WO2008025076A1

WO2008025076A1 - An optical amplifier, a laser and methods of manufacture thereof

Info

Publication number: WO2008025076A1
Application number: PCT/AU2007/001250
Authority: WO
Inventors: Graham David Marshall; Martin Ams; Peter Dekker; James Austin Piper; Michael John Withford
Original assignee: Macquarie University
Priority date: 2006-08-29
Filing date: 2007-08-29
Publication date: 2008-03-06

Abstract

An optical waveguide device (2000) comprising at least one optical waveguide (2002) formed within a bulk optical material (2001), the waveguide having a central longitudinal axis (2004), and a grating formation (2005) within the bulk optical material (2001) co-axial with the axis of at least one optical waveguide (2004). The device (2000) may be used, for example, as an amplifier or laser when combined with a pump means.

Description

AN OPTICAL AMPLIFIER, A LASER AND METHODS OF MANUFACTURE THEREOF

TECHNICAL FIELD

[ 0001 ] The present invention relates to devices for optical communication systems and in 5 particular to an optical amplifier, a laser and methods of manufacture thereof.

[ 0002 ] The invention has been developed primarily for use as a waveguide optical amplifier and/or waveguide laser and methods of manufacture thereof and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use. i o BACKGROUND OF THE INVENTION

[ 0003 ] Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.

[ 0004 ] The recent explosion in the use of the Internet has demanded vastly higher bandwidth 15 performance in short- and medium-distance applications. Optical communication systems based on glass optical fibers (GOF) allow communication signals to be transmitted not only over long distances with low attenuation, but also at extremely high data rates, or bandwidth capacity. This capability arises from the propagation of a single optical signal mode in the low-loss windows of glass located at the near-infrared wavelengths of 850, 1310, and 1550 nm. 0 [ 0005 ] In an optical waveguide, a higher refractive index core surrounded by a lower refractive index cladding can transmit a large amount of optical information over long distances with little signal attenuation.

[ 0006 ] However, as the optical network nears the end user at the LAN stage, the network is characterized by numerous splittings of the input signal into many channels. This feature5 represents a fundamental problem for optical networks. Each time the input signal is split, the signal strength per channel is reduced. As optical telecommunication networks push further and further toward the end user, there is thus an ever growing need for compact and low cost optical amplification devices. [ 0007 ] An optical amplifier amplifies an optical signal directly in the optical domain without converting the signal into an electrical signal and reconverting the electrical signal back to an optical signal.

[ 000 S ] Rare earth doped optical amplifiers are emerging as the predominant optical signal amplification device for every aspect of optical communication networks spanning from repeaters, pre-amplifiers, signal conditioners and power boosters to in-line amplifiers for wavelength division multiplexed (WDM) systems.

[ 0009 ] Waveguides can be written inside bulk glass by using a tightly focused beam of a femtosecond laser having sufficient intensity to cause the glass to undergo various processes which result in permanent Small increases in the refractive index of the glass. The degree to which this happens depends on the pulse energy and scan rate of the laser beam.

[ 0010 ] U.S. Pat. No. 5,039,191 (Blonder et al.), U.S. Pat. No. 6,043,929 (Delavaux et al.), U.S. Pat. No. 5,119,460 (Bruce et al.), PCT Publication WO 00/05788 (Lawrence et al.), and J. Schmulovish, A. Wong, Y. H. Wong, P. C Becker, A. J. Bruce, R. Adar "Er Glass Waveguide Amplifier at 1.55 μin on Silicon," Electron. Lett., Vol. 28, pp.1181-1182, 1992 all disclose straight line waveguides.

[ 0011 ] US patent No 6,650,818 discloses an optical channel waveguide and an optical amplifier. The amplifier includes a substrate and an optical waveguide channel disposed in or on the substrate. The optical waveguide channel includes a first generally spiialing portion having a first ftee end and a first connected end, a second generally spiraling portion having a second free end and a second connected end, and a transition portion. The transition portion has a first transition section connected to the first connected end, a second transition section connected to the second connected end, and an inflection between the first and second transition sections. An amplifier assembly incorporating the channel waveguide and a method of amplifying a light signal are also disclosed.

[ 0012 ] US Patent No 6,661,567 discloses an optical amplifier comprising a slab defining at least one optical waveguide extending across the slab through a doped region of the slab. The slab is at least partially transparent and configured to distribute, through the slab, pump light incident on a side wall of the slab. This pump light irradiates the optical waveguide over its length. The slab is made of borosilicate, sulfide or lead glass. The optical waveguide is formed in the slab by using a pulsed femtosecond laser beam which is focused within the slab while the focus is translated relative to the substrate along a scan path, at a scan speed effective to induce an increase in the refractive index of the material along the scan path. Substantially no laser induced physical damage of the material is incurred along the scan path. M. Ohashi and K. Shiraki, "Bending Loss Effect on Signal Gain in an Er Doped Fiber Amplifier," IEEE Photon. Technol. Lett., Vol. 4, pp.192-194, 1992 disclose a curved zigzag shaped channel waveguide to increase the channel length. However, this approach creates the problem of high bending losses at turning regions in the curved waveguide. The bending radius is Rbemuπg ~ (D /2Jn) x Rsubstme where n is the number of channel waveguide curve turning regions. Due to the high bending curvature, or small bending radius, the bending loss of such waveguide is extremely high, resulting in low signal gain and limited usable waveguide channel length,

[ 0013 ] In addition, femtosecond laser pulses have been used for inscribing optical components inside transparent materials. In particular, it has been demonstrated that femtosecond Ti: Sapphire laser pulses can induce a change in the refractive index inside bulk glasses without damage by K. M. Davis, et al., Opt. Lett 21, 1729-1731 (1996). Using this mechanism, optical waveguides have been written in various bulk media.

[ 0014 ] K. Hirao and K. Miura, J. Non-Crystl. Sol 235, pp. 31-35, 1998 disclose a "direct- write" laser method of forming optical waveguides within a glass volume that is transparent to the wavelength of a femtosecond laser. In this method, a 120 fs pulsed 810-nm laser is focused within a polished piece of germanium-doped silica as the glass is translated perpendicular to the incident beam through the focus. Increases in refractive index of the order of 10^"2 were reported for a specific condition in which the focus was scanned ten times over the exposed area.

[ 0015 ] Minoshima, K, et al: Fabrication Of Coupled Mode Photonic Devices In Glass By Nonlinear Femtosecond Laser Materials Processing, Optics Express 645, 29 July 2002, describe the fabrication of coupled mode devices in transparent glasses by nonlinear materials processing with femtosecond laser pulses. In another article by Minoshima, K, et al: Photonic Device Fabrication In Glass By Use Of Nonlinear Materials Processing With A Femtosecond Laser Oscillator, Optics Letters, Vol. 26, Issue 19, pp. 1516-1518 October 2001, three dimensional configurations of waveguides are disclosed. These articles disclose the coupling of two or more signal waveguides. However, the aforementioned publications describe the use of passive waveguides to process a signal by coupling from one waveguide to another. As a result, it is of the utmost importance for this coupler to preserve the mode of the signal during the transfer. For telecomraunications purposes, the mode is typically a single mode (TEMoo)- For this type of coupler, the direction of guiding also has to be preserved. Furthermore, this coupler has a length dependent interaction zone. If this zone is too short and the coupling ratio is overly small, the coupling is inefficient. If it is too long, the signal couples back to the original waveguide. [ 0016 ] One potential problem with a direct write process of forming waveguides in bulk glass using short-pulse focused lasers is that of over-exposure. Irradiation with too much energy can lead to physical damage in the glass. Physical damage results in undesired attenuation of optical signals transmitted through the glass.

[ 0017 ] Another problem in direct write methods of making optical structures relates to the trade-off between the stability of the writing device, e.g., the laser, and the energy necessary to induce the desired refractive index change in the substrate material.

[ 0018 ] There is thus a need for a practical direct write method of creating optical devices having a sufficiently increased refractive index at an acceptably high write rate. Such a method could be used to write continuous light-guiding waveguide patterns connecting any two points within a continuous block of a suitable material, or to make other optical devices such as Bragg gratings for wavelength division multiplexing or gain flattening.

[ 0019 ] Active optical waveguides may be compared to diode pumped lasers. Diode pumped lasers fall into two categories, namely end-pumped (coaxial/collocated) or side pumped lasers, each of which has advantages and disadvantages. For example, laser end pumping schemes offer longer interaction lengths or "mode overlap" between the pump beam and the gain region of the laser medium. Consequently, higher efficiencies are available for end pumped waveguide geometries relative to side pumped geometries. However, a disadvantage with end pumping geometries is that they are cumbersome, due to the additional requirements of couplers and isolators in micro-waveguide lasers. [ 0020 ] By contrast, conventional side pumping configurations are poorly suited to the extraction of high powers from compact, micro-waveguide lasers due to their lower efficiencies.

[ 0021 ] End pumping is thus used in the majority of cases where efficient laser sources (eg telecommunications) are required. Erbium doped fibre amplifiers, for example, can be pumped by light from diode lasers (98Onm) which are end coupled into a length of high gain erbium doped fibre using conventional couplers and isolators. The additional requirement of couplers and isolators contributes to the size and expense of these devices.

[ 0022 ] There is consequently a need for a method to improve the efficiency at which laser materials can be side pumped. [ 0023 ] There is also a need for a waveguide provided in a medium in which a large refractive index differential change can be provided, and a method of producing such a waveguide without physical damage to the medium.

[ 0024 ] Silicon (including silicon on insulator) and glass are the two main optical platforms used in the fabrication of planar photonic chips. Silicon is typically processed using a combination of lithography and etching methods to microstructure its surface. By comparison, glass is processed using ion exchange methods (where sodium atoms axe replace with silver) to create waveguides below the surface of glass substrates or, in some cases, chemical vapour deposition processing to once again produce surface features. Each technology has strengths and weaknesses. Silicon technology can readily accommodate a range of photonic components such as waveguides, splitters and gratings for wavelength selectivity, however, it cannot support gain so it does not lend itself to simple amplification or lasing schemes compatible with modem communication networks. Also, silicon waveguide fabrication techniques typically result in asymmetric waveguides which exhibit undesirable birefringence. Furthermore, the cost of silicon devices is dominated by the complex packaging requirements needed to make them efficient and stable, a cost that has slowed the uptake of this technology. By comparison, glass based optical chips offering amplification can be readily manufactured, however, the processing methodologies used here are poorly suited to the fabrication of wavelength control structures such as non-polarisation sensitive Bragg gratings, thus limiting their functionality. For example, waveguides fabricated using ion implantation techniques often require a Bragg grating structure to be formed on the surface of the glass sample, i.e. the grating structure is not co-located with the waveguide itself, which then results in a non-uniform interaction between the grating and the waveguide introducing undesirable birefringence into the device.

[ 0025 ] The present invention advantageously provides a new photonic microfabrication platform and devices that combine the strengths of both silicon and (ion exchanged) glass photonics. In particular, a photonic fabrication toolkit that can produce; low loss waveguides and guided wave components (ie. splitters etc); high, fidelity gratings enabling wavelength control; and compact waveguide amplifiers and laser devices.

SUMMARY OF THE INVENTION

[ 0026 ] The inventors have developed a state-of-the-art fabrication platform that enables the creation of novel photonic devices such as optical waveguides in a bulk optical medium for example a glass, polymer or crystal medium. The techniques disclosed herein have been developed to capitalise on a mechanism whereby intense, tightly focussed laser radiation can locally modify the refractive index inside glass samples. Sophisticated system engineering is applied to coordinate the operating parameters of an ultrashort pulsed laser and high precision motion stages. A unique feature of this platform is that it provides dual waveguide and grating writing capability, and ability to directly fabricate embedded devices. Examples of photonic devices fabricated with this platform include low loss waveguides and waveguide arrays in a range of passive and active glasses, the world's first directly written waveguide Bragg grating, world leading high power-fully integrated fibre lasers, two- and three- dimensional (2-D and 3-D) waveguide splitters and waveguide amplifiers. Some basic photonic integrated chips (PICs) have also been demonstrated with this technology.

[ 0027 ] According to a first aspect there is provided an optical device comprising: at least one optical waveguide formed within a bulk optical medium; and at least one grating formation within the bulk substrate co-axial with the at least one optical waveguide. [ 0028 ] The optical device may be a monolithic optical device. The bulk medium may be & bulk medium and may be either a glass, polymer, ceramic, polycrystalline or crystalline medium. The medium may or may not be a photosensitive material.

[ 0029 ] The optical device may be a passive optical device and the optical medium may be a bulk passive optical medium and may be for example fused silica, phosphate glass or borosilicate among others, as well a large range of polymer or crystalline medium as would be appreciated by the skilled addressee.

[ 0030 ] The optical device may be an active optical device and the optical medium may be a bulk active optical medium and may be for example Er doped phosphate glass, Er-Yb doped phosphate glass, Yb-doped silica or Tm-doped fluoride glass among others, as well a large range of glass, polymer or crystalline media doped with an active ion such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm)₃ samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho)₁ erbium (Er), thulium (Tm), ytterbium (Yb), titanium (Ti), cerium (Ce), or chromium (Cr) among others or any combinations of these as would be appreciated by the skilled addressee. In other arrangements, the active ion may be a Raman active ion and the optical device may be a Raman optical device. The Raman optical device may be fused to a laser device such that waveguides in the optical media are aligned, to form a Raman laser device.

[ 0031 ] The optical device may be a nonlinear optical device and the optical material may be a bulk nonlinear optical material and may be for example arsenic free chalcogenide, As-Se based Chalcogenide and Pd-doped silica among others. The optical device may be a passive nonlinear or an active nonlinear optical device.

[ 0032 ] The active optical device may be a waveguide amplifier device. The active device may be a waveguide laser device.

[ 0033 ] The grating may be a Bragg grating and may be a distributed feedback (DFB) grating, The grating may be a Bragg grating and may be a distributed Bragg reflector (DBR) grating. The grating may be directly coupled with the waveguide. The grating may be co-axial with the waveguide. The grating may be distributed throughout the length of the waveguide or alternatively may be positioned at desired locations along the length of the waveguide. There may be two, three, four five or more gratings associated with the waveguide. [ 0034 ] The grating may be configured for one or more resonance frequencies, for example the grating may have one, two, three, four, five or more resonance frequencies corresponding to desired conditions. The grating may possess variations in phase alone the length of the grating structure, for example the grating may possess a desired phase-change (e.g. π/2, π/4 or π/8 etc) at desired locations as required for various resonance characteristics thereof. [ 0035 ] The phase change may be implemented either directly during the writing process, or added afterwards via an externally disposed heating apparatus (for example a heating element such as a Peltier or similar device attached to a surface of the bulk device in disposed so as to interact with the grating in the embedded waveguide). The heating apparatus may be located with respect to the waveguide so as to modify the properties of either the refractive index of the waveguide, or the average refractive index of a grating formation in the optical device. The extemal heating device may modify the average refractive index of a grating formation in the waveguide to assist in the absorption of a pump beam in the waveguide by active ions in the waveguide, thereby to provide for a desired absorption of the pump beam along the length of the waveguide. [ 0036 ] Arrangments of the first aspect may be termed "an intra-active waveguide DFB laser in a bulk material", wherein the laser comprises:

1) Intra-active waveguide DFB: refers to the fact we have a co-axial grating and waveguide structure - note other DFB forms have the grating on a separate plane to the active (gain) region, and are reliant on evanescent coupling between the two, in the present arrangements, the grating and the waveguide may be directly coupled.

2) In bulk material: intra-active core lasers have been demonstrated by the fabrication method described herein, where a grating is written inside a bulk optical material (which also is taken to encompass planar optical materials alos)

[ 0037 ] A classification of "intra-active waveguide DFB laser" also serves as a useful umbrella term for more specific embodiments such as, among others: a) tunable DFB laser, for example based on the Nd : Er co-doped scheme b) multiple wavelength lasers / frequency combs by suitable configuration of the grating written in the device c) integrated waveguide lasers, for example integrated with 980nm rejection filters, 1 x 32 amplified and wavelength selective splitters among others d) waveguide lasers with novel pump geometries to engineer against photothermal instabilities.

[ 0038 ] According to a second aspect there is provided an optical waveguide device comprising a bulk optical medium including; a signal inlet and a signal outlet; a signal waveguide extending from the signal inlet through the optical medium to the signal outlet, said signal waveguide being capable of transmitting light from the signal inlet to the signal outlet; a pump inlet and a pump outlet; and a pump waveguide extending from the pump inlet through the optical medium to the pump outlet, said pump waveguide being capable of transmitting a pump beam from the pump inlet to the pump outlet; wherein the optical waveguide amplifier is adapted to cause at least a portion of the pump beam transmitted in the pump waveguide, in use, to leak from the pump waveguide and to interact with and amplify light in the signal waveguide

[ 0039 ] In one arrangement of the second aspect, the optical waveguide device may be an waveguide amplifier device wherein the signal waveguide is adapted to transmit a signal beam of light and the interaction of the pump beam with the signal waveguide causes an amplification of the signal beam.

