JP2891240B2 - Multi-wavelength simultaneous oscillation optical fiber laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an excitation type solid-state laser, and more particularly to an optical fiber type laser capable of simultaneously outputting multi-wavelength laser light.

[0002]

2. Description of the Related Art Several methods for extracting a plurality of laser beams from one laser have been known.

For example, Japanese Patent Application Laid-Open No. 51-031195 discloses that the same energy level can be obtained by forming a plurality of resonators for one laser gain medium, utilizing the fact that the frequency spread of laser gain is large. A method of extracting laser outputs of a plurality of wavelengths from different positions is described. In this method, continuous oscillation is possible even with a single wavelength, and the sum of the output powers of the plurality of wavelengths is substantially constant.

Japanese Patent Application Laid-Open No. Hei 5-259558 discloses that a laser transition from the same energy level excited by discharge to two different lower levels is performed using copper vapor as a laser medium. A method for obtaining laser oscillation of a wavelength is described. In this method, since the laser gain is divided into two wavelengths, it is better to oscillate at a single wavelength when increasing the output power of the target wavelength, and to lower the laser lower level at both wavelengths. Continuous oscillation was difficult because the lifetime was longer than the lifetime of the laser upper level.

As a multi-wavelength laser based on a completely different principle from these methods, Applied Physics Letters (Appl. Phys. Lett. Vol. 6) has been proposed.
2 (6) No. 6 (1993) p. 543)
A method is described in which a laser transition from the lower level of the first laser to the lower level is performed to extract the second laser light, thereby obtaining laser light of two wavelengths in a cascade. However, when the lower level of the second laser is the ground level (the second laser is a quasi-three-level laser), since the re-absorption loss from the lower level of the second laser is large, the strong excitation exceeds the re-absorption loss. And laser oscillation is difficult, and there is a problem that a sufficient laser output cannot always be obtained.

In the case of a self-terminating laser transition in which the lower-level lifetime of a laser transition that is normally intended is longer than the upper-level lifetime, a gain that lasts over time cannot be obtained.
It is difficult to perform continuous oscillation operation, and it is also difficult to increase output in a pulse operation mode. Therefore, an attempt was made to lower the lower level density by some method.

As one method of reducing the laser lower level number density, as shown in FIG. 2, a lower level 15 of a laser transition 12 of a laser ion 10 is applied to an excitation laser diode (L).
D) There is a method in which an excited level absorption transition 21 is made to the upper level 18 by irradiation of light or light prepared separately. However, in this method, it is often difficult to excite the excitation level so that the excitation LD wavelength reduces the lower level density of the laser, and the apparatus becomes complicated by using an excitation light different from the excitation LD. There was a point. Further, there is a problem that heat distortion is generated in the solid medium due to heat generated by radiationless relaxation.

As another method of reducing the laser lower level number density, as shown in FIG. 2, a sub-addition ion 11 having an energy level 19 substantially equal to the laser lower level 15 is used.
Is co-doped with the laser ions 10 in a solid-state laser medium, and the energy is transferred 27 in a resonant manner. But,
In practice, it is often difficult to select sub-addition ions, so that two ions are added into the solid-state laser medium, so that the flow of energy is complicated and it is difficult to determine an appropriate addition concentration. Was. Further, in this case as well, there is a problem that heat generation due to radiationless relaxation causes thermal distortion in the solid medium.

[0009]

SUMMARY OF THE INVENTION The present invention enables continuous oscillation and high power output thereof even when the laser transition is of a self-terminating type, or high power output in a pulse mode, and at the same time provides a useful second power source. It is an object of the present invention to provide a multi-wavelength simultaneous oscillating optical fiber type laser capable of emitting multiple wavelengths by extracting laser light.

[0010]

According to the present invention, a second laser transition from a lower level of a first laser transition to a lower energy level is performed using an optical fiber in which laser ions are added to a glass material. A multi-wavelength simultaneous oscillating optical fiber type laser that outputs at least two wavelengths by forming at least one pair of resonators satisfying a resonance condition with respect to the wavelengths of the first and second laser transitions. About.

[0011]

FIG. 1 shows the energy levels of laser ions that generate laser light. For example, using a laser diode (LD) as excitation light, ions are excited from a level 1 to a laser upper level 3 or a level 4 higher than the upper level. When the first laser transition 8 is a self-terminating type, the laser lower level 2 has a longer life than the laser upper level 3, so that the population inversion required for laser oscillation hardly occurs.

Therefore, in the present invention, by providing a resonator so as to simultaneously oscillate the second laser transition 9, the lower level 2
Forcefully reduce the number density of As a result, continuous oscillation of the first laser 8 and high output thereof, and high output in high repetition pulse oscillation can be realized. The energy flow is cascaded, and two types of laser light can be extracted simultaneously, so that useful multi-wavelength laser light can be obtained at the same time. As described above, since the transition from the level 2 to the lower level 1 is not a nonradiative relaxation but a laser transition, the heat generation in the solid medium is small, the thermal distortion can be suppressed, and the laser operation can be stabilized. Can be done.

In the present invention, by using an optical fiber as the laser medium, cascade oscillation can be performed more efficiently for the following reasons.

That is, when the second laser lower level 1 is the ground level, the second laser is called a quasi-three level laser, and the loss due to reabsorption from the second laser lower level 1 is large. However, strong excitation exceeding the re-absorption loss is required, and it has been difficult for a laser using a conventional solid crystal to oscillate. However, in the fiber laser of the present invention, the excitation light can be confined in the optical fiber at a high density, so that the laser ions can be strongly excited.

Further, since a glass material such as fluoride glass is used as the material of the optical fiber, the excitation life of the second laser upper level 3 can be extended since the material-specific phonon energy is small. Since the fiber material is amorphous, the Stark spread of each energy level of laser ions increases, so that laser oscillation can be easily obtained. High-efficiency oscillation of the laser becomes possible.

In the case of the fiber type, unlike the case where a crystal is used, the calorific value per unit volume is small because the fiber is long. For this reason, there is an advantage that the performance is hardly deteriorated due to deformation due to heat generation or the like. However, a more stable laser operation can be obtained by cascading laser oscillation to prevent heat generation.

Next, the laser of the present invention will be described more specifically with reference to FIGS.

[Embodiment 1] FIG. 3 shows an embodiment of the present invention. A single clad optical fiber 31 having a core 32 and a clad 33 to which laser ions 10 for generating laser light were added was formed. A mirror 34, which transmits the LD light 28 and has a high reflectivity to the first laser 29 and the second laser 30, is provided at the end of the optical fiber on the side of the pumping light incident side. A partial reflection mirror 35 having a reflectance suitable for the laser wavelength was provided, and a fiber laser resonator was constituted by the two mirrors. The first laser 29 and the second laser 30 oscillate simultaneously when the LD light enters from the mirror 34 side.

Here, the optical fiber does not need to maintain a straight line, and if the optical fiber is long, it can be reduced in size by winding it. It is not always necessary to closely contact both end surfaces of the optical fiber 31 and the mirrors 34 and 35. When the core is a multi-mode, by providing a gap, the number of modes propagating through the optical fiber can be reduced.

Further, although different depending on various conditions such as the pumping LD power and the magnitude of optical propagation loss in the optical fiber, the laser resonator is constituted only by the Fresnel reflection at the end face of the optical fiber by removing the output extraction mirror 35. In some cases. Also, separate mirrors can be used for each wavelength.

[Embodiment 2] The embodiment shown in FIG. 4 is different from the embodiment 1 shown in FIG.
1, a Q switch element 36 is provided.

The Q switch element 36 performs a switching operation on both the first laser and the second laser, and a switching operation on one of the first laser and the second laser. For example, if the Q switch operation is performed only on the second laser, the generated Q switch pulse of the second laser sharply decreases the excitation density of the lower level 2 of the first laser. A pulse can be generated.

The Q switch element 36 can be installed between the high reflection mirror 34 and the optical fiber laser medium 31. The mirror 34 or 35 on the side where the Q switch element is inserted has a curvature suitable for the reflecting surface so that the reflected laser beam returns to the core of the fiber.

[Embodiment 3] FIG. 5 shows still another embodiment of the present invention. In the first embodiment, the reflection mirror is provided separately from the optical fiber. However, the reflection mirror can be formed directly on the optical fiber. The dielectric multilayer coat 37 having the same characteristics as the high reflection mirror 34 in the first embodiment is directly applied to the end face of the optical fiber laser medium 31 on the LD light incident side, and the partial reflection in the first embodiment is similarly applied to the other end face. By applying a dielectric multilayer coat 38 having the same reflectance as that of the output take-out mirror 35, a resonator can be formed.

[Embodiment 4] FIG. 6 shows another embodiment. In the laser device shown in the third embodiment, a reflection mirror is formed by gradient index fiber gratings 39 and 40 instead of the dielectric multilayer coat. The refractive index distribution fiber gratings 39 and 40 are written into the core 32 of the optical fiber by ion implantation or irradiation of ultraviolet light so as to have a reflectivity suitable for the first laser and the second laser. You don't have to be at both ends.

In the first to fourth embodiments, the reflection mirror for forming the optical fiber laser resonator has been described.
Laser resonators can also be configured by combining different types from among the mirrors, dielectric multilayer films, and gradient index fiber gratings of the first to fourth embodiments. For example, LD
The refractive index distribution fiber grating 39 is used as the high reflection mirror on the light incident side, and the partial reflection extraction mirror 35 can be used on the first laser and second laser extraction sides.

[Embodiment 5] FIG. 7 shows still another embodiment of the present invention. This is a multi-wavelength simultaneous oscillation optical fiber type laser using a double clad optical fiber instead of the single clad optical fiber shown in the first to fourth embodiments. The double clad optical fiber includes a core 32 doped with laser ions and a first clad 4 through which pumped LD light propagates.
2. It is composed of a second clad 43 that totally reflects the pump LD light into the fiber, excites laser ions added to the core 32 over a long fiber length, and the first clad 42 has a large sectional area and a large numerical aperture. There is an advantage that light can be more easily incident on the first cladding 42 than directly incident on the core. Utilizing the large numerical aperture of the second clad, LD light can be made to enter the first clad obliquely from the side of the optical fiber instead of the end face of the optical fiber.

[0028]

EXAMPLE Using the structure shown in the fifth embodiment, holmium ions (concentration: 0.25% by weight) were added to the core as laser ions, and the core diameter was 10 μm and the first cladding diameter was 250 μm.
μm, second cladding diameter 260 μm, length 200-100
An optical fiber of 0 cm was manufactured. By pumping with the LD light having an excitation wavelength of 890 nm or 1.1 μm, a first laser having a wavelength of 3 μm and a second laser having a wavelength of 2 μm could be obtained.

FIG. 8 shows the relationship between the continuous wave laser output power and the absorption LD power. The second laser started to oscillate with an increase in the absorption LD power due to the laser ions, and immediately or at the same time the target first laser started to oscillate, and the total output of both increased as the absorption LD power increased.

Although the configuration of the fifth embodiment is used here, similar results can be obtained by the configurations shown in the first to fourth embodiments.

Although holmium ions have been described as laser ions, other laser ions such as erbium, thulium and praseodymium may be used. In addition, an LD is usually used as excitation, but is not limited to this.

The glass material used for the optical fiber of the present invention is transparent to the excitation light and the oscillation laser light, has a low phonon energy inherent to the material, has a long excitation life for each level, and has a low A laser beam having such a property that the Stark spread of each energy level of a laser ion caused by a local electric field is wide is used.

Examples of such a material include fluoride glass, chalcogenite glass and quartz glass. Phonon energy is small and 2
Fluoride glass and chalcogenite glass are preferred because absorption at μm or more is small, and especially 2-3 μm
Fluoride glass with small propagation loss in the region is preferred.

The fluoride glass may be zirconium fluoride,
A glass containing several kinds of fluorides such as barium fluoride, lanthanum fluoride, aluminum fluoride, sodium fluoride, and hafnium fluoride, and preferred are zirconium fluoride, barium fluoride, lanthanum fluoride, and aluminum fluoride. And ZBLAN glass containing sodium fluoride as a component and AZF glass containing zirconium fluoride and aluminum fluoride as components.

In the above description, the two-wavelength oscillation has been described as an example. However, when the lower level of the second laser transition is not the ground level, the laser transition to a lower level is performed. Oscillation over the wavelength is also possible.

[0036]

According to the present invention, when the laser transition is of a self-terminating type, continuous oscillation and high output thereof or high output in the pulse mode can be achieved at the same time.
By extracting useful second laser light, a laser capable of emitting light of multiple wavelengths can be provided.

It is another object of the present invention to provide a laser capable of reducing the thermal distortion of the solid-state laser medium by extracting the energy which has been unavoidably converted into heat in the solid-state laser medium by non-radiative relaxation as a laser, and performing stable operation. Can be.

[Brief description of the drawings]

FIG. 1 is a flow chart of the excitation energy of the present invention.

FIG. 2 is a flow chart of excitation energy in a conventional example.

FIG. 3 shows an embodiment of the present invention.

FIG. 4 shows an embodiment of the present invention.

FIG. 5 shows an embodiment of the present invention.

FIG. 6 shows an embodiment of the present invention.

FIG. 7 shows an embodiment of the present invention.

FIG. 8 shows a two-wavelength laser output and absorption L when the multi-wavelength simultaneous oscillation optical fiber type laser of the present invention is pumped using an LD.
It is a figure showing the relation of D power.

[Explanation of symbols]

1-4 Energy level 5, 6 Excitation by LD light 7 Non-radiative relaxation transition 8 First laser transition 9 Second laser transition 12 Laser transition 13-18 Energy level of laser ion 19 Energy level of sub-addition ion 11 20 Excitation by LD light 21 Excitation level absorption 23, 24, 25, 26 Non-radiative relaxation transition 27 Energy transfer 28 LD light 29 First laser light 30 Second laser light 31 Laser ion-doped single clad fiber 32 Core 33 Clad 34 No. High-reflection mirror of first laser and second laser 35 Output mirror of first laser and second laser 36 Q-switch element 37 Dielectric multilayer coat 38 Dielectric multilayer coat 39 Refractive index distributed fiber grating 40 Refractive index distributed fiber grating 41 Laser Ion-doped double clad phi 42 first cladding 43 second cladding

Claims (10)

(57) [Claims] An optical fiber in which laser ions are added to a glass material is used to generate a first laser transition from a lower level of the first laser transition to a lower energy level. A multi-wavelength simultaneous-oscillation optical fiber type laser that outputs at least two wavelengths by forming at least one pair of resonators satisfying a resonance condition with respect to the wavelength of the second laser transition. 2. The first laser transition is a self-terminating transition in which the lifetime of a lower level is longer than the lifetime of an upper level.
The multi-wavelength simultaneous oscillation optical fiber type laser according to the above. 3. The multi-wavelength simultaneous oscillation optical fiber laser according to claim 1, wherein a lower level of the second laser is a ground level. 4. Simultaneously oscillating at both ends of the optical fiber
The multi-wavelength simultaneous oscillation optical fiber type laser according to any one of claims 1 to 3, wherein a resonator is provided by providing a mirror having an optimum reflectance for the laser wavelength and the excitation wavelength. 5. The multi-wavelength simultaneous oscillation optical fiber type laser according to claim 4, further comprising a Q-switch element between said optical fiber and a mirror. 6. The resonator according to claim 1, wherein a dielectric multilayer coat having optimum reflectance for two or more laser wavelengths and excitation wavelengths simultaneously oscillated at both ends of the optical fiber is provided to form the resonator.
4. The multi-wavelength simultaneous oscillation optical fiber laser according to any one of 3. 7. A resonator according to claim 1, wherein a periodic refractive index distribution fiber grating having an optimum reflectivity for two or more laser wavelengths and pump wavelengths oscillating simultaneously in the optical fiber is formed. The multi-wavelength simultaneous oscillation optical fiber type laser according to any one of the above. 8. The optical fiber according to claim 4, wherein the optical fiber is a single clad type fiber having a core doped with laser ions.
The multi-wavelength simultaneous oscillation optical fiber type laser according to any one of the above. 9. The optical fiber according to claim 4, wherein said optical fiber is a double clad optical fiber having a core doped with laser ions.
The multi-wavelength simultaneous oscillation optical fiber type laser according to any one of the above. 10. The laser according to claim 1, wherein the laser ions are holmium ions, are excited by light having an excitation wavelength of 890 nm or 1.1 μm, and oscillate at two wavelengths of 3 μm and 2 μm. 2. A multi-wavelength simultaneous oscillation optical fiber type laser according to item 1.