength and highly transmissive at the desired output wavelengths (in the case of a KGW Raman-active medium, this would be reflective at 1064- 1272nm and transmissive at 532-636nm). The mirror 1120 is therefore capable of functioning as an output coupler for the Z cavity 1100 at visible wavelengths of output laser light. A laser material 1125 is provided between the mirrors 1105 and 1120, which may be Nd:YAG. A flashlamp 1130 is provided extracavity for side pumping the laser material 1125, as described for Figure 18. A Light emitting diode 1150 is also located in the cavity 1100 so as to be capable of seeding the Raman- active medium 1135 with wavelengths corresponding to the first Stokes wavelengths of the Raman- active crystal 1135 (ie 1158 or 1176nm) The Raman-active medium 1135 is located in the Z cavity 1100 between the mirrors 1110 and 1115, and is as described as for Figure 15 as KGW. A non¬ linear medium 1140 is located in the cavity 1100 between the Raman-active medium 1135 and the mirror 1115. The non-linear medium 1140 may be capable of SHG or SFG or other parametric frequency conversion although for illustration purposes in this particular example is a frequency doubling crystal. Although not shown in Fig. 19 for reasons of simplicity, the non-linear medium 1140 may be provided with means for tuning according to any one of the possibilities described in the present specification. Thus for example the non-linear medium 1140 may be provided with an angle tuner, or a temperature tuner, or may comprise a plurality of individual non-linear media and may be provided with a plurality of temperature and/or angle tuners. Alternatively it may comprise a periodic structure which forms a grating within the medium, and the means for tuning may be a motor or similar means for translating the non-linear medium 1140 relative to the laser beam resonating in the cavity 1100. Thus pump radiation from the flashlamp 1130 causes the laser material 1125 to produce a fundamental laser beam in the Z cavity 1100. At least two different Stokes wavelengths are capable of being produced from the fundamental beam by Raman-active material 1135. The fundamental and Stokes wavelengths resonate in the Z cavity 1100. When the non-linear medium 1140 is tuned to shift the selected wavelength of laser light, a visible beam is produced by the non-linear medium 1140, which is coupled out of the cavity 1100 by the mirror 1120, acting as an output coupler.

In order to select a particular wavelength of 579 or 588nm, the LED 1150 is operated in a similar manner as described above in Figure 18. The laser shown in Figure 19 may comprise at least one polariser (not shown) which may be included in the Z cavity 1100. The polariser may be one or two plates of glass at Brewsters angle and/or a cube, one or more angle rods/crystals or other polariser known in the art. The polarisers are believed to cause the fundamental to lase on only one linear polarisation. The polariser may be positioned in a location in the cavity 1100 where there is no significant visible field.

The examples described above may further comprise a step of providing an intracavity etalon. The intracavity etalon may prevent generation of parasitic laser wavelengths. The oscillation of these parasitic wavelengths with the fundamental may be the cause of very high laser fields (spiking) that surpass the damage threshold of the laser optics. The step of providing an intracavity etalon may be particularly relevant to flashlamp pumped Q-switched cavities although an intracavity etalon may be used in all cavities as described in the above examples. Modifications and variations such as would be apparent to a skilled addressee are deemed to be within the scope of the present invention. It is to be understood that the invention should not be restricted to the particular embodiment(s) described above.

Claims

Claims:

1. A laser system capable of selecting between at least two different frequencies of output laser light, said system comprising: a) at least one resonator cavity comprising at least two reflectors; 5 b) a laser material located in the resonator cavity; c) a pump source located outside of the resonator cavity for pumping the laser material with a pump beam to generate a cavity laser beam; d) a Raman-active medium located in the resonator cavity for shifting the frequency of the cavity laser beam such that at least two different Raman-shifted frequencies are generated; Q e) a frequency selector for selecting a single frequency of output laser light by either deselecting all but one of the at least two different frequencies or by selecting one of the at least two different frequencies; and f) an output coupler for outputting the single frequency of output laser light.

2. The laser system of claim 1 , wherein one of the two or more reflectors functions as the s output coupler.

3. The laser system of claim 1, wherein the Raman-active medium comprises two separate Raman-active media for shifting the frequency of the cavity laser beam such that at least two different frequencies are generated.

4. The laser system of claim 3, wherein each of the two different Raman-active media Q generates one or more different Stokes frequencies.

5. The laser system of claim 1 , further comprising a Q-switch.

6. The laser system of claim 1 , wherein the frequency selector comprises a seeding device to seed the Raman-active medium with a seed beam in order to cause the Raman-active medium to produce predominantly a single frequency of laser light with a Raman-shifted frequency ₅ corresponding to the wavelength of the seed beam.

7. The laser system of claim 1 , further comprising a non-linear medium disposed in the cavity for converting the frequency of a laser beam selected from the group consisting of a laser beam produced by the laser material and a laser beam which has been frequency shifted by the Raman-active medium. 0

8. The laser system of claim 7, wherein the non-linear medium is selected from the group consisting of a frequency doubler and a sum frequency generator.

9. The laser system of claim 7, wherein the output laser beam is selected from the group consisting of the Raman-shifted frequency and a laser beam that has been converted by the non¬ linear medium.

10. The laser system of claim 1, wherein one or more of the laser material, the 5 Raman-active medium and the non-linear medium is solid.

11. The laser system of claim 1, wherein the pump source is selected from the group consisting of a pump source capable of end pumping the laser material and a pump source capable of side pumping the laser material.

12. The laser system of claim 6, wherein the seed beam is polarised. o

13. The laser system of claim 1 , wherein the Raman-active medium is birefringent.

14. The laser system of claim 13, wherein the Raman-active medium is capable of generating two different polarisations of laser light spatially separated from each other, each having a different Stokes frequency.

15. The laser system of claim 14, wherein the selector is capable of selecting the single s frequency of Raman-shifted laser light by realigning one of the reflectors and/or the Raman-active medium.

16. The laser system of claim 1, additionally comprising a birefringent element, whereby the Raman-active element is capable of generating Raman-shifted frequencies of laser light, and the birefringent element is capable of separating the Raman-shifted frequencies Q according to their polarisation.

17. The laser system of claim 1, wherein the frequency selector comprises a polarisation selector.

18. The laser system of claim 17, wherein the polarisation selector is selected from the group consisting of a mechanically rotatable selector, a Faraday rotator and an electro-optic ₅ rotator.

19. The laser system according to claim 1, wherein the frequency selector comprises a wavelength tunable element for selecting a single frequency of laser beam.

20. The laser system of claim 19, wherein the wavelength tunable element is selected from the group consisting of an optical filter, a prism, a grating, an etalon and an interference filter 0 21. A method for providing an output laser beam from a laser system, said laser beam having a frequency which may be selected from two or more frequencies, said method comprising: a) generating a laser beam within a resonator cavity by pumping a laser material located in the cavity with a pump beam from a pump source located outside the cavity; b) shifting the frequency of at least a portion of the laser beam by passing the laser beam through a Raman-active medium such that at least two different Raman-shifted frequencies are generated; c) selecting a single frequency of output laser light by either deselecting all but one of the at least two different Raman-shifted frequencies or by selecting one of the at least two. different

Raman-shifted frequencies; and d) outputting from the laser system the single frequency of output laser light.

22. The method of claim 21, wherein the step of selecting comprises seeding the Raman- active medium using a seed beam with a frequency which is capable of causing the Raman-active medium to generate predominantly a single frequency, said single frequency being the same as the frequency of the seed beam.

23. The method of claim 21 , further comprising the step of passing the output laser beam from the Raman-active medium through a non-linear medium, thereby frequency converting said laser beam.

24. The method of claim 21 , wherein the Raman-active medium is birefringent, and the step of selecting comprises orienting at least one of the Raman-active medium and a reflector so that one of the at least two different frequencies of Raman-shifted laser light is capable of resonating in the cavity more efficiently than the other different frequency(s) of Raman-shifted laser

25. The method of claim 21, wherein the Raman-active medium is capable of shifting different polarisations of incident light to different Stokes frequencies, and the step of selecting uses a polarisation selector.

26. The method of claim 25, wherein the step of selecting comprises tuning a wavelength tunable element.

27. A method of using a laser system for treating, detecting or diagnosing a selected area on or in a subject requiring such diagnosis or treatment, comprising: a) generating a laser beam within a resonator cavity by pumping a laser material located in the cavity with a pump beam from a pump source located outside the cavity; b) shifting the frequency of at least a portion of the laser beam by passing the laser beam through a Raman-active medium such that at least two different Raman-shifted frequencies are generated; c) selecting a single frequency of output laser light by either deselecting all but one of the at least two different Raman-shifted frequencies or by selecting one of the at least two different Raman-shifted frequencies ; d) outputting from the laser system the single frequency of output laser light; and e) illuminating the selected area with the output laser light.