[ 0040 ] In another arrangement of the second aspect, the optical waveguide may be a waveguide laser device wherein the signal waveguide comprises optical reflectors to form a laser resonator cavity, and the pump beam interacts with, the laser cavity to generate a laser beam in the laser resonator cavity. In one arrangement of the waveguide laser, the laser resonator cavity may comprise two optical reflectors which may be optical reflectors either adjacent to or co-axial with the signal inlet and the signal outlet In another arrangement of the waveguide lasei, the optical reflectors may be the interface with the signal waveguide between the bulk optical medium and another optical medium (for example free space (air) or a second optical medium such as a glass, crystal or polymer medium adjacent the optical medium comprising the signal waveguide).

[ 0041 ] The refractive index of the optical medium outside the pump waveguide at the wavelength of the pump beam may be different to the refractive index of the optical medium in the pump waveguide at the wavelength of the pump beam (i.e. may be different to the pump refractive index). The refractive index of the optical medium outside the signal waveguide at the wavelength of the light transmitted in the signal waveguide may be different to the refractive index of the optical medium in the signal waveguide at the wavelength of the light transmitted in the signal waveguide (i.e. may be different to the signal refractive index). The pump waveguide and the signal waveguide may, independently, be active or passive. [ 0042 ] To enhance or facilitate the leaking of light from the pump waveguide, or alternatively the coupling strength between the pump and signal waveguides (i.e.evanescent coupling for example) the pump waveguide may be bent or curved over at least a portion of its length, Alternatively or additionally, its internal surfaces may be roughened or rifled or treated in some other way to enhance the leaking of light therefrom. The bending of the pump waveguide with respect to the signal waveguide may be adapted to provide varying level of interaction or coupling between light in the pump waveguide with the signal waveguide. The varying level of coupliπg may be adapted to provide a desirable amount of coupling between the light in the pump waveguide with the signal waveguide along at least a portion of the length of the signal waveguide, The desirable coupling strength maybe an approximately equal amount of coupling (ie. a substantially constant coupling coefficient along a desired interaction length) along the length of the signal waveguide. The curved portion of the pump guide relative to the signal waveguide may be adapted to compensate for varying levels of absorption of the pump waveguide by active ions in an active signal waveguide to provide for an approximately equal level of pump light absorption in the signal waveguide along at least a portion of the length of the signal waveguide. [ 0043 ] To facilitate the extent to which light that has leaked from (or is evanescently coupled with) the pump waveguide can enter the signal waveguide and interact with light transmitted therein, at least a portion of the pump waveguide may extend for at least a portion of its length in close proximity to the signal waveguide. Alternatively or additionally, the pump waveguide may intersect with or be combined with or co-axial with the signal waveguide. The pump waveguide may be combined with or be co-axial with the signal waveguide along the entire length of the signal waveguide.

[ 0044 ] The pump and signal waveguides may have separate outlets, or they may have the same outlet. Thus, the signal and pump waveguides may start off as separate waveguides, whereafter they may be combined, along their lengths, to become co-axial. The pump and signal waveguides may be co-axial along their entire length in the optical medium.

[ 0045 ] The signal inlet and the signal outlet may be associated with a first and second extremity of the optical medium respectively. The pump inlet and pump outlet may be associated with a third and fourth extremity of the optical medium respectively, In order to facilitate connection of the pump and signal waveguides to their respective optical circuits, the signal inlet and the pump inlet may be spaced from one another. For instance, in an arrangement in which the optical medium is in the form of a cube or block having surfaces that are substantially flat and that are disposed at right angles to one another, the signal inlet may be disposed in a first end surface of the cube or block whilst the pump inlet may be disposed in a side surface of the block, one that forms a right angle with the first end surface. In this arrangement, the signal outlet and the pump outlet may both be provided in a second end surface of the block, with the signal waveguide being substantially straight and the pump waveguide being bent through an angle of about 90'.

[ 0046 ] The optical waveguide device conveniently may also include a grating or gratings associated with the signal and/or the pump waveguides for selecting and shaping the desired s wavelength and spectrum supported by the waveguide. The one or more gratings may form reflectors of the laser resonator cavity. The grating(s) may be Bragg grating(s) and may be distributed Bragg reflector (DBR) grating(s). The grating may be directly coupled with fhe waveguide. The grating may be co-located with the waveguide. The grating may be distributed throughout the length of the waveguide or alternatively may be positioned at desired locations

JO along the length of the waveguide. There may be two, three, four five or more gratings associated with the waveguide.

[ 0047 ] In this and other arrangements in which the pump waveguide is bent, it may be located in the same place as the signal waveguide.

[ 0048 ] According to a third aspect there is provided a waveguide amplifier comprising an ] 5 optical medium including: a signal inlet and a signal outlet; a signal waveguide extending from the signal inlet through the optical medium to the signal outlet, said signal waveguide being capable of transmitting light from the signal inlet to the signal outlet; a pump inlet and a pump outlet; and a pump waveguide extending from the pump inlet through the optical medium to the pump outlet, said pump waveguide being capable of transmitting a pump beam from the pump 20 inlet to the pump outlet; wherein the pump waveguide intersects with the signal waveguide, causing the pump beam leaking from the pump waveguide, in use, to interact with and amplify the light transmitted in the signal waveguide.

[ 0049 ] The waveguide amplifier conveniently may also include a grating or gratings for selecting and shaping the desired wavelength and spectrum in a resonator cavity. The waveguide5 amplifier may be a waveguide laser.

[ 0050 ] According to a fourth aspect there is provided a waveguide amplifier comprising an optical medium including: a signal inlet and a signal outlet; a signal waveguide extending from the signal inlet through the optical medium to the signal outlet, said signal waveguide being capable of transmitting light from the signal inlet to the signal outlet; a first pump inlet and a first0 pump outlet; a first pump waveguide extending from the first pump inlet through the optical medium to the first pump outlet, said first pump waveguide being capable of transmitting a first pump beam from the pump inlet to the pump outlet; a second pump inlet and a second pump outlet; and a second pump waveguide extending from the second pump inlet through the optical medium to the second pump outlet, said second pump waveguide being capable of transmitting a second pump beam from the pump inlet to the pump outlet, whereby light leaking from at least one of the first pump waveguide and the second pump waveguide, in use, couples with and amplifies the light transmitted in the signal waveguide.

[ 0051 ] The first pump waveguide may be located in a first plane that intersects with or accommodates the signal passage. The first pump waveguide may be curved, in said first plane, in a region proximal to at least a portion of the signal passage, The second pump waveguide may be located in a second plane that intersects with or accommodates the signal passage. The second pump waveguide may also be curved, in said second plane, in a region proximal to at least a portion of the signal passage. The refractive index of the optical medium outside the first pump waveguide at the wavelength of the first pump beam may be different to the refractive index of the optical medium in the first pump waveguide at the wavelength of the first pump beam (i.e. to the first pump refractive index). The refractive index of the optical medium outside the second pump waveguide at the wavelength of the second pump beam may be different to the refractive index of the optical medium in the second pump waveguide at the wavelength of the second pump beam (i.e. to the second pump refractive index). The refractive index of the optical medium outside the signal waveguide at the wavelength of the light transmitted in the signal waveguide may be different to the refractive index of the optical medium in the signal waveguide at the wavelength of the light transmitted in the signal waveguide. The first pump refractive index and the second pump refractive index may be the same or may be different.

[ 0052 ] According to a fifth aspect there is provided a waveguide amplifier comprising a signal waveguide and at least partially curved pump waveguide located in an optical medium, wherein the curvature of the pump waveguide and the proximity of the pump waveguide to the signal waveguide are selected such that, in use, signal light transmitted in the signal waveguide is side-pumped by pump light leaking or evanescently coupled from pump light transmitted in the pump -waveguide.

[ 0053 ] The signal waveguide may be provided with a pair of longitudinally spaced reflectors defining a resonator cavity in the signal waveguide and a gain medium positioned in the resonator cavity, so that the gain medium may be pumped by one or more pump beams, to emit a stimulated signal beam. Thus the waveguide amplifier of the invention may constitute a laser beam generator or a portion thereof. The longitudinally spaced reflectors may be one or more grating formations in the signal waveguide. The grating formations may be Bragg grating formations. The grating formations may comprise a. region of alternating sub-regions of different refractive index along a portion of the signal waveguide. The grating formations may be a single grating formation at either a distal or proximal end of the signal waveguide. The longitudinally spaced reflectors may comprise a single grating formation at either a distal or proximal end of the signal waveguide and a reflector comprising an end facet of the signal waveguide at an interface at either the proximal or distal end respectively of the signal waveguide in the optical material. The single grating formation may be formed along substantially the entire length of the signal waveguide. The average refractive index of the one or more grating formations may vary in different locations along the signal waveguide.

[ 0054 ] The waveguide amplifier conveniently may also include a grating or gratings for selecting and shaping the desired wavelength and spectrum in a resonator cavity. The waveguide amplifier may be a waveguide laser,

[ 0055 ] The waveguide amplifier may further comprise a light emitting diode for generating the pump light to be transmitted in the pump waveguide. The diode may be provided in the pump waveguide or as a separate unit.

[ 0056 ] The gain of the signal beam transmitted through the signal waveguide of a waveguide amplifier device fabricated according to processes disclosed herein may exceed the loss of light form the signal beam to the environment. The gain of the signal beam may exceed about 1 dB, and may exceed about 2, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 20OdB. The gain may be between about 1 and about 20OdB, or between about 1 and 100, 1 and 50, 1 and 20, 1 and 10, 1 and 5, 1 and 2, 10 and 200, 50 and 200, 100 and 200, 10 and 200, 10 and 100, 10 and 50, 50 and 150, 50 and 100 or 100 and 15OdB, e.g. about I₁ 2, 3, 4, 5, 10, 15, 2O₃ 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100,110, 120, 130, 140, 150, 160, 170, 180, 190 or 20OdB. The gain may be between about 0.1 and IOdB/cm, or between about 0.1 and 5, 0.1 and 2, 0.1 and 1, 0.1 and 0.5, 0.5 and 10, 1 and 10, 2 and 10, 5 and 10, 0.5 and 5, 1 and 5, 2 and 5, 1 and 3 or 1 and 2, for example about 0.1, 0.2, 0.3, 0.4, 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, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9. 9,5 or IOdB/cm. [ 0057 ] In other arrangements, the pump and signal waveguides may be aligned to be co- directional over a desired length. It is to be noted that they are not necessarily co-directional at the input face. The waveguides are typically co-axialcoplanar (i.e. where the pump and the signal waveguides do not overlap in the optical medium however are located within a particular plane of the medium) over at least a portion of the lengths in the optical medium. They may also be coaxial (i.e. where the pump and the signal waveguides overlap in the optical medium with a common axis) over at least a portion of the lengths in the optical medium, In other arrangements there may be a plurality of pump waveguides associates with one or more signal waveguides, and the signal and pump waveguides may be non-co-planar. For example in one example arrangement there may be a single waveguide surrounded by a ring of pump waveguides, where there may be 2, 3, 4, 5, 6, 7, 8, 10, 12, 16, 32, 64 or more pump waveguides surrounding the signal waveguide. The pump and signal waveguides may have different diameters.

[ 0058 ] Where the pump and signal waveguides are coplanar, they may be in close proximity with each other so that the propagating pump mode overlaps with the signal waveguide, causing them to be coupled for example via evanescent coupling.

[ 0059 ] According to an sixth aspect of the invention, there is provided a method of amplifying an optica) signal, including the steps of: transmitting the optical signal through a signal waveguide provided in an optical waveguide amplifier comprising an optical medium, the signal waveguide extending from a signal inlet, through the optical medium, to a signal outlet; transmitting pump light through a pump waveguide, said pump waveguide extending from a pump inlet through the optical medium to a pump outlet; and causing at least a portion of the pump light transmitted in the pump waveguide, in use, to leak from the pump waveguide and to couple with and amplify signal light transmitted in the signal waveguide.

[ 0060 ] The pump waveguide may have a pump refractive index for the pump light that differs from a refractive index for said pump light in the optical medium outside the pump waveguide. The signal waveguide may have a signal refractive index for said signal light that differs from a refractive index for said signal light in the optical medium outside the signal waveguide.

[ 0061 ] In an arrangement there is provided a method of amplifying an optical signal, including the steps of: providing a waveguide amplifier according to the present invention; transmitting the optical signal through the signal waveguide of the waveguide amplifier to the signal outlet; transmitting pump light through the pump waveguide of the waveguide amplifier to the pump outlet; and causing at least a portion of the pump light transmitted in the pump waveguide, in use, to leak from the pump waveguide and to couple with and amplify signal light transmitted in the signal waveguide.

[ 0062 ] The present invention also encompasses the use of a waveguide amplifier according to the invention for amplifying an optical signal in the signal waveguide.

[ 0063 ] According to a seventh aspect there is provided a system for fabrication of an optical device comprising: a pulsed laser source; a pulse control means for modifying the pulses from the laser source; a pulse focussing system for directing pulses from the laser source to a fabrication location in space; a sample stage for holding and positioning a sample in which the optical device is to be fabricated; and a stage controller means for controlling the location of the sample stage with respect to the fabrication location.

[ 0064 ] The system may further comprise a synchronisation means for, in operation, synchronising the stage controller means and pulse control means positioning of the sample stage with pulses from the laser source. [ 0065 ] The pulse control means may control the repetition rate and/or the pulse energy of pulses from the laser source. The pulse control means may enable operation of the laser source in burst mode. The sample stage may be a translation stage and the stage controller may provide translation and positioning in two- and/or three-dimensions. The pulse focussing system may comprise a slit for formation of an elliptical focal spot size of pulses from the laser source at the fabrication location.

[ 0066 ] The system may be adaptable for fabrication of an optical device comprising a at least one optical waveguide formed within a bulk optical material; and at least one grating formation within the bulk substrate coaxial with the at least one optical waveguide. The system may be adaptable for fabrication of the optical waveguide and the grating simultaneously. [ 0067 ] According to a eighth aspect there is provided a method of forming an optical device using the system of the seventh aspect comprising; placing a bulk optical material within which the device is to be fabricated onto the sample stage; configuring the pulses of the pulsed laser system and the positioning of the sample stage for a desired waveguide device structure; providing a pulsed laser signal to the optical material at the fabrication location; and translating the sample on the sample stage in accordance with requirements, wherein the position of the sample stage and the pulsed laser signal are synchronised wherein the laser pulses modify the refractive index of the optical material in the fabrication location to form the optical device.

[ 0068 ] The optical device may be a bulk or monolithic optical device. The method may be configured to form a waveguide and a grating in the optical material in a single pass. The energy and/or the pulse repetition rate of the laser pulses may be periodically modified during fabrication of the optical device to provide a periodically varying refractive index modification in the optical material in accordance with requirements. The pulsed laser source may be operated in burst mode wherein, as the sample is translated with respect to the fabrication location during formation of the optical device, different locations within the optical material are exposed to different number of pulses from the laser source, thereby resulting in a difference in the refractive index modification of the difference locations within the optical material to form both the waveguide and the grating in a single pass. The pulse laser signal may be a circularly polarised pulsed laser signal. In other arrangements the waveguide and the grating may be written in more than one pass. [ 0069 ] The method may comprise fabrication of an optical device comprising at least one optical waveguide formed within a bulk optical material; and at least one grating formation within the bulk substrate co-axial with the at least one optical waveguide, The method may comprise fabrication of the optical waveguide and the grating simultaneously.

[ 0070 ] The power of the laser source at the focus of the laser beam in the medium may be sufficient to change the refractive index of the medium by at least IxIO^"4, or by between about IxIO^ and IxIO^"1, IxIO^"4 and IxIO^ , IxIO^"4 and IxIO^"3 or lxlO^"3 and IxIO^'1, e.g. about IxIO^"4, IxIO^*3, 5XlO^'3, IxIO^*2, 5xlO^'2 or IxIO^'1. The power of the laser may be insufficient to cause damage (including for example optical damage) in the medium.

( 0071 ] The speed at which the laser beam is moved through the optical medium may be adjustable. Where the optical medium is glass, the speed at which the focus of the laser beam is translated within the optical medium may vary from about lOμm to about 100mm per second. The speed may depend on the characteristics of the laser beam and the particular optical medium in which the device is to be fabricated, and may for example between about 10 microns/second and about lmm/second or about 10 and 500, 10 and 100, 10 and 50, 100 and 500 or 200 and 800 microns/second or between about 1 and 10, 2 and 10, 5 and 10 or 1 and 2 mm/sec, or between about 100 microns/second and lmm/sec or between about 500 microns/sec and Imm/sec, for example about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 20O₃ 250, 300, 350, 400, 450, 500, 600, 700, 800. or 900 microns/second or about 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 mm/second. In some arrangements, the sample may be translated with a constant velocity (or speed) during me fabrication process. In other arrangements, the speed that the sample is translated my vary when

, s writing different portions of the optical device. For example in one arrangement of a waveguide splitter device fabricated with the present system, each waveguide arm of the splitter may be written in a separate pass of the laser beam, and where the arms of the splitter share a common waveguide portion (i.e. a single input waveguide portion), the sample may be translated at a greater speed than in the separated portions of the waveguide arms so that the refractive index io change in the optical medium in the single waveguide portion is built up in each pass. Writing the single input portion at a greater speed also minimises the possibility of void formation in the portion which is written with the laser beam numerous times.

[ 0072 ] The shape of the focus of the laser beam may be circular or elliptical in perpendicular cross-section and may be selected in accordance with the motion of the sample transition state is thereby to fabricate a substantially circularly symmetric waveguide, as is described in greater detail below. Alternatively, it may be shaped according to the modes that are present. In the event that it is elliptical in cross-section, more light can leak out than, for instance, when it is circular in cross-section. In an arrangement where the pump and signal waveguides are not coaxial, it may be advantageous for the pump waveguide to have an elliptical cross-section as the

20 increased leakage from the pump waveguide could be utilized to provide increased interaction of the pump radiation with the signal radiation in the signal waveguide. The shape of the focus of the writing laser beam may be adjustable. The shape of the focus of the writing laser beam may be variable from a circular cross-section to an elliptical cross-section.

[ 0073 ] The pulse energy of the writing laser may also be adjustable. The pulse energy may 25 typically be variable from about lOOnJ to about lOμJ depending on a number of factors for example the particular device being fabricated, the optical medium in which the device is to be fabricated and the beam quality of the writing laser beam. For example in one arrangement, the pulse energy may be about 10 to 30, 10 to 50, 50 to 10, 30 to 10 or 30 to 8OnJ, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90 or 10OnJ₃ although other energies may be used. In other arrangements, 30 the pulse energy may be in the range 0.1 to 10 μj, or 0.1 to 5 0.1 to 4, 0.1 to 3, 0.1 to 2, 0.1 to 1 , 1 to 10, 1 to 5, 1 to 4, 1 to 3, 1 to 2 μj and may be approximately 0.1, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, or 10 μJ.

[ 0074 ] The pulse repetition rate of the laser writing beam may be in the Hz, the kHz regime, the MHz regime, or the GHz regime. For example the pulse repetition rate may be in the range of 1 to 100 kHz, or it may be inthe range of O.l to lOO MHz. It will be appreciated that a writing beam operating in the MHz regime would be able to fabricate a waveguide device as described herein many orders of magnitude faster than a writing beam operating in the kHz regime. Similarly a laser writing beam operating at GHz pulse repetition rates would be able to fabricate a waveguide device as described herein many orders of magnitude faster than a writing beam operating operating at MHz repetition rates.

[ 0075 ] The polarization and polarization direction of the incident laser beam, used to inscribe the waveguide may be controlled, For linearly polarized light the direction of polarization may be maintained either perpendicular or parallel to the optic axis of the waveguide by rotating the target sample or rotating the polarization direction. Alternatively, the incident laser beam may be circularly polarized oτ may be converted to circularly polarized light to provide this level of control. Circularly polarized light also offers other advantages for the fabrication process such as lower propagation losses and elevating the energy threshold for void formation.

[ 0076 ] The pathway described by the laser is preferably adjustable so as to be able to form the various shapes and configurations mentioned above. To achieve this, the optical medium or laser or both may be translated using a computer controlled motorised device that is capable of moving the optical medium in one, two or three orthogonal dimensions (along x, y and/or z axes) or in rotational directions or in both, whilst maintaining a fixed laser focal spot inside the optical medium.

[ 0077 ] The laser system may be a femtosecond mode locked oscillator or a regeneratively amplified femtosecond laser. The power of the pulses of a pulsed laser beam may be in the range of 0.1 to 4 mJ, although the damage threshold of the medium will vary depending on a large number of factors for example the type of medium and the beam quality of the laser beam, and the power of the laser beam may be adjusted accordingly taking these factors into account as would be readily appreciated by the skilled addressee. [ 0078 ] The laser focus may be computer controlled. The computer may be programmed to control movement of the focus in a x-direction and in a y-direction, It is optionally programmed to also control movement of the focus in a z-direction.

[ 0079 ] The femtosecond laser system may be capable of generating a train of optical pulses with temporal widths of 10-500 fs or more, for example about 10 to 400, 10-300, 10-200, 10- 100. 10-50, 50-500, 100-500, 50-200 or 50-lOOfs, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450 or 500fs,.

[ 0080 ] The repetition rate and the energy per pulse of the femtosecond laser may vary depending on the particular system used. [ 0081 ] The femtosecond laser system may include an oscillator (a system that delivers energy) from which the pulse train may be developed. The maximum energy per pulse may be about 1 nJ to 10 μJ (e.g. about 1-50 nJ, 1-25, 1-10, 5-15, 25-100, 50-100, 20-80, or 30-70 rJ, for example in one arrangement the pulse energy may be about I₃ 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 nJ, or in another arrangement the pulse energy maybe about 0,1 to 10 μJ, 0.1 to 5 0.1 to 4, 0.1 to 3, 0.1 to 2, 0.1 to 1, 1 to 10, 1 to 5, 1 to 4, 1 to 3, 1 to 2 μJ and may be about 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5. 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, or 10 μJ.

[ 0082 ] If more energy is needed, an amplifier section may be added. In this way, one may achieve mJ levels of energy albeit at the expense of the repetition rate. [ 0083 ] The repetition rate of the pulses, which may be related to the round trip time in the cavity, may be very high. Depending on the specific amplification scheme and ultimate pulse width, the repetition rate may range from 1 kHz to about 100 MHz, or about 1 to 500, 1 to 200, 1 to 100, 1 to 50, 1 to 20, 1 to 10, 1 to 5, 1 to 500, 50 to 500 or 100 to 500kHz, or about 1 to 100, 10 to 100, 50 to 100 or 10 to 50MHz, or about 10kHz to 100MHz. lOOkHz to 100MHz, 500kHz to 100MHz, 500kHz to 50MHz, 50OkHz to 10kHz or 50OkHz to 1MH2, for example aboutl, 2, 3, 4, 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, 600, 700, 800 or 90OkHz or 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, 5O₃ 60, 70, 80, 90 or 100MHz. A range of from about 1 kHz to about 250 kHz can work well. [ 0084 ] The laser may be directly controlled by the motion control system and may adjust position (in the X. Y and Z directions) and other laser beam parameters such as power and focus size, shape and rotation. The laser pulse rate and timing may be controlled by the motion control system to facilitate the writing of Bragg or long-period grating structures with a fixed or varying pitch (e.g. chirp) and fixed or varying refractive index change (e.g. apodization). The motion control system may provide synchronisation of the laser pulses of the laser beam deposited in the optical medium with the location of the medium on the translation stage to minimise the positional error of the deposited pulses with respect tot the waveguide being fabricated. The positional error of the pulses may be controlled to be within the range of approximately 0 to 12%_: or may be in the range of 0 to 11, 0 to 10, 0 to 9, 0 to 8, 0 to 7, 0 to 6, 0 to 5, 0 to 4, 0 to 3, 0 to 2, 0 to 1%, and may be typically in the range of approximately 3 to 7 % or 4 to 6%, and may be approximately 0, 0.1, 1, 2, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 75, 8.9, 10, 11, or 12%. The positional error may relate to regions of different refractive index in the optical medium comprising a grating formation, which may be a Bragg grating formation, wherein the position error of the different refractive index locations (or the central point of the different refractive index locations) with respect to the Bragg condition (described below), and particularly with respect to the grating period Λ of the Bragg condition, is in the range of approximately 0 to 12%.

[ 0085 ] The advantage of this technique is that the laser pulses can be directly synchronized to a physical position of the sample during the waveguide writing process or applied to the waveguide region before or after the waveguide writing process.

[ 0086 J Dimensions of the optical device

[ 0087 ] The optical medium may be dimensioned so that it has the approximate size of a microchip.

[ 0088 ] The optical medium may be in the form of a slab or other bulk material. The slab may be substantially square or rectangular and it may have sides that are from about 0.5 to about 10 cm, or about 1 to 10, 2 to 10, 5 to 10, 0.5 to 5, 0.5 to 2 or 1 to 5cm, for example about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or 10cm, and may be about 1 cm long. Its surface area may be from about 0.1 cm²to about 10 cm², or from about 0.5 to 10, 1 to 10, 2 to 10, 5 to 10, 0.1 to 5, 0.1 to 2, 0.1 to 1, 0.1 to 0.5, 0.5 to 5, 1 to 5 or 0.5 to 2cm², for example about 0.1, 0.2, 0.3, 0.4, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or lOcm². [ 0089 ] The slab may be from about SO μm to about 5000 μm thick, oi about 50 to 1000, 50 to 2500, 50 to 4000, 100 to 2500, 500 to 1500, 750 to 1250, or 900 to 1100 μm, for example about 50, 75, 100, 200, 300, 400, 500, 600, 700, 800, 900, 950, 975, 1000, 1025, 1050, 110O₃ 1200, 1300, 140O₃ 1500, 1600, 1700, 1800, 1900, 2000, 2500, 3000, 3500, 4000, 4500, or 5000 μm thick.

[ 0090 ] The or each pump waveguide and or the or each signal waveguide may have a diameter of from about 0.1 μm to about 50μm, or between about 1 to 50, 5 to 50, 10 to 50, 20 to 50, 0.5 to 20, 0.5 to 10, 0,5 to 5, 0.5 to 2, 0.5 to 1, 1 to 20, 1 to 10 or 10 to 20 μm for example about 0.1, 0.2, 0,3, 0.4, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 910, 15, 20, 25, 30, 35, 40, 45 or 50 μm. In an example arrangement, the (each) pump waveguide diameter may be from about 3μm to about 5μm. The diameter may be for example between about 3 and 4, 4 and 5 or 3.5 and ^• 4.5 μm, and may be about 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9 or 5 μm. In one arrangement, the (each) diameter of the signal waveguide may be about 8μm to about lOμm. The refractive index of the pump waveguide may be smaller than that of the signal waveguide so that its mode field diameter of the pump waveguide is larger than that for the signal waveguide.

[ 0091 ] In an arrangement of the optical device comprising separated pump and signal waveguides, the separation between the pump and signal waveguides in the region in which they overlap may be from about 0.5μm to about 20μm, or about 1 to 20, 5 to 20, 10 to 20. 0.5 to 10, 0.5 to 2, 1 to 10, 1 to 5 or 5 to 10 μm, for example about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 1,5 16, 17, 18, 19 or 20 μm. The separation may be of the order of about a mode size, e.g. between about 0.5 and 2 mode sizes, or 0.5 and 1, 1 and 2 or 0.8 and 1.2 mode sizes.

[ 0092 ] The waveguides may be as long as is required to achieve a desired amplification of the signal beam. Because of their small diameters and because they may be wound up in the form of a coil (which also includes a coil that may have one or more linear sections) inside the optical medium, the coil (or bends in the coil between linear sections) having a diameter of from about 1 mm to about 20 mm, it is possible to fit a waveguide having a length of as long as a 1 meter or even several meters into the optical medium. The coil may have a diameter of between about 1 and 20mm, or between about 1 and 10, 1 and 5, 5 and 20, 10 and 20 or 5 and 10mm, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11₇ 12 ,13, 14, 15, 16, 17, 18, 19 or 20mm. The diameter of the coil (or the radius of curved portions of the \vaveguide(s) in the medium) may depend on the size of the optical medium. The waveguide length (pump and/or signal) may be between about 0.01 and 10m, or between about 0.01 and 5, 0.01 and 2. 0.01 and I₅ 0.01 and 0.5, 0.01 and 0.2, 0.01 and 0.1, 0.01 and 0.05, 0.02 and 0.1, 0.02 and 1, 0.5 and 10, 1 and 10, 5 and 10, 0.5 and 5, 0.5 and 2, s 0.5 and 1, 1 and 5, 1 and 2m, 0.1 and 5, 0.1 and 2, 0.1 and 1, 0.1 and 0.5, 0.5 and 10, 1 and 10, 5 and 10, 0.5 and 5, 0.5 and 2, 0.5 and l_s 1 and 5 or 1 and 2m for example about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 Or IOm.

[ 0093 ] In one arrangement, the waveguides are about 10 cm long. In other arrangements, theyo may be longer, depending on the application. They may for example be about 20, 30, 40, 50, 60, 70, 80 or 90 cm long, or about 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5m long. In other arrangements, the waveguide may be less than 10 cm long and the waveguide length may be in the range of 5 mm to 100mm, The waveguide length may be for example approximately 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 mm long. s [0094] Optical medium

[ 0095 ] The optical medium may be made of or comprise silicate, borosilicate, sulfide, fluoride, germanium, lead or phosphate glass. The glass may be a silicate, a borosilicate or a vitreous glass.

[ 0096 ] Alternatively, the optical medium may be a plastics material such as a suitable0 polymer. The polymer may be selected from the group consisting of linear polymers (including polyamide (PI), PET and PMMA), non-linear polymers (including polydiacetylenes and poly (p- phenylene vinylene) (PPV) derivatives), polymers with gain (including poly(para-phenylene) (PPP) and doped PMMA) and photosensitive polymers (including perfluorcyclobutyl (PFCB) and benzocyclobutene (BCB)). s [ 0097 ] The optical medium may have a refractive index between about 1.2 and 1.8, or between about 1.3 and 1.8, 1.4 and 1.8, 1.5 and 1.8, 1.6 and 1.8, 1.7 and 1.8, 1.2 and 1.7, 1.2 and 1.5, 1.3 andd 1.7, 1.31 and 1.65, 1.4 and 1.6, 1.5 and.l .7 or 1.3 and 1.5, for example about 1.2, 1.25, 1.3, 1.31, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75 or 1.8 or may have a refractive index greater than 1 , 8 or less than 1.2. Refractive indices for suitable polymers and other suitable0 materials may be found in "Handbook of Optical Materials," (Marvin J. Weber, CRC Press 2002, ISBN: 0849335124). [ 0098 ] The optical medium may be substantially transparent to the signal and pump light beams. Alternatively, it may be substantially opaque to the wavelengths of the signal and pump light beams whilst the materials) of the signal and pump waveguides is (are) substantially transparent to the signal and pump light beams respectively. [ 0099 ] The optical medium may be doped with a dopant (active ion) that becomes optically active upon being stimulated will, incident radiation. The dopant may be a metal or metals. The metal or metals may be selected from the group including Erbium, Neodymium, Ytterbium, Holmium, Praseodymium, Titanium, Cerium, Chromium, Thulium, Lanthanum, Promethium, Samarium, Europium, Gadolinium, Terbium, Dysprosium or any combination of these elements. [ 00100 ] The optical medium may comprise a glass doped with a rare earth ion, such as Nd^{3^}, Er³⁺ Or Yb³⁺.

[ 00101 ] The optical medium may be doped with the rare earth by means of any one of a variety of processes such as modified chemical vapor deposition, ion exchange, photolithography, flame-hydrolysis, reactive ion-etching, ion implantation etc. The dopant concentration may be from about 1 x 10²⁰ to about 5 x 10²⁰ ions per cubic centimeter (e.g. 1 x 10²⁰ to 4 x 10²⁰ , 1 x 10²⁰ to 3 x 10²⁰, 1 x 10^Mto 2 x 10²⁰, 2 x 10²⁰K) 5 x 10²⁰, 3 x 10²⁰to 5 x IO²⁰, 4x 10²⁰to 5 x 10²⁰, 2 x 10²⁰Io 4 x 10²⁰or 3 x IO²⁰ to 4 x 10²⁰ ions per cubic centimeter, for example 1 x IO²⁰, 1.5 x IO²⁰, 2 x IO²⁰, 2.5 x IO²⁰, 3 x 10²⁰, 3.5 x 10²⁰, 4 x IO²⁰, 4.5 x 10²⁰Or 5 x 10²⁰ ions per cubic centimeter), or from about 1 % by weight to about 5% by weight, e.g. 1 to 4, 1 to 3, 1 to 2, 2 to 5, 3 to 5, 4 to 5, 2 to 4 or 3 to 4%, for example 1, 1.4, 2, 2.5, 3, 3.5, 4, 4.5 or 5% by weight.

[ 00102 ] In the event that Erbium doped phosphate glass is used as the optical medium, the concentration of Erbium may be larger than about 1x10¹⁸ ions per cubic centimeter, and may advantageously be larger than about IxIO²⁰ ions per cubic centimeter. A concentration of Erbium of around 4 x IO²⁰ ions per cubic centimeter has been found to work well.

[ 00103 ] Alternatively, the optical medium may be made of or comprise a ceramic material which may be a vitreous ceramic material and which may be doped as aforementioned. Alternatively it may be doped with another laser active material. As another alternative, it may be silver doped alumina, silver doped glass, Forturan or photosensitive glass. In any one of the above aspects, the optical medium may be a composite optical medium which may be formed for example by diffusion-bonding of different bulk media, or active and passive media, wherein, during fabrication of an optical device in such a composite media the writing parameters of the fabrication system may be suitable configured for the different bulk media (for example to take account of different refractive indicies of the differing bulk media).

Hie optical medium may be a laser glass such as those that are commercially available from Kigre Incorporated of the USA, including those designated Q-98, Q-100, Q-246, QE-7S, QE-7, QX/Nd, QX/Er, QX/Yb and QG-108.

[ 00104 ] A high concentration of rare earth ions can lead to problems such as ion clustering and lifetime quenching, which in turn reduce the amplifier performance,

[ 00105 ] The second material may be a solid ox a liquid and may be of the same substance as the optical medium or it may be a different substance.

[ 00106 ] The material inside the waveguide may be a liquid. The liquid may be capable of solidifying to provide a second material which differs from the material of the optical medium.

[ 00107 ] The waveguides may be hollow passages extending through the optical medium, and the passages may be backfilled with a suitable backfill material which may be silica gel, a sol- gel, a liquid or polymer doped or stained with an optically active material. The backfill material may be doped with a rare earth ion as aforementioned for the optical medium, The backfill material may be an epoxy resin.

[ 00108 ] In order to increase the gain of the signal beam, an apparatus according to the invention may comprise a gain medium in the signal waveguide, with the inlet and outlet surfaces of the signal waveguide being made reflective to light having a wavelength within a first range and transmissive to light having a wavelength within a second range. The first range may be from about 8O8nm to about 1 OOOπm, and the second range may be from about lOOOnm to about 1700nm.

[ 00109 ] The gain medium may be a rare earth doped glass as hereinbefore described. [00110] Signal Beam

[ 00111 ] The signal waveguides may be adapted to carry more than one wavelengths of light, so as to be capable of carrying more than one channel of information. The wavelength of the signal beam may be in the near-infrared region of the spectrum. The wavelength of the signal beam may be from about δOOnmto about 1700mn, or about 800 to 1500nm, 800 to 1200nm, 800 to lOOOnm, 1000 to 1700nm, 1200 to 1700nm, 1500 to l700nm, 1000 to 1500nm or 1000 to 1300nm. The wavelength may be selected from about 850 nm, about 900 nm, about 950 nin, about lOOOnm, about 1100 ran, about 1200 nm, about 1300 nm, about 131OnUi₃ about 1400 nm, about 1450nm, about 1550nm, about 1500nm, about 1600 nm, about 1650nm or combinations thereof. The signal waveguides may carry light of one or more wavelengths in the dense wavelength-division-multiplexing (DWDM) or coarse wavelength-division-multiplexing (CWDM) specifications of the International Telecommunications Union (ITU). For example the signal waveguides may carry light that conforms to ITU standards ITU-T G.694.1 (DWDM) or G.694.2 (CWDM). [ 00112 ] Each channel may be adapted to carry up to 10 gigabits of data per second or more, or up to about I₅ 2, 3, 4, 5, 6, 7, 8 or 9 gigabits (e.g. about 1, 2, 3, 4, 5, 6, 7, 8 ,9 or 10 gigabits). In other examples, each channel may be adapted to carry up to 40 gigabits of data per second, or up to about 1, 2, 3, 4, 5, 6, 7, 8 or 9, 10, 15, 20, 25, 30, 35 or 40 gigabits (e.g. about 1, 2, 3, 4, 5, 6, 1₁ 8 ,9, 10, 15, 20, 25, 30, 35 or 40 gigabits). In further examples each channel may be adapted to carry up to 100 gigabits of data per second.

[ 00113 ] Where the waveguide is to be used with a 60-channel DWDM system, the signal transmission rate may be as much as 1 terabit (10¹² bits) per second or more.

[ 00114 ] More than one signal waveguide may be provided.

[ 00115 ] The or each signal waveguide may be individually tailored for single or multi-mode propagation of a signal wavelength or range of wavelengths.

[ 00116 ] The signal beam may be optically regenerated using a non-linear optical process and amplification in accordance with the invention.

[00117] Pimp Beam

[ 00118 ] The wavelength of the or each pump beam may be in the range of from about 800 nm to about 1200 nm, or about 800 to 1000, 1000 to 1200 or 900 to 1 lOOnm. It may be selected from 808nm, 980 nm, 1060 nm, or 1480 nm, The first and second pump beams may have the same wavelength or they may have different wavelengths.

[ 00119 ] Alternatively or additionally, the first and second pump beams may have the same or different intensities. [ 00120 ] In one arrangement, the or each pump beam is supplied by a diode, and the wavelength of the or each pump beam is about 975nm for an Yb-doped system, about 800nm for a Nd-doped system or other pump wavelength depedinging on the active dopant ion or transition of the active ion to be pumped as would be appreciated by the skilled addressee. The or each pump wavelength is typically less than about 1 OOOnm although it will be appreciated that pump wavelengths longer then lOOnro may also be used for appropriate active ions or targeted pump transitions thereof.

[ 00121 ] The or each pump beam may be capable of amplifying the signal beam by from about 1 dB to about 30 dB, or about 1 to 20, 1 to 10, 1 to 5, 5 to 30, 10 to 30, 20 to 30, 5 to 20 or 10 to 2OdB₅ for example about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25 or 3OdB, or more than 3OdB.

[ 00122 ] Conveniently, the pump beam is capable of amplifying the signal beam by from about 0.1 dB per meter to about 1000 dB per meter, conveniently from about 1 to about 1000 dB per meter, and may be advantageously from about 10 to about 1000 dB per meter, from about 100 to about 1000 dB per meter, or from about 100 to about 500 dB per meter. It has been found that an amplification of from about 200 to about 300 dB per meter is particularly suitable.

Suitable amplification may be for example about 0.1, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 10O₅ 150, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 350, 400, 450, 500, 600, 700, 800, 900 or lOOOdB/m.

[ 00123 ] The pump beam may be modulated so as to improve the quality of the signal. [ 00124 ] Where more than one pump waveguide is present, each pump waveguide may be individually tailored for single or multi-mode propagation of a pump wavelength or range of wavelengths,

[ 00125 ] In order to improve the efficiency of pumping of the signal beam by the pump beam, xhe single-mode cutoff wavelength for the pump waveguide may be set to lie between the pump and signal wavelengths. It is to be understood that the signal mode cutoff is dependent on both the waveguide radius and the refractive index contrast between the core and the cladding. Consequently there is no one solution. However, if a refractive index contrast of 1 x 10^*3 is generated iα the optical medium, then a I6μm diameter step index waveguide will have a cut-off wavelength of approximately 1200 run. [ 00126 ] The pump beam may be spatially and/or temporally modulated. The pump beam may be modulated by pulsing the driver circuit for the pump laser. Signal amplification may be modulated either spatially or temporally, or both spatially and temporally. This may be achieved by modulating the coupling between the pump and signal waveguides. To facilitate modulation, the pump waveguide may overlap the signal waveguide.

[ 00127] Refractive Index

[ 00128 ] The retractive index of the optical medium may be different to the refractive index of the signal waveguide. It may be less than the refractive index of the signal waveguide.

[ 00129 ] The refractive index differences may be greater than 1x10"*, and may be between about 1x10"⁴ and 1x10"'. Suitable refractive index differences may be between about IxIO^"4 and IxIO^'2, IxIO⁴ and IxIO^'3, IxIO^'3 and IxIO^*1, IxIO^'3 and lxW² or 5x10^ and 5x10^"', for example about lxlθΛ 5XlO-⁴, lxlO^"3, 5xlO^'3, W², 5xlO^"2 or 10^"1.

[ 00130 ] For example, it has been reported by Saliminia et al: Writing Optical waveguides in fused silica using 1 kHz femtosecond infrared pulses; Journal of Applied Physics, Vol. 93, No 7 (1 April 2003), p 3724, that irradiation with pulse energies of 2 μJ and 7 μJ will produce refractive index changes of 2 x 10^"3 and ~5 x 10^"3 respectively in fused silica.

[ 00131 ] The refractive indexes of the signal waveguide and the or each pump waveguide may be the same or different.

[ 00132 ] The refractive index of the optical medium may be greater than the refractive index of the or each pump waveguide and/or of the signal waveguide. The amplifier may have one or more pump waveguides, and may have between one and a plurality of pump waveguides.

[ 00133 ] Shapes and Configurations of Waveguides [ 00134 ] The signal waveguide may be straight.

[ 00135 ] At least one of the pump waveguides may have a portion that is straight and runs parallel to the signal waveguide over at least a portion of the length of the signal waveguide. Alternatively, they may be co-axial.

[ 00136 ] Alternatively, the pump and signal waveguides may be coiled, and the coils may run substantially parallel to and in close proximity to one another, for at least a portion of their lengths, for a substantial portion, and potentially over substantially their entire lengths, so as to maximize the extent to which the pump wave can interact with and amplify the signal wave. In this way_: it is possible to provide an amplifier that has the approximate dimensions of a microchip, yet offers the potential of centimeters over which pump light can interact with and amplify signal light. [ 00137 ] The pump waveguide and the signal waveguide may be located in the same plane and the pump waveguide may be curved.

[ 00138 ] In one arrangement, the pump waveguide has a curved portion and a straight portion and the signal waveguide is straight, and is disposed parallel to the straight portion of the pump waveguide. [ 00139 ] In another arrangement, both the pump waveguide and the signal waveguide are at least partially curved and extend substantially parallel to each other over at least part of their lengths.

[ 00140 ] In a further arrangement, at least one of the pump and signal waveguides is nonlinear with the waveguides having one or more regions where they are in close proximity of each other or where they overlap or cross over, so that the pump light can interact with and amplify the signal light.

[ 00141 ] The pump and/or signal waveguides may, independently, comprise coils, and the coils may be planar, helical or spiral. Alternatively, they may be shaped like a corkscrew, a cylinder or a sphere. [ 00142 ] The waveguide amplifier may comprise alternating layers of waveguide wherein layers of signal waveguide alternate with layers of pump waveguide. The layers of signal waveguide may be optically connected and the layers of pump waveguide may be optically connected or they may be optically isolated, or some may be optically connected and some may be optically isolated. [ 00143 ] The waveguides in adjacent layers may have corresponding shapes, which may be coils, with the waveguide of each layer being separated at a substantially constant distance along a major portion of their lengths. The pump waveguides may be leaky, whilst the signal waveguides may be non-leaky, and the light leaking from the pump waveguides may interact with and amplify the signal(s) transmitted in the signal waveguide(s). [ 00144 ] The signal waveguide may have at least one straight portion and at least one curved portion, and the curved portion may be bent or it may be coiled and may have one or more coils, and may have between about 1 and 1000 coils, or 1 and 500, 1 and 100, 1 and 50, 1 and 20, 1 and 10, 10 and 100O₇ 50 and 1000, 100 and 1000, 500 and 1000, 10 and 500, 100 and 500, 10 and 50, 50 and 500 or 50 and 200, for example about 1, 2, 3, 4, 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, 600, 700, 80O₅ 900 or 1000 coils, and the coils may be coplanar or they may be non-coplanar. At least a portion of the waveguides may be co-axial,

[QOl 45 J Leakimss ( 00146 ] The or each pump waveguide may be leaky with respect to a wavelength at least a portion of pump light passing therethrough. Leakiness in the sense of the present application generally relates to coupling between the pump guide and the signal guide such that, when the pump guide is in close proximity to a signal waveguide, light in the pump guide may "leak" into the signal guide. An example of the type of coupling envisaged is evanescent coupling between the pump and signal waveguides, although other coupling methods as would be appreciated by the skilled address would also be encompassed. The "leakiness" as described in this application is therefore to be distinguished from leakiness concerning a loss mechanism whereby light which "leaks" from a waveguide is lost to the device, a typical example being bend losses due to a curved waveguide, where the curve is too great to support the mode of the light in the waveguide.

[ 00147 ] In some arrangements, it may be advantageous to increase the extent to which light can leak out of the pump waveguide, by providing the waveguide with corrugations in at least a portion of its wall. Where the waveguide has a diameter of about 20 microns the corrugations may be between about 0.2 and 5 microns in length, for example between about 0.5 and 5, 1 and 5, 2 and 5, 0.2 and 2, 0.2 and 1 or 0.5 and 2 microns, e.g. 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0,8, 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 microns in length. The ratio of the diameter of the waveguide and the length of the corrugations may be between about 4 and about 100 (i.e. 4:1 and 100:1), or between about 10 and 100, 20 and 100, 50 and 100, 4 and 50, 4 and 20, 4 and 10, 10 and 50, 10 and 20 or 20 and 50, for example about 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 or 100. [ 00148 ) The or each waveguide may be as long as required to achieve a desired amplification, which may be from about lmm to about Im, or about 10mm to Im, 100mm to Im, 1 Omm to 500mm, lmm to 500mm, lmm to 100mm, lmm to 10mm, 5mm to 500mm, ΪOmm to 200mm, . 10mm to 100mm, 10mm to 50mm, 50mm to 200mm or 50mm to 500mm. [ 00149 ] The signal waveguide may have no corrugations in its wall and is preferably not leaky with respect to signal light passing therethrough.

[ 00150 ] The pump beam(s) may leak from about 5% to about 100% of their energy between the pump waveguide inlet and the pump waveguide outlet, or between about 5 and 90, 5 and 70, 5 and 50, 5 and 30, 1.0 and 100, 10 and 90, 10 and 50, 30 and 100, 30 and 90, 30 and 50, 50 and 100, 50 and 90 or 50 and 70%, for example about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98 or 99% of their energy. In some arrangements of the above aspects, the pump beam(s) may be adapted to leak more than 50%, more preferably close to 100%, say more than 90%, alternatively approximately 98%, or 99% of their energy between the pump waveguide inlet and the pump waveguide outlet. [ 00151 ] Losses may occur as a result of curves and roughness in the pump waveguide. Losses may also be associated with the end facets (typically 5% at each face), whilst intrinsic absorption losses associated with the material itself may be as little as 10% and as high as 80%, depending on the dopant concentration and quality of laser glass,

[ 00152 ] The present invention is not particularly concerned with the preservation of the mode of the pump beam. The invention relies on the leakage of at least a portion of the pump radiation into the signal waveguide.

[ 00153 ] Furthermore, the efficiency of pumping is relatively insensitive to the direction of the pump and signal beams. The efficiency of pumping generally increases with Jength of overlap. However, the losses from the pump waveguide are generally not directly dependent on the length of the pump beam, but rather on its shape and the degree of curvature of the pump waveguide at any particular point.

[ 00154 ] If the waveguide according to the invention is designed with a reasonable degree of care and skill, such as for instance when it is optimised for a laser pump beam having a wavelength of less than about 1 micron, the signal radiation will not leak back into the pump wavegυide to any significant degree, because its diameter is too small to sustain single mode guidance of the signal (which typically has a wavelength from about 1400 to about 1600nm).

[ 00155 ] Single mode guidance in waveguides is a function of the numerical aperture (NA) of the waveguide (the square root of the difference of the squares of the refractive indices for the core and the cladding) and the wavelength (lambda).

[00156] Manufacture

[ 00157 ] According to a ninth aspect there is provided a process for making a waveguide amplifier comprising providing a signal waveguide and a pump waveguide in an optical medium, whereby at least a portion of the lengths of the pump waveguide and the signal waveguide are located in a region proximal to one another so that, in use, light leaking from the pump waveguide can interact with and amplify light transmitted in the signal waveguide.

[ 00158 ] The signal and pump waveguides may be provided in the optical medium by writing, drilling etching or any other suitable manner.

[ 00159 ] Each of the pump waveguide and the signal waveguide may, independently, be written into the optical medium by using a focused laser beam which optionally is a femtosecond laser beam, with the focal point of the laser beam being moved from one point on the surface of the optical medium to another point on the same or another surface of the optical medium. The laser beam may be pulsed and may be focused within the optical medium while the focus is translated, within the optical medium, along a scan path, at a scan speed effective to modify the refractive index of the optical medium along the scan path.

[ 00160 ] In other arrangements, at least one of the pump and signal waveguides may, independently, be formed by etching, using a suitable etchant. The etching may be reactive ion etching. The etchant may be Ar⁺, O₂, HF, CF₄, C₂F₆ or SFg, and other reactive species capable of removing the contents of the proposed waveguides. [ 00161 ] The laser beam may be designed or tuned such that substantially no laser induced physical damage of the optical medium is incurred along the scan path. Alternatively, the material of the optical medium may be converted to a form that can be easily removed by dissolution or chemical reaction. [ 00162 ] According to a tenth aspect there is provided a process for making a waveguide amplifier, comprising the steps of: writing a signal waveguide passage in an optical medium using a focused laser beam, the material inside the waveguide pathway being converted into a form that is chemically or physically different from the material of the optical medium and may be removed by dissolution by a solvent or chemical reaction with an etchant: removing the material in the waveguide by dissolution or etching; and replacing the material in the pathway with a second material having a refractive index different to the refractive index of the optical medium.

[ 00 J 63 ] Waveguide amplifier and Oscillator [ 00164 ] A waveguide oscillator may be provided in the amplifier according to the invention, or else it may be provided as a separate oscillator. Where the waveguide oscillator is to form part of the amplifier, a single distributed reflector or a pair of spaced reflectors and an optical medium may be provided inside the signal waveguide, with the reflectors being selected such as to have appropriate reflectivities and transmissivities and so as to form an optical cavity capable of amplifying intracavity radiation incident on the optical medium. The reflectors may be in the form of gratings or discontinuities introduced in the waveguide. The single distributed reflector may extend substantially the entire length of the waveguide oscillator in the optical medium. Alternatively, they may be created by locating a reflective material in desired locations outside the waveguide, [ 00165 ] The overall stability of the amplifier may be greater in arrangements that embody an optical oscillator,

[ 00166 ] Where the optical medium is glass, large changes in the refractive index, such as >10^"4, can be achieved in a reasonable amount of writing time. The formation of laser-induced physical damage should preferably be avoided. [ 00167 ] In one arrangement, a spiral type waveguide with a plurality of 90' bends is written into the optical medium, to reduce the amount of area required for the waveguide. However, because of the tight bend radius at each of the 90° bends, a substantial amount of light may be lost at each bend. In another arrangement, additional refractive index modified sections surrounding the bends may be included to reduce bend losses. Thus the waveguide of the present invention may have a minimum bend radius such that bend losses are acceptable. Bend losses may be less than about 50%, or less than about 5, 2, 1, 0,5 or 0.1%. The maximum radius of curvature of a bend may be between about 1 mm and 50 mm, or between 1 and 40, 1 and 30, 1 and 20, 1 and 10, 5 and 20, 10 and 15, 8 and 12 mm, eg about 1, 2, 3, 4, 5₅ 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 , 25, 30, 40 or 50 mm.

[ 00168 ] After writing of the waveguides in the optical material, the waveguide amplifier may be subjected to annealing in order to improve the smoothness of the walls of the waveguides and/or increase the refractive index difference between the optical material and the waveguide or waveguides. The annealing may be performed for a period from about 1 hour to about 100 hours (or. about 1 to 50, 1 to 20, 1 to 10, 1 to 5, 1 to 2, 5 to 100, 10 to 100, 50 to 100, 10 to 50 or 10 to 20 hours, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 or 100 hours), and may be at a temperature of from about 5*C to softening point, and even to just below the melting point of glass. The annealing temperature may thus be several hundreds of degrees Centigrade. The softening point may be for example between about 400 and about 700⁰C, or between about 500 and 700, 600 and 70O₃ 400 and 600, 500 and 600 or 500 and 57O⁰C, e.g. about 400, 450, 500, 510, 520, 530, 535, 540, 550, 560, 570, 580, 590, 600, 650 or 700 ⁰C. [ 00169 ] Another approach is the so-called "back-fill" approach. With this approach, the material inside each waveguide or any one or more of them is removed after writing. To achieve this objective, the material inside the waveguide is preferably converted, during writing of the waveguide, into a form which is liquid or at least pourable, flowable or soluble in a suitable solvent. [ 00170 ] After removal of the material from the waveguide(s). the waveguide(s) may be backfilled with a suitable material having a desired refractive index.

[00171] Advantages

[ 00172 ] The invention has the advantage that it provides a waveguide amplifier and/or waveguide laser comprising a co-axial waveguide and grating formed in bulk optical material. The waveguide and the grating may have different axial dimensions.

[ 00173 ] The invention has the advantage that it provides a waveguide amplifier or laser in a bulk optical material which is not polarisation dependent, i.e. the optical waveguide in the waveguide amplifier or laser has minimal if any birefringence such that light propagating in the waveguide does not experience difference optical parameters in orthogonal directions perpendicular to the propagation direction in the waveguide (i.e. the axis of the waveguide). [ 00174 ] The invention has the advantage that a waveguide amplifier and/or waveguide laser may be formed in bulk optical material by a direct write process.

[ 00175 ] The invention has the advantage that a waveguide may be formed that is curved in 2 or 3 dimensions and that is contained in a relatively small area or volume in a substrate, hence increasing the amplification channel waveguide length and reducing the overall size of the amplifier.

[ 00176 ] The invention further has the advantage that it provides a waveguide amplifier wherein a signal guide is decoupled from a pumping mechanism at the chip level.

[ 00177 ] In one arrangement, the invention also has the advantage that it enables the efficient transfer of energy from a pump beam to a signal beam while utilizing a side pumping geometry. In another arrangement, the invention has the advantage that it provides a waveguide laser in a bulk optical material utilising an end-pumping arrangement

[ 00178 ] In addition, the invention has the advantage that thermal loading associated with fabricating side pumped micro-waveguide lasers can be relaxed, so that amplifiers suitable for generating moderate output powers can be made.

[ 00179 ] The invention furthermore has the advantage that it benefits from the lower cost and complexity of side pumped systems as compared to end pumped systems.

[ 00180 ] This invention couples the advantages of highly efficient co-axial end pumped schemes with the simpler engineering requirements of side pumped ones for laser written micro- waveguide lasers.

[ 00181 ] The invention permits efficient coupling between the pump beam and the gain region in solid state lasers, while at the same time allowing full flexibility in the manner by which pump light is coupled into the gain medium.

[ 00182 ] Where the waveguide alters the propagation direction of a side coupled pump beam until it is co-axial with, and overlaps, the propagation direction of the signal beam, the mode overlap is increased and this improves the efficiency of the waveguide amplifier.

[ 00183 ] According to a further aspect there is provided an optical waveguide device comprising: at least one optical waveguide formed within a optical material, the waveguide having a central longitudinal axis; and; at least one grating formation within the bulk optical material co-axial with the axis of at least one optical waveguide.

[ 00184 ] The optical material may be a bulk optical material and tiie at least one optical waveguide may be disposed within the bulk optical material. The optical material may be a bulk optical material and the at least one optical waveguide may be embedded in the bulk optical material. In use, light propagating in the at least one optical waveguide experiences minimal birefringence, wherein minimal birefringence implies that the light propagating in the waveguide expereinces substantially the same properties in orthogonal transverse directions perpendicular to the direction of propagation, [ 00185 ] According to a further aspect there is provided an optical waveguide device comprising: a bulk optical medium comprising a signal inlet and a signal outlet and a pump inlet and a pump outlet; a signal waveguide extending from the signal inlet through the optical medium to the signal outlet, said signal waveguide being capable of transmitting light from the signal inlet to the signal outlet; and a pump waveguide extending from the pump inlet through the optical medium to the pump outlet, said pump waveguide being capable of transmitting a pump beam from the pump inlet to the pump outlet wherein the optical waveguide device is adapted to cause at least a portion of the pump beam transmitted in the pump waveguide, in use, to be coupled with the signal waveguide.

[ 00186 ] The signal inlet and the signal outlet may be associated with a first and second extremity of the optical medium respectively. The signal inlet and the signal outlet may be associated with a third and fourth extremity of the optical medium respectively. The signal inlet and the pump inlet may be spaced from one another. The pump waveguide may be adapted to be evanescently coupled with the signal waveguide.

[ 00187 ] The signal and the pump waveguide each may have minimal polarisation dependence for light propagating therein in use.

[ 00188 ] The signal and the pump waveguides may each comprise a corresponding central longitudinal axis, and wherein either the signal waveguide, the pump waveguide or both the signal and the pump waveguide may each comprise at least one grating co-axial with the corresponding central longitudinal axis over at least a portion of its length. The grating may be a Bragg grating. The grating may be a distributed feedback Bragg grating. The grating may comprise alternating adjacent regions of different refractive index co-axial with the axis of the waveguide.

[ 00189 ] The optical device is an optical amplifier wherein the optical medium may be doped with an active ion optical device such that, in use, light launched in the pump inlet is coupled to s the signal waveguide and interacts with a light signal propagating in the signal waveguide thereby to amplify the light signal.

[ 00190 ] The optical device may be a waveguide laser wherein the optical medium is doped with an active ion and the signal waveguide comprises at least on optical reflector therein defining a resonator cavity. The optical reflector may be at least one grating formation co-axial ) o with the signal waveguide. The optical reflector may be at least one distributed feedback Bragg grating formation co-axial with the signal waveguide. The grating may be co-axial with and symmetrical about the central longitudinal axis of the waveguide,

[ 00191 J According to a further aspect there is provided a laser comprising: an optical waveguide embedded in a bulk optical material, the waveguide having a central longitudinal 15 waveguide axis: at least one grating formation in the waveguide co-axial with the waveguide axis; and a pump means adapted to deliver a pump beam to the optical waveguide.

[ 00X92 ] The co-axial grating may either a single distributed reflector or a pair of spaced reflectors, the coaxial grating defining an optical resonator in the waveguide, and wherein the coaxial grating is adapted for refection of a desired wavelength of light such that light of the0 desired wavelength is capable of oscillating within the resonator cavity.

[ 00193 ] The coaxial grating may be a single distributed grating defining a resonator cavity in the waveguide, wherein the single distributed grating extends substantially the entire length of the waveguide. The grating may be a Bragg grating and may be adapted for either reflection or transmission of a desired wavelength of light according to the Bragg condition. The grating may5 be a distributed feedback Bragg grating. The grating may comprise alternating adjacent regions of different refractive index co-axial with the central longitudinal axis of the waveguide. The grating may be a first-order Bragg grating (»?=l)for a desired wavelength λ according to the Bragg condition m . λ = n_eff A , where n_eg is the effective refractive index of the grating and A is the grating period. The spacing between the alternating adjacent regions of different refractive0 index co-axial with the central longitudinal axis of the waveguide may be configured according to the Bragg condition m . λ - n^. A for a desired wavelength λ where m is the order of the grating , n_eff is the effective refractive index of the grating and A is the grating period. The positional error in the spacing between the alternating adjacent regions of different refractive index co-axial with the central longitudinal axis of the waveguide may be less than 12% of the s optimal spacing determined by the grating period A The grating may be co-axial with and symmetrical about the central longitudinal axis of the waveguide.

[ 00194 ] According to a further aspect there is provided a system for fabrication of an optical device comprising: a pulsed laser source; a pulse control means for modifying the pulses from the laser source; a pulse focussing system for directing pulses from the laser source to a io fabrication location in space; a sample stage for holding and positioning a sample in which the optical device is to be fabricated; and a stage controller means for controlling the location of the sample stage with respect to the fabrication location.

[ 00195 ) The pulse control means may be capable of synchronising the positioning of the sample stage with respect to the pulse control means of the pulse laser source. is [ 00196 J The pulse control means may be capable of configuring of the pulses of the laser system with respect to the positioning of the sample stage with respect to: configuring the pulse energy of the pulses, configuring the repetition rate of the pulses, configuring the duty cycle of the pulses; configuring the path the sample stage takes during fabrication; configuring the number of times the sample stages traverses a path or a portion thereof, configuring the speed or

M velocity of the sample stage for desired portions of the waveguide structure.

[ 00197 ] The pulse control means may be capable of configuring the laser pulses for the formation of a grating formation co-axial with a waveguide formation in the optical device. The pulse control means may be capable of configuring the laser pulses for simultaneously forming a grating formation co-axial and a waveguide formation in the optical device.

2s [ 00198 ] According to a further aspect there is provided a method of forming an optical device comprising the steps of; placing a bulk optical material within which the device is to be fabricated onto a translation stage; configuring the pulses of a pulsed laser system and the positioning of the sample stage foτ the writing of a desired waveguide device structure within the bulk optical material; providing a pulsed laser signal to the optical material at a fabrication0 location within the optical material; and translating the sample on the sample stage in accordance with requirementS_j wherein the position of the sample stage and the pulsed laser signal are synchronised wherein the laser pulses modify the refractive index of the optical material in the fabrication location to form at least one waveguide within the optical material to form the optical device. [ 00199 ] The configuring of the pulses of the laser system may comprise one or moie of: configuring the pulse energy of the pulses, configuring the repetition rate of the pulses, configuring the duty cycle of the pulses.

[ 00200 ] The configuring of the positioning of the sample stage may comprise one or more of: configuring the path the sample stage takes during fabrication; configuring the number of times the sample stages traverses a path or a portion thereof, configuring the speed or velocity of the sample stage for desired portions of the waveguide structure.

[ 00201 ] The step of configuring the pulses of apulsed laser system and the positioning of the sample stage for the writing of a desired waveguide device structure within the bulk optical material may be adaptable for the writing of a grating formation co-axial with the central longitudinal axis of a waveguide formed within the optical material

[ 00202 ] The step of configuring the pulses of a pulsed laser system and the positioning of the sample stage for the writing of a desired waveguide device structure within the bulk optical material may be adaptable for the simultaneous writing of a grating formation co-axial with the central longitudinal axis of a waveguide formed within the optical material BRIEF DESCRIPTION OF THE DRAWINGS

[ 00203 ] Arrangements of the optical waveguide amplifier and waveguide laser devices, and fabrication methods for producing such devices will now be described, by way of an example only, with reference to the accompanying drawings wherein:

[ 00204 ] Figures IA and IB are schematic depictions of the fabrication platform and system; [ 00205 ] Figure 2 is a photograph of a waveguide being written inside a bulk glass sample using the fabrication method described herein;

[ 00206 ] Figure 3 is a graph of the insertion loss for waveguides written in bulk optical media with the fabrication system of Figure 1 for a linearly (horizontal and vertical) and circularly polarized writing beam; [ 00207 } Figure 4 is a schematic representation of a curved waveguide in a bulk medium used for characterisation of the insertion loss as a function of the writing beam polarisation;

[ 00208 ] Figure S is a three-dimensional representation of one arrangement of an optical amplifier in accordance with the invention;

5 [ 00209 ] Figure 6 is a three-dimensional representation of another arrangement of an optical amplifier in accordance with the invention;

[ 00210 ] Figure 7 shows a photograph of a region of refractive index change (i.e. a waveguide) induced in a silica glass sample, as described in Example 1, with an inset showing the cross section and the main figure showing the region of refractive index change in the plane JO of translation of the laser beam/sample, in which the waveguide was written without the beam shaping method; and

[ 00211 ] Figure 8 shows a photograph of a region of refractive index change (i.e. a waveguide) induced in an Erbium doped phosphate glass sample, as described in Example 1, with an inset showing the cross section and the main figure showing the region of refractive i s index change in the plane of translation of the laser beam/sample, in which the waveguide was written with beam shaping.

[ 00212 ] Figure 9 A shows schematic and phase contrast images of 1 x 2 waveguide splitter devices written using the fabrication system and an example beam profile of light exiting an example one of the 1 x 2 splitters; 0 [ 00213 ] Figure 9B shows a phase contrast image of a plurality of curved waveguide formations formed in a bulk optical medium;

[ 00214 ] Figure 1OA shows a schematic of a 1 x 4 waveguide splitter with a single junction written using the fabrication platform and an output beam profile for light launched into the inlet of the waveguide which has been split into each of the 4 output ports; 5 [ 00215 ] Figure 1OB shows a schematic representation of a 1 x 8 waveguide splitter demonstrating the 3-dimensionai freedom permitted by the present fabrication system and methods;

[ 00216 ] Figure 1OC shows an end-on schematic of the 1 x 8 waveguide splitter of Figure 1OB; [ 00217 J Figure 1OD shows an end-on schematic taken at plane A of the 1 x 8 waveguide splitter of Figure 1OB;

[ 00218 ] Figure 1 IA shows a number of composite waveguide/Bragg grating stnictres written at different layers within a fused silica sample s [ 00219 ] Figure 1 IB shows a phase contrast image of one of waveguide/grating structures of Figure HA;

[ 00220 ] Figure 12 shows a graph of the spectrum of a triple band filter produced in off-the- shelf telecommunications fibre (SMF 28) by the present fabrication system;

[ 00221 ] Figure 13 A shows a spectrum of a grating structure written in the core of an opticalo fibre consisting written using the present fabrication system

[ 00222 ] Figure 13B shows a phase-contrast image thegrating written in the core of an optical fibre of Figure 13A;

[ 00223 ] Figure 14 is a graph of a transmission spectrum of a grating written using the present system inside the core of a Yb-doped fibre; s [ 00224 ] Figure 15 shows a schematic representation of a complex waveguide amplifier device geometry with independent signal and pump waveguides capable of being fabricated using the present system;

[ 00225 J Figure 16 is a photograph of a waveguide amplifier written inside a Er: Yb co-doped phosphate glass sample exhibiting upconversion fluorescence in response to a pump beam;o [ 00226 ] Figure 17 shows an a schematic setup used to characterise the waveguide amplifier of Figure 16;

[ 00227 ] Figure 18 is a graph of the gain spectrum of the amplifier of Figure 16;

[ 00228 ] Figure 19 shows an example of a side-pumped waveguide laser device geometry capable of being written with the present fabrication system; 5 [ 00229 ] Figure 20 is a schematic depiction of a waveguide laser written in a bulk glass sample; [ 00230 ] Figure 21 and 22 respectively are graphs of the reflection and transmission spectra of the waveguide laser of Figure 20 in an unpumped state; and

[ 00231 ] Figure 23 shows a spectra of the laser output of the waveguide laser of Figure 20.

DETAILED DESCRIPTION ( 00232 ] Disclosed herein are arrangements of waveguide amplifiers and waveguide lasers devices in bulk optical materials and methods for the fabrication thereof.

[ 00233 ] Fabrication Methodology

[ 00234 ] The presently described arrangement of the fabrication platform includes the following control features: [ 00235 ] Spatial control) The system designed to enable fabrication of photonic devices with a relatively large real estate, spanning areas at least 25 x 25mm. Positional errors over this area to in the range of between 1 run to less than 0.5 μm , and typically between 1 nm and 0.1 μm in order to satisfy basic requirements for high quality Bragg gratings.

[ 00236 ] Temporal and.phase control: In order to produce complex grating structures good phase control is a must. For example, sampled Bragg grating devices are generated by introducing phase offsets, typically half a period, at regular points along the length of the grating. This requires excellent timing phase between the pulses of the writing beam and the location of the bulk material in which the grating is to be written, a requirement which has been achieved in the present arrangements of the fabrication platform and direct write system (see for example Figures IA and IB) as evidenced by the examples below.

[ 00237 ] Optical waveguides can be written by direct write systems in bulk glasses using two translation schemes, the waveguides are either written in a direction that is parallel or perpendicular to the laser beam direction. It is common practice to use a high quality microscope objective as the focusing lens. In the parallel writing geometry the maximum length of a waveguide is limited by the working distance of the focusing element however the advantage of this technique is that it automatically produces waveguides with a refractive index profile of cylindrical geometry. Where the laser beam is scanned in a transverse direction there is no limit to the length of waveguide achievable however the focal region of the writing laser beam (which ultimately describes the shape of the waveguide) is no longer circular in the direction of the waveguide's axis. The effect of the asymmetry of the laser focal region on the waveguide is such that, without mediation through other experimental techniques, a waveguide is produced with strong aspect ratio asymmetry. This asymmetry has a deleterious effect on the performance of the waveguide due to poor device mode-matching to the circularly symmetric mode profile from optical fibres used for input and output signal coupling and high transmission losses. Several techniques have been used to modify the shape of the resultant refractive index region and to improve its symmetry. They include multi-passing of the writing laser beam over the written region in several slightly offset scans, using a very high magnification and numerical aperture (NA) objective such that each laser pulse produces a spherical modified region (of size that is defined by the diffusion of the laser pulse's energy into the glass matrix) or by introducing an astigmatism into the writing beam before it enters the focusing objective. Another technique, formerly used in the fabrication of microfluidic devices, is whereby a slit is introduced into the writing beam before the microscope objective. The effect of this slit, which is placed with its long axis parallel to the direction of the laser beam translation, is to expand the shape of the laser focus in one direction. This technique has several significant advantages over other methods of waveguide shape correction in that it is exceptionally simple, allows single pass writing and can be applied to objectives with long working distances allowing many layers of waveguides to be written above each other. This method enables to fabrication of highly symmetrical waveguides in bulk optical media. [ 00238 ] Reproducibility: The system was designed to maximise reproducibility. Expectations are a short term reproducibility of less than 2% variation in the standard optical characteristics (ie. propagation loss, mode profile, Bragg wavelength etc) between consecutively fabricated devices, and long term reproducibility (over 6 months) exhibiting less than 5% variation.

[ 00239 ] Flexibility: Development of advanced processing techniques for inscribing photonics devices in a range of glass types with 2-D and 3-D fabrication capability.

[ 00240 ] The micro-optical properties of the devices formed using the above fabrication platform desirably have the following features:

( 00241 ] Waveguides: A fundamental achievement of the present system is the demonstration of single mode waveguides operating at wavelengths within the telecommunication C-Band. The challenge overcome here has been to constrain the laser induced localised refractive index change to less than 20 μm in size and produce refractive index increases greater than 1 x 10Λ [ 00242 ] Another performance realisation of the present system was the production of waveguides with propagation losses that are one-fifth of the peak gain per unit length available from commercially available active glasses and comparable to those reported for silicon photonics. [ 00243 ] Photonic devices, particularly those formed using common silicon waveguide fabrication techniques, incorporating asymmetric waveguides exhibit undesirable polarisation dependence. The present system desirably is capable of fabricating circular waveguides with less than 10 % ellipticity and low polarisation dependence.

[ 00244 ] Gratings: Key achievement in the fabrication of Bragg grating structures included fabrication of gratings exhibiting linewidths (less than lnm FWHM) and off resonant losses (less than IdB) comparable to gratings written using conventional inscription methods employing ultraviolet laser sources, phase masks and photosensitive glasses.

[ 00245 ] The new photonic fabrication platform described herein represents a paradigm shift in the way micro-fabricators approach PIC production. In particular, the prevailing mind set is that standard lithographic methods, or adaptions thereof such as the ion exchange method, are the methodology of choice when undertaking PIC production. This is largely because these fabrication methodologies leverage 20 years of development by the semiconductor industry and are readily scalable to mass production, However, as pointed out above conventional lithographically generated silicon PICs and ion exchanged glass PICs have key shortcomings that limit the range of functionalities on offer. Furthermore, complex planar circuit designs must resort to using intersecting waveguides to link different photonic components. These crossing points can produce unwanted loss and cross-talk between discrete photonic components. In conventional microelectronics these links are made via copper interconnects located on a different plane, typically the backside of the circuit board. Similarly, the 3-D fabrication capability enabled by the new platform offers the same flexibility and opens up a suite of new photonic designs. Not only can cross talk be eliminated but photonic circuit designs capitalizing on a 3D format will be able to minimize the number of optical connections at the input / output faces of the chip, therefore reducing packaging time and cost.

[ 00246 ] Furthermore, the embedded nature of the devices produced by this platform. In particular, the glass host serves as a self contained "clean room" reducing the device' s susceptibility to humidity and reducing the requirement for expensive packaging. The ability to modify the sub-surface properties of optical materials also has implications beyond photonics. For example, the same system is used to weaken the sub-surface structural integrity of optical materials in order to generate cleavage planes for debris free laser dicing of microelectronic wafers. [ 00247 ] The Fabrication Platform System

[ 00248 ] The fabrication platform comprises a Direct Write laser system (usually associated with microprocessing via material removal) for forming micron-scale structures in a bulk device (see for example Figures IA and IB). Processing such micron-scale structures places a number of constraints on the laser and the manner in which the laser beam is delivered onto the target face. In particular, conventional beam scanning techniques such as those used in laser marking systems result in inadequate control between the focal point of the laser beam and the target surface. The practical solution in this case is to operate with a fix beam path and move the target with precision positioning stages. Fixing the beam path also reduces pointing instability thus improving resolution at the target. Figure 2 is a photograph of a waveguide being written inside a bulk glass sample 201. The writing beam is focused to a spot 203 in the sample 201 by a microscope objective lens 205. The tightly focused laser beam undergoes non-linear absorption within a localised region and induces a refractive index change. A white light continuum is generated as a by-product of this process.

[ 00249 ] The new fabrication system presented here represents a significant departure from conventional laser processing where material is physically removed via ablation. For photonic applications transparent media (glass) are used, and tightly focussed ultrashort laser pulses are used to increase the refractive index in a region confined to the focal spot. Several physical mechanisrns are responsible for these changes in the refractive index, however, the simplest intuitive explanation is that non-linear absorption within the focal spot causes localised melting of the glass followed by densification when it resolidifies. The requirement for strong non-linear absorption necessitates the use of ultrashort pulsed lasers. It is also critical that the laser wavelength falls within the transmission window of the glass hosts of interest. In the present system, a Ti: Sapphire regeneratively amplified laser producing 120 femtosecond long laser pulses at a wavelength of 800nm in a 1 kHz frequency pulse train is used. Typical average powers used during fabrication are between 1-3 mW although the particular operating characteristics of the writing laser are highly dependent upon a number of factors as would be appreciated by the skilled addressee for example the beam quality of the laser beam and the particular medium in which the device is to be fabricated (i.e. glass, polymer, crystal etc)..

[ 00250 ] Ia some arrangements, substantially no laser induced physical damage of the material is incurred along the scan path, the only variation induced in the optical medium being a change 5 in the refractive index thereof where it has been exposed to the radiation of the writing beam.

[ 00251 ] Gratings written in optical fibres or bulk optical material where there is no physically induced damage (e.b. voids in the optical medium caused by localised heating in response to the laser pulse) are generally known as a Type I grating. In some arrangements, however, it may be advantageous to form a grating by the generation of laser-induced physical damage (e.g. voids)o generally known as Type II gratings. This class of grating generally is able to withstand much larger operating temperatures of tbe optical fibre or bulk material (e.g. greater than 800⁰C to typically 1100⁰C or greater) before the grating experiences any "washing out" effects.

[ 00252 ] The beam delivery system of the present fabrication platform 100 is shown in Figure IA and comprises a laser 102 which may be a femtosecond laser capable of producings femtosecond laser pulses in a laser beam 101 (although in other arrangements, the laser may be a pulse laser capable of produces laser pulses of different pulsewidths or in still further arrangements, the laser may be a continuous wave laser), an online imaging device 104 for example a CCD camera for observing the writing of the device, a dynamic beam attenuator 106 which may for example consisting of a waveplate and polarizer and rotating motion control stageo combination for selecting the average and/or instantaneous power in the writing beam 101 for fabrication of the optical device. The delivery system also comprises laser mirrors as required (for example 108 and 110) for steering the writing beam 101 to the sample 112, which is held on a motion control stage 114. The laser beam 101 is focused on the sample 112 by focussing optics 118 such as a lens or focusing microscope objective (in the examples described herein, the5 focussing optics were are microscope objective with a numerical aperture of NA = 0.46, although depending upon the laser system and the device to be fabricated other focussing optics may be used as would be appreciated by the skilled addressee). The beam delivery system may also comprise beam shaping elements 116 such as an adjustable slit to modify the shape of the beam at the focus in the sample for example to compensate for the motion of the sample on theo motion control stage to assist in the writing of circularly symmetric waveguides in the sample. The beam delivery system may also consists of online diagnostics (for example a frequency resolved optical gating or FROG beam analyser, and/or other beam quality analysis devices 120 as shown in Figure IB) to monitor the laser beam 101 so as to maintain a desired set of optical parameters of the writing beam 101 during fabrication of the optical device in the sample 112.

[ 00253 ] The motion control system 114 ofthe fabrication platform 100 of the present arrangement incorporates two types of translation stage that are combined to give a flexible and cost effective system. For example, in the present arrangement used for fabrication ofthe devices in the examples below, the motion control stage comprised a single axis air bearing stage with one nanometre resolution and 200mm traverse. This type of motion control stage is primarily used in the manufacture of devices such as gratings where light interacts resonantly with the laser fabricated structures. The two other cartesian axes of movement in the present arrangement are provided through conventional mechanical stages that offer 10 nm resolution and 50mm of travel.

[ 00254 ] Furthermore, the same motion control system can also be used to select the laser power through the use of a rotation stage and polarisation based optical attenuator. These systems, sourced from Aerotech Inc. ofthe USA, are programmed using a form ofthe conventional G-code, used world wide for the manufacture of components by CNC machine. This enables one to design complex photonic devices in a familiar CAD environment or to work with external user supplied schematics. However, the present system is primarily controlled via an Integrated System computer. This centralized system controls the laser pulse frequency, phase, power along with the position of the stage system and is responsible for measuring the laser beam quality.

[ 00255 ] Of importance to the system is the ability to synchronise the movement ofthe stage systems to the frequency and phase controllers and this can be achieved through a timing system(s) electronic control modules which may be purpose built or adapted by a skilled person. An example arrangement ofthe electronic control module may comprise one or more frequency generator devices connected via control circuitry to the pulse firing control circuitry ofthe laser system. The one or more frequency generator devices may be configured to control the number of laser pulses that are deposited in the bulk device in a particular region to form or evoke a refractive index change in the optical medium at the laser focus therein, wherein the amount that the refractive index is modified is determined by the total amount of energy deposited in a particular region ofthe optical medium. The control module may modify the timing ofthe laser pulses in a number of ways for example by a modification of the amplitude of the laser witing beam, modification of the on-off duty cycle of the laser, to limit the repetition rate of the laser - which may be accomplished by actually modifying the firing repetition rate of the laser or blocking or selecting particular pulses from the free running repetition rate of the laser - such that, as the sample is translated, different regions of the optical medium experience different numbers of laser pulses, thereby evoking a different amount of refractive index change in those regions. The different regions may be determined in terms of the Bragg condition for grating formations, such that, alternating portions of the waveguide being febricated experience alternative regions of refractive index variation, wherein the spacing of the different refractive index regions is governed by the Bragg condition for a desired operating wavelength as described below.

[ 00256 ] In some arrangements, the electronic control module may be configured for fabrication of gratings with complex functions for example multiple wavelength gratings, chirped gratings, gratings having at least one phase change at a desired position along its length (for example a π/2 phase change at a desired location as exemplified in Figure I3B), and other complex features as would be appreciated by the skilled addressee.

[ 00257 ] In some arrangements, the electronic control modules may also be able to synchronise the firing of the writing laser system with the location of the sample on the translation stage to assist in minimisation of the positional error of the pulses of the writing beam deposited in the optical medium.

[ 00258 ] These timing systems are adapted to allow one to rapidly reconfgure the frequency and phase controllers to automate the creation of a wide range of devices, a technique that improves repeatability and reduces manufacturing times. Furthermore, aspects of this system can be reconfigured programmatically, which significantly extends the flexibility of the system. [ 00259 ] Particular attention must also be made to sample handling during fabrication. In the present arrangement, bulk glass samples are held on a custom made vacuum chuck mounted onto the X-Y-Z translation stage. The sample is translated through a fixed laser focal spot, Fibre glass samples are located within a conventional fibre chuck mounted onto a computer controlled stage. The stage enables alignment of the fixed laser focal spot with the core of the optical fibre. The free end of the fibre is affixed to the X-Y-Z translation stage and drawn through the chuck at a fixed velocity. For both bulk and fibre glass samples the pulse phase and frequency of the laser system can be synchronised and manipulated with respect to sample position to create unique grating structures.

[ 00260 ] Samples which have been processed include the following passive, active and nonlinear glasses: • Passive: Fused silica (bulk and fibre), Phosphate glass (bulk) and borosilicate

(bulk).

• Active: Er doped phosphate glass (bulk), Er-Yb doped phosphate glass

(bulk), Yb-doped silica (fibre) and Tm-doped fluoride (fibre).

• Non-linear: Aisenic free chalcogenide (bulk), As-Se based Chalcogenide (bulk) and Pd-doped silica (bulk).

[ 00261 ] The polarisation of the writing beam of the fabrication platform may be controlled by the control systems. Recently it has been shown that optical waveguides written in fused silica using the direct write technique contain polarisation-dependent nanocracks or nanoporous structures. These nanostructures have been found to be self-ordered and periodic (with a size and period as low as 20 om and 140 run respectively) while being orientated in a perpendicular direction to the electric field vector of a linearly polarised writing beam. These embedded oxygen-depleted periodic structures may be visualised using a scanning electron microscope (SEM), Auger electron spectroscopy techniques and selective chemical etching. Two explanations for the formation of these nanostructures have been postulated: firstly that the interference between the incident light field and the electric field of the bulk electron plasma wave results in a periodic modulation of electron plasma concentration and permanent structural changes in the glass network; or alternatively that the evolution of nanoplasmas into disc shaped structures due to high non-linear ionisation creates the nanostructures.

[ 00262 ] A quite different modification morphology, however, is produced with circularly polarised femtosecond radiation. Instead of the self-ordered nanostructures typical of linear polarisation, circular polarisation produces embedded disordered nanostructures that vary in size. This result led us to investigate the effect of laser writing beam polarisation on the propagation losses of femtosecond laser written optical waveguides in bulk fused silica and may be used to fabricate both straight and curved waveguides. [ 00263 ] In operation, to fabricate a waveguide in a bulk optical medium, the form of the waveguide may initially be drawn up in a three-dimensional engineering software package (e.g. Autocad or similar package), and the technical specifications of the three-dimensional drawing may be translated into software code for operation of a three-dimensional translation stage using

5 common software conversion packages for such a purpose. Next the operating parameters of the writing laser beam are specified (eg the pulse energy, repetition rate, pulse amplitude, polarisation etc) are selected with respect to the particular optical medium in which the waveguide is to be fabricated, wherein different portions of the waveguide may be selected to be fabricated with different operation parameters with respect to the nature of the waveguide itself. io The system is then set into writing mode wherein the laser is turned on and the translations stage is moved according to the stage software code such that the optical medium attached to the stage is moved with respect to the focal location of the laser system to fabricate the waveguide.

[ 00264 ] In operation, to form a waveguide in a bulk optical medium as above, which also includes a grating formation therein (for example waveguide 2002 in monolithic bulk optical i s medium 2001 of Figure 20 (which may be doped with active ions, e.g. Pr, Er, Yb, Nd, Ho Th, or any combination of two or more thereof), the grating formation 2010 comprising alternating regions 2005 and 2006 of different retractive index with a period Λ according to the Bragg condition), the same procedure is carried out, however the procedure for selecting the operating parameters of the laser is more complicated, wherein, in the portion of the waveguide where the0 grating formation is to appear, the operating parameters of the laser (for example using the electronic control modules described above) must be selected such that alternating regions of the waveguide experience a different total amount of laser energy deposited therein (thereby causing altematiixg regions of different retractive index within the guide), This is performed in practice by forming the waveguide with laser pulses of different power levels (eg. pulse energy or pulses amplitude, or total number of laser pulses deposited in a desired region) thereby establishing a particular amount of laser energy deposited in selected regions of the bulk material to obtain a desired refractive index change of the bulk material in the desired region. For example, during fabrication of the waveguide 2002 and grating formation 2010, as the optical material is translated on the stage at a constant velocity, portions 2005 and 2006 of grating 2010 may see for0 example alternately 20 and 40 pulses of the writing beam, i.e. the duty cycle of the witing beam is modified from its ftee running repetition rate of IkHz (giving the 40-pulse sections) to for example 500 kHz (giving the 20-pulse sections) thereby to form alternating regions of different refractive index to form a continuous waveguide 2002 having a continuous grating formation 2010. In glass the pulse energy of the laser write beam may be of the order of 190-22OrJ (eg about 20OnJ) whereas in fused silica the pulse energy of the laser write beam may be of the order of2-5μJ. [ 00265 ] This laser writing method is not a point-by-point method of writing the waveguide and grating in the bulk material. Rather it is a phase modulated method, which is achieved by modulating the amplitude of the laser pulse used for writing the grating in the bulk medium (such as a bulk glass which may be a glass doped with active ions), for example. This results in a continuous waveguide (as opposed to a point by point waveguide), The write wavelength of the laser writer may be about 800nm, for example. The laser light pulses of the wrtite beam may be circularly polarised.

[ 00266 ] In this manner, both the continuous waveguide and the grating may advantageously be formed simultaneously in the bulk optical material in a single pass: the waveguide is defined by the region disposed within (i.e. embedded) the bulk material having a central longitudinal axis through the bulk material and an effective refractive index n^ and the grating is formed by the adjacent alternating (although complex grating structures are also contemplated here) regions of different refractive index formed by the deposition of differing amounts of laser energy in different regions along the centra] longitudinal axis. The method of forming embedded waveguide and grating formations in bulk optical material in a single pass has the advantage that the waveguide and grating are coaxial about the central longitudinal axis of the waveguide and the grating is symmetrical or substantially symmetrical about that central longitudinal axis. On the other hand in a multiple-pass operation where the grating is formed in a pass after the waveguide is formed the grating may be displaced with respect to the waveguide such that it is not co-axial and/or it is not symmetrical or substantially symmetrical about the central axis of the waveguide.

[ 00267 ] Fabrication of a continuous waveguide 2002 in this manner, the waveguide having a central axis 2004, with a grating therein extending along at least a portion of along the whole or entire length of the waveguide 2002 provides for an embedded grating in the bulk optical medium 2001, wherein the central axis of the grating formation 2010 is symmetrically disposed or substantially symmetrically disposed (i.e. within about 1 to 15% of being symmetrically disposed) about the central longitudinal axis 2004 of the waveguide 2002 itself, thus forming an embedded waveguide having a coaxial grating. Once formed continuous waveguide 2002 with grating formation 2010 may be formed into a laser system (provided medium 2001 is doped with active ions) by operatively associating it with a light pump source (such as for example as shown in Figure 17). The waveguide 2002 may be pumped with light from the pump source, at one or both ends 2011 and 2012, whereby laser light is emitted from waveguide 2002. The operating wavelength of the laser may be determined at the time of fabricating waveguide 2002 by suitable selection of the grating period between regions of 2005 and 2006 and suitable choice of medium 2001 doped with an appropriate active ion dopant. The operating laser may be tuned by, for example heating the medium 2001 to a suitable temperature to modify the effective refractive index of grating 2010 thus modifying the resonance wavelength of the laser, Thus a laser system may include medium 2001, which is an active medium doped with at least one active ion, with waveguide 2002 and grating 2010 disposed therein, a pump source (not shown) to pump waveguide 2002 and a heater (not shown) to heat medium 2001. In alternate arrangements, the laser system may comprise a pump waveguide disposed within the medium 2001 in proximity to the waveguide 2002 such as for example shown in any one of Figures 5₃ 6, or 19. In further arrangements, the grating 2010 may be a complex grating formation for example comprising multiple grating formations giving rise to multiple resonance wavelengths as required, a phase modulated grating, a grating with one or more π/2 phase changes or shifts therein, a frequency chirped grating or some other complex grating. The wavelength of the pump source is selected on the basis of the active ions doped in medium 2001 and may be about 800nm for Nd dopant and about 980nm for Yb dopant, for example. When medium 2001 is glass, crystalline, polymer, polycrystalline or ceramic based the laser system of the invention using one of these medium 2001 does not exhibit polarisation dependence whereas silocon and/or silica based laser devices do, The laser system of the invention may or may not include reflectors (not shown) at either end of waveguide 2002. The reflector of the laser system of the invention may comprise a distributed Bragg reflector disposed along the whole length or part thereof of the waveguide. The longer the grating 2010 and waveguide 2002 the higher the Q of the cavity defined by the grating reflector. If reflectors are used at either end of waveguide 2002 the cavity defined by the reflectors and waveguide 2002 would tend to have a broader Q (i.e. which would exhibit a broader transition). [ 00268 ] Temporal anaphase control

[ 00269 ] The master electronic system offers fine control over the phase relationship between the pulse frequency of the laser and the position of the motion control stages. Nanometric adjustments can be introduced by the master control system to introduce phase offsets as small as 10 urn or changes in the laser frequency of just 1 part in 10⁶. This level of control enables access to a full grating fabrication tool-kit:

• Period => full control over the target Bragg wavelength

• Amplitude => enables apodised gratings

• Phase =J> enables sampled gratings

• Frequency =-> enables chirped gratings [ 00270 ] Spatial control

[ 00271 ] In an arrangement of the present fabrication platform, an initial system included motion control stages with 25 x 25 x 25 mm full translation. The insights gleaned from that system permitted scaling up to 50 x 50 x 200 mm with a second motion control stage. Furthermore, the beam delivery system was designed to utilise long working length optics, permitting embedded photonic devices to be written at depths spanning 100 μm to 2mm below the surface of a standard glass sample. A typical waveguide with dimension of 10 μm diameter will be isolated from its neighbours if separated by 20μm. As a result, up to 60 discrete layers may be inscribed in a bulk glass sample, making possible true 3-D photonic fabrication.

[ 00272 ] Figure 4 shows a comparative study of the effects of femtosecond laser polarisation on the optical insertion loss of direct written waveguides in fused silica. Waveguides were written with circularly polarized light 201 and linearly polarized light where the orientation of the polarization was parallel 202 and perpendicular 203 to the direction of traversal of the sample during the writing process. It can be seen that the optical transmission of light through a waveguide may be increased significantly by writing waveguides using circularly polarized light.

[ 00273 ] The optical transmission of 1535 nm light through a range of waveguides fabricated using a fixed slit width and various pulse energies and polarisations is shown. For the case of linearly polarised radiation, the optical transmission was similar when the electric field vector of the writing beam lay either parallel or perpendicular to the direction of sample translation. Clearly, the optical transmission of light through a direct written waveguide can be increased by fabricating waveguides using circularly polarised femtosecond radiation, In fact, the absolute optical transmission increased by a factor of 3 at a pulse writing energy of 5 μJ. Beyond this value the transmission through a waveguide increases at a reduced rate as a function of pulse energy. This observation is attributed to the waveguides becoming asymmetric and multimode. This waveguide core asymmetry at high fabrication pulse energies can be seen by noting the near- field profiles shown in Figure 4. The elongated structures are a direct result of self- focussing of the writing beam due to Kerr lensiixg at high pulse energies. However, the waveguide asymmetry caused by these self-focussing effects can be compensated for by adjustment of the slit width W_y. In particular, the following equation which governs the aspect ratio of the slit, W/W_x, required to fabricate waveguides with symmetrically shaped cross- sections:

W_x n V 3 ^x >

[ 00274 ] NA is the numerical aperture of the focussing objective, n the refractive index of the glass substrate and W_x 1he unapertured beam waist. A propagation loss study was performed using six different types of fabricated waveguides.

[ 00275 ] For a fixed pulse energy of 3.5 μJ a waveguide was fabricated using linear perpendicular and circular polarisations, a waveguide overwritten S times with the same pulse energy again using linear perpendicular and circular polarisations, a waveguide using 45° linearly polarised light and a waveguide using 45« elliprically polarised light with a major to minor axis ratio of 2:1 , The propagation loss of these 6 waveguides are shown in the following table of propagation losses for a variety of directly written waveguides (polarisation angle is relative to the direction of sample translation):

[ 00276 ] Once again the results show that the lowest loss waveguides are fabricated using circularly polarised light. This loss decreases when the multiple pass technique is employed. The single pass linearly polarised written waveguides exhibit similar propagation losses. The propagation loss of the elliptically written waveguide was between that of the linearly and circularly polarised written waveguides as expected.

[ 00277 ] It is believed that the relatively high propagation losses associated with waveguides fabricated using linearly polarised radiation is related to the underlying mechanism that produces the periodic nanostructures. This relationship is consistent with the improved propagation loss of the waveguides written with circularly polarised light and the reduced presence of these periodically aligned nanostructures when circular polarisation is used. In a follow up qualitative study the influence of laser polarisation on bend losses was also investigated. The curved waveguide 400 device shown in Figure 4 was fabricated using each of the three states of polarisation outlined above. The waveguide 401 consisted of a shallow bend with radius 55 cm over a length of 15 mm giving an output deviation of 0.15 mm from the optical axis of the input beam. To help facilitate the visual monitoring of bend losses, single mode curved waveguides were fabricated for use at 635 am.

[ 00278 ] Similar transmissions for the curved waveguides fabricated using both states of linear polarisation was measured. The curved waveguide føbricated using circularly polarised radiation resulted in a throughput approximately 3 times greater than the value achieved for the linearly polarised written curves. These results are consistent with those shown in Figure 4A for the case where straight waveguides were written with three different states of polarisation. 1» these studies of bend losses the resulting tangent between the input and output axes was approximately 1. Writing curved waveguides with greater angles and a fixed slit orientation would lead to asymmetries in the waveguide cross-section. To mitigate this effect the long axis of the slit can be synchronously rotated as a function of waveguide angle. An alternative methodology is to use MHz laser systems without the optical slit where cumulative heating effects automatically generate circular waveguide cross-sections. [ 00279 ] Importantly, it can be seen that there are clear device advantages to using a circularly polarised writing beam when fabricating curved waveguides with a kHz laser system. At this stage it is not clear whether the same advantages apply to straight or curved waveguides written using a MHz laser system,

[ 00280 ] The beam delivery system developed as part of the new platform uses simple beam shaping techniques to match the horizontal and vertical dimensions of the waveguide for any host glass. Circular waveguides (similar to that of Figure 8) with less than 8 % variation in the cross-section diameter and negligible polarisation dependence.

[ 00281 ] Reproducibility

[ 00282 ] Design principles and operating protocols have been employed to ensure good reproducibility. These include vibration isolation via the use of floated optical tables; laminar flow hoods to reduce dust contamination of the optic delivery system; high precision motion control stages (1 micron resolution) combined with protocols for tuning them to πώώnise resonances and positional offsets; and a laser with good beam quality (M² less than 1.2), low pointing jitter, good pulse to pulse energy stability (+/- 2%) and stabilised pulse frequency (to 1 part in 10⁶). Examples of reproducibility include routine writing of Bragg gratings with less than 10 pπv variation between the target wavelength and less than 0,02 dB variation in the off resonant loss, waveguides in bulk glass with less than 0.05 dB variation in the propagation loss.

[ 00283 ] Flexibility The fabrication platform can be readily adapted for writing waveguides hi a range of passive, active and nonlinear glasses. [ 00284 3 Waveguides: Waveguides with dimension of 8 to 10 μm diameter, compatible for single mode guiding, are readily fabricated with the platform described herein, although waveguides with other dimension are also readily fabricated. Furthermore, 1^st order Bragg structures (i.e. m = 1 in the Bragg condition m . λ = ttø. A), with 500 nm size features, targeting the telecommunication C-band can be fabricated with this system. Refractive index increases up to 5 x 10^"3 have also been realised using advanced processing techniques.

[ 00285 ] Waveguides fabricated with the new platform have propagation losses less than 0.4 dB /cm in both fused silica and phosphate glasses, well with the desirable parameters for commercial applications.

[ 00286 ] Gratings: Gratings fabricated by the microfabrication platform presented here typically exhibit superior linewidths of 100 pm FWHM and comparable off resonant losses of less than IdB (see Figure 14). Most importantly, this grating writing method can be utilised with both photosensitive and non-photosensitive glasses, and planar and fibre based waveguides. Compared to conventional gratings these femtosecond laser written gratings have superior temperature stability. Indeed, no degradation was observed at elevated temperatures of 600 ⁰C EXAMPLES [00287] Example ]

[ 00288 ] Referring to Figure 5, there is shown a schematic of an example integrated waveguide optical amplifier 500 capable of being directly written in a bulk optical material by the fabrication methods and system described above. The waveguide optical amplifier 510 comprises an optical gain medium in the form of a block of silica based glass 512 doped with a rare earth (RE) element b the form OfEr³⁺WMCh has been deposited in the glass by modified chemical vapor deposition. The block of glass has a top 512.1, a bottom 512.2, a first end 512.3 and a second end 512.4.

[ 00289 ) Using a femtosecond laser (not shown), a signal waveguide 114 and two pump waveguides 516, 518 have been written into the optical gain medium 512. The signal waveguide extends from a signal inlet 514,1 to a signal outlet 514.2. The first pump waveguide 516 extends from a first pump inlet 516.1 located in the surface of the top 512.1 to a first pump outlet 516.2 located in the surface of the second end 512.4, whilst the second pump waveguide 518 extends from a second pump inlet 518.1 located in the surface of the bottom 512.2 to a second pump outlet 518.2 located in the surface of the second end 5Ϊ2.4. The first and second pump waveguides 516, 518 are thus bent through 90°, with their inlets located in opposing surfaces of the optical medium 512. This has the advantage that they can more easily be connected to other components in an optical network system. The signal waveguide 514 is substantially straight, and enters the optical medium 512 at the first end 512.3 whilst it leaves the optical medium at the second end 512.4.

[ 00290 ] In use, a signal beam is transmitted through the signal waveguide 514. The signal wavelength is about 1530 nm. Pump lasers (not shown) generate pump beams, each having a wavelength of approximately 980 nm. The pump beams are transmitted through the pump waveguides 516, 518. Light leaking from the pump waveguides 516, 518 interacts with the signal beam in the signal waveguide 514 and amplifies the signal beam.

[ 00291 ] Optical isolators (not shown) are provided in the signal waveguide 514 and the pump waveguides 516, 518 to prevent back-reflected signal amplification in the rare earth (RE) doped channel waveguide 514 from being transmitted to the network.

[00292] Example 2 [ 00293 ] Referring to Figure 6, there is shown a further example of an optical device capable of being written by the fabrication system above. The present example is of an arrangement of an integrated waveguide optical amplifier 600 comprising an optical gain medium in the form of a block of silica based glass 612 doped with a rare earth (RE) element in the form OfEr³⁺ which has been deposited in the glass by modified chemical vapor deposition. The block of glass 612 has atop 612.1, abottom 612.2, afirst end 612.3 and a second end 612Λ s [ 00294 ] Using a femtosecond laser (not shown), a signal waveguide 614 and a pump waveguide 616 have been written into the optical gain medium 612. The signal waveguide 614 extends from a signal inlet 614.1 to a signal outlet 614.2. The pump waveguide 616 extends from a pump inlet 616.1 located in the surface of the first end 21-2.3 to a pump outlet 616.2 located in the surface of the second end 612.4. io [ 00295 ] Bach of the signal andpump waveguides 614, 616 is shaped as a spiral coil, most of which lies in a plane with the plane of me signal waveguide lying above the plane ofthe pump waveguide and spaced therefrom by about 5 to 20 μm (typically about lOμm).

[ 00296 ] In use, a signal is transmitted through the signal waveguide 214. The signal wavelength is about 1530 run. A pump laser (not shown) generates a pump signal having a i j wavelength of approximately 980 nm. The pump beam is transmitted through the pump waveguide 616. Light leaking from the pump waveguide 616 interacts with the signal beam in the signal waveguide 614 and amplifies the signal.

[ 00297 ] Optical isolators (not shown) are provided in the signal waveguide and the pump waveguide to prevent back-reflected signal amplification in the RE doped channel waveguide0 214 from being transmitted to the network.

[00298 J Example 3

[ 00299 ] The present example describes an optical waveguide fabricated using a regeneratively amplified Tksapphire femtosecond laser system (Hurricane) from Spectra- Physics. This system produces 800nm wavelength 120 fs period pulses at a maximum repetition5 rate of 1 kHz. Laser pulses from this laser were focused through a 20^χ microscope objective (Olympus UMPlanFL, NA 0.46) and brought to a focus inside a polished 5x5x5 mm phosphate glass sample (Toplent Photonics). The waveguide, shown in Figure 7, was fabricated using a sample translation speed of 40 μm/s, a laser pulse energy of 0.24 μJ and a pulse repetition frequency of 1 kHz. The polarization ofthe writing beam was linear and aligned parallel to the0 direction of sample traverse. Because ofthe elliptical cross-section ofthe region of refractive index modification the experimenters were unable to couple light into this device, Figure 7 is a top phase contrast and cross sectional view of a waveguide written without the beam profiling method (i.e. achieved by inserting a slit in the beam). Figure 8 shows the same views for a waveguide written with beam profiling. In this case a circular cross section was obtained, which is important for low loss guiding,

[00300 ] Example 4

[ 00301 ] The present example describes an optical waveguide fabricated using the same laser system cited in Example 3. Laser pulses of 800 nm wavelength were focused through a 20^χ microscope objective (Olympus UMPlanFL, NA 0.46) and brought to a focus inside a polished 5x5*3 mm erbium-doped (4XlO²⁰ ions per cubic cm) phosphate glass sample (Toplent

Photonics). A 500 μm width optical slit was inserted proximate to the focusing objective in order to modify the shape of the laser beam focal region inside the medium (further details of which may be found in M. Ams et aL, Opt. Exp., vol. 13, pp. 5676-5681, 2005 which is wholly incorporated herein by reference). A sample translation speed of 40 μm/s and laser pulse energy of 1.5 μj (measured after the laser beam had propagated through the slit) was used.

[00302] Example s

100303 ] The present example describes an optical waveguide fabricated using the same laser system cited in Example 1. The laser pulses were tightly focused using a 2Ox microscope objective (Olympus UMPlanFL, NA 0.46) and brought to a focus inside a polished 30*15x3 mm fused silica glass sample (Schott). Before entering the microscope objective, the polarisation of the laser beam was adjusted using a Berek compensator (New Focus Model 5540) and the physical shape of the laser pulses were modified by a 500 μm horizontal slit aperture. The waveguide was fabricated using a sample translation speed of 25 μm/s, various laser pulse energies and a pulse repetition frequency of 1 kHz. [ 00304 ] The above examples 3 to 5 describe the fabrication of an optical waveguide. The laser pulses of the writing beam were focused through a microscope objective and brought to a focus inside a glass sample that can be either passive or active, Passive glass (e.g. phosphate glass) does not offer gain whereas active glass (e.g. erbium doped phosphate glass) does offer gain. Included in the signal or pump waveguide regions may be a periodic grating structure of either Bragg or long-period format. The purpose of this grating is to act as a reflector, gain flattener or pump-to-signal waveguide coupler for one or a range of working wavelengths. This grating region may be written either before, during or after the signal and/or pump waveguide inscription process. In one arrangement, a WDM amplifying waveguide includes a Bragg grating structure to act as a gain flattener across the range of amplified frequency channels. In another arrangement, a waveguide laser device may be formed with an amplifying waveguide device that includes one or more reflectors such as grating formations and or structures such as Bragg or DFB grating formations to form a laser resonator cavity. In one arrangement of the waveguide laser, the laser resonator cavity may comprise two optical reflectors which may be optical reflectors either adjacent to or co-axial with the signal inlet and the signal outlet. In another arrangement of the waveguide laser, the optical reflectors may be the interface with the signal waveguide between the bulk optical medium and another optical medium (for example free space (air) or a second optical medium such as a glass, crystal or polymer medium adjacent the optical medium comprising the signal waveguide). In an example arrangement of the waveguide laser the laser may be a micro-laser device and may comprise a highly reflecting laser cavity mirror, a lower reflectivity output-coupling mirror and a pump-to-signal waveguide coupling structure. In other arrangements (as shown in Example 12 below) the resonator cavity may be defined by a single grating formation in a portion of the signal waveguide, which may extend substantially the entire length of the signal waveguide). The pump-to-signal waveguide coupling structure may take a variety of forms, and example arrangements are shown in the examples below (e.g. in Example 11).

[00305] Example 6

[ 00306 ] Figure 9 A shows a series of two dimensional 1 x 2 splitters which have been fabricated inside fused silica samples using the present arrangement in both a schematic three- dimensional depiction (left-hand image) and phase-contrast images of the actual devices as fabricated (right-hand images). The measures output from one of the 1 x 2 splitters after launch of light into the single-end input port clearly shows two separated and distinct output beams. Figure 9B shows a number phase-contrast images of examples of curved waveguides that have been fabricated inside fused silica samples with the present system.

[ 00307 ] Figure IOA shows a schematic of a 1 x 4 waveguide splitter with a single junction. Conventional planar 1 x 4 splitters have to be fabricated from sets of 1 x 2 splitters to ensure 50 : 50 power sharing in each port. The intensity profile of the output ports (~ 10 μm diameter) in the 1 x 4 three dimensional splitter is shown in the false colour image (left-hand image). Note that these can be easily brought back to a common plane within the bulk device to facilitate pigtailing to a standard fibre array such as for example in the example schematic of a 1 x 8 waveguide splitter as shown in Figure 1OB where a single input waveguide portion 1010 is split at a junction point 1012 into 8 output portions 1014. The output portions 1014 are equi-radially spaced at the junction 1012 - see Figαre 1OD showing an end-on schematic view taken as plane A (1018 of Figure 10B) of the equi-radially spaced output portions. This arrangement ensures that light in the single input portion to be substantially equally distributed into each of the output portions 1014. The output portions 1014 are then brought into a single plane 1016 for pigtailing to a linear fibre array (not shown). . Embedded waveguide splitter devices such as those shown in Figures 1OA and 1OB (i.e waveguides disposed within the bulk optical material) have the significant advantage that they are insensitive to external factors for example vibration and humidity and they can be directly fabricated by the processes and systems described herein without any alignment procedures necessary, which is often a significant time and cost for prior waveguide splitter devices. As will be appreciated, a waveguide splitter such as those described in this example can be fabricated with any arrangement and any number of splits as desired Le. a waveguide splitter may be fabricated to split a single primary waveguide into for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 16, 32, 64, 128 or any other number of secondary waveguides as required by a particular application. Also, it will be appreciated that any one of the secondary waveguides may also be split into any number of tertiary waveguides and so on.

[ 00308 ] The bulk waveguide splitter device as described herein also has the advantage that it can be made to be significantly smaller than prior waveguide splitter devices, for example, the 1x8 splitter of Figure 1OB can be fabricated in a bulk medium with a length of about 10 to 115 mm compared with alternate 1x8 splitters which are typically of the order of 25 to 35 mm. [ 00309] Example 7

[ 00310 ] Figures llA and HB shows composite waveguide / Bragg grating structures 1102 written at different layers within a fused silica sample 1104. Figure 11B shows a phase contrast image of one of waveguide/grating structures 1102 showing the alternating regions 1106 and 1108 of different refractive index in the waveguide of the DFB Bragg grating structure formed in the waveguide 1110 fabricated in the optical material 1104. As can be clearly seen, the grating formation in the guide occupies the full extent of the waveguide in the transverse direction perpendicular to ihe length of the waveguide, thereby ensuring a significant mode overlap (of the order of 70 to 80% or greater) between the grating and the guide so that the majority of light travelling through the waveguide interacts with the grating, leading to a significantly more efficient Bragg effect on the propagating optical field and minimal polarisation dependence.

5 [00311 J Example 8

[ 00312 ] To demonstrate the temporal and phase control of the fabrication platform, Figure 12 shows the transmission spectrum of a triple band filter which was fabricated with the present fabrication system by writing Bragg grating formations directly in the core of an off-the-shelf telecommunications fibre (SMF 28). This filter consists of three consecutively written, co-axial io and adjacent (neighbouring) gratings each targeting different wavelengths. The grating targeting 1549 ntn was written over a longer length in the optical medium than the other two gratings, demonstrating control over the peak reflectivity of a particular wavelength as desired. Similar control over the peak reflectivity can also be exercised by controlling the degree to which the refractive index is modulated. This may ba accomplished by control of a number of factors in the is fabrication of the device, such as the power of the writing beam, the number of pulses deposited at a particular location, the speed of translation of the sample during fabrication etc. The custom triple band filter which was used to produce the spectrum of Figure 12 was written with the fabrication system described herein in less than 1 minute.

[ 00313 ] Similarly, Figures 13A and 13B respectively show a spectrum and phase-contrast 20 image of a grating written directly in the core 1302 of an optical fibre consisting of square wave amplitude alternating regions of different refractive index 1304 to the fibre core 1302 Induced by the writing beam of the fabrication platform described above, and also incorporating a π/2 phase modulation 1306 shown in the expanded region 1308 of the phase-contrast image.

[ 00314 ] To write the present gratings in the core of an optical fibre, the outer protective ι$ coating of the optical fibre was removed prior to fabrication of the grating, however, direct writing in the core of an optical fibre without first removing the outer coating may be possible under certain conditions (i.e. suitable transmission characteristics of the coating with respect to the writing beam etc). Figure 14 shows a further spectrum from a grating written inside the core of a Yb-doρed fibre. The Bragg resonance has a linewidth of 60 pm FWHM.

3o [00315] Example 9 [ 00316 ] Waveguides and waveguide Bragg gratings have been successfully written inside a range of active glass hosts (see Figure 16, which is a photograph of an active waveguide exhibiting upconversion fluorescence as a result of an optical input pump beam). The host demonstrating superior qualities for this application is Er: Yb co-doped phosphate glass (Kigre, Inc.). Gains in a 4 mm long waveguide amplifier of 7.3 dB/cm have been measured. This value compares well with the 3 to 5 dB/cm available from commercial planar waveguide amplifiers. Of greater importance are recent successful demonstrations showing narrow linewidth Bragg gratings can be integrated into the waveguide amplifier using the same fabrication platform. Target end user applications for these devices include I x N power splitters for Fibre-To-The- Home (FTTH).

100317] Example 10 - Complex devices

[ 00318 ] It will be clearly evident to the skilled addressee that the present fabrication system is readily capable of fabricating complex devices directly in a bulk material comprising a variety of waveguide devices such as splitters, amplifiers, specialised gratings, pump guides, etc. [ 00319 ] Figure IS shows a schematic of a possible waveguide amplifier geometry 1500 with independent signal and pump waveguides (1502 and 1504 respectively). The signal waveguide comprises a 1 x 2 splitter 1504 where a portion of the signal 1506 that enters the device 1500 at signal inlet 1508 is directed in a secondary signal waveguide arm 1510 to a first signal outlet 1512, where the portion 1514 of the signal is output from the device 1500 and may be connected to another device, for example for testing of the input signal 1506. The remaining portion of the signal is directed by the 1 x 2 splitter to the primary signal waveguide where it is amplified. The primary signal waveguide comprises a coiled structure 1516 in 3 -dimensions within the bulk material 1518 where it is amplified be pump light 1520 which in operation is launched into pump inlets 1522 and 1524 of the pump waveguide. The light in the primary signal waveguide interacts with and is amplified in the linear sections 1525 of the coiled structure 1516 where it is adjacent to corresponding linear sections 1526 of the pump waveguide 1504. The amplified signal 1528 is then output from the device 1500 through a second signal outlet 1530 amplifying. The device may also incoiporate one or more grating structures anywhere in any of the waveguide sections of the device, for example an isolation filter can be written into the primary signal waveguide for rejection of unwanted pump light in the light output from the second signal outlet 1530. [ 00320 ] As can be seen in Figure 15, the signal and pump waveguides 1502 and 1504 may have different have different diameters which can be realised with the fabrication system described above by suitable adjustment of the focal spot sixe of the laser beam used for writing of the waveguides. In the same manner, it will be appreciated that tapered waveguides may also s be fabricated by the fabrication system described above by suitable modification of the focal spot size of the writing beam during the fabrication of the device.

[ 00321 ] Reflective Bragg structures exhibiting linewidths of 100pm FWHM have also been inscribed into a waveguide amplifier. This demonstration validates the new fabrication platform described herein combining the strengths of both silicon and (ion exchanged) glass photonics. In io particular, the platform permits the fabrication of low loss waveguide devices with high fidelity gratings in a gain host.

[ 00322 ] As an exmple demonstration that the present system is capable of fabricating wveguide devices in active optical media for the implementation of a waveguide amplifier similar to that of Figure 15, Figure 16 shows upconversion in a waveguide amplifier written j 5 inside a Er: Yb co-doped phosphate glass sample. The amplifier is butt coupled to 2 optic fibres in this example. Figure 17 shows an a schematic setup used to characterise the waveguide amplifier (the Device-Under-Test or DUT) of Figure 16 using a 980 run pump laser to amplify a signal beam at a wavelength of 1535 run. The resulting gain spectrum is shown in Figure 18.

[ 00323 ] Example 11 - Waveguide laser 1 ■ 0 [ 00324 ] The coupling structure between the pump and the signal waveguide may take one of a variety of aiτangements. For example, the pump and signal waveguides may be separated in the optical medium but located in close proximity over a desired pumping region so that pump light in the pump waveguide interacts with the signal waveguide to generate a laser beam in the resonator cavity. However, it has been found that this particular arrangement may be 5 disadvantageous for long interaction/pumping regions due to the decrease in pump intensity along the pump waveguide in the pumping regions as it interacts with the signal waveguide. That is, different regions of the signal waveguide may interact with and absorb different amount of pump radiation. Where the signal waveguide also comprises a grating in the pumping region, the average effective refractive index n_β/of the grating may vary along the length of the pumping0 region (i.e. via the Kjramers-Kronig relationship) and may also cause the resonance wavelength(s) of the grating to change in accordance with the Bragg condition m. λ - r^g. Λ (where m is the grating order, λ is the resonance wavelength, and A is the grating period).

[ 00325 ] An alternate arrangement 1900 is shown in Figure 19 (which is an example of a side- pumped geometry) which has the advantage that the level of interaction in the pump guide 1902 varies along the length of the pumping region 1904 with the signal waveguide 1906, which in the present arrangement comprises reflectors 1908 and 1910 at either end of the waveguide in the bulk optical medium 1912 to form a resonator cavity 1920. The device 1900 is thus a waveguide laser device. In operation, pump light 1914 is launched into the pump waveguide 1902 and interacts with the resonator cavity 1920 to generate a laser beam in the resonator cavity, a portion of which is output from one or both ends of the signal waveguide 1906 as output laser beam 1916.

[ 00326 J Example 12 - Waveguide Laser 2

[ 00327 ] This example demonstrates the fabrication and operation of a waveguide laser device, represented schematically in Figure 20, incorporating a waveguide and grating structure in a single Er/Yb co-doped bulk glass optical medium (i.e the optical medium is an active optica medium). The laser structure was based on a DPB architecture and produced a single-frequency laser output that was stable in wavelength.

[ 00328 ] The waveguide laser was fabricated with the presently described system using a regeneratively amplified, low-repetition rate Ti: sapphire femtosecond laser system (Hurricane) from Spectra-Physics. This system produces less than 120 fs, 1 kHz pulses and can deliver an average power of 1 W. Laser pulses at 800 ran were focussed through a 20* microscope objective (Olympus UMPlanFL, NA 0.46) and injected into polished -30x15*3 mm Er Yb co- doped phosphate glass samples from Kigre (QX). Before entering the microscope objective, the polarisation of the laser beam was adjusted using a Berek compensator (New Focus Model 5540) and the physical shape of the laser beam was modified by a 0.500 mm horizontal slit aperture. The polarization controller generated circularly polarized light from the linearly polarized laser beam. The slit (which was orientated with its long dimension in the direction of sample translation) served to expand the laser focus in the direction normal to the laser beam propagation and sample translation. This enabled waveguides with circular symmetry to be written using a low magnification long working distance objective. [ 00329 J The beam exiting the femtosecond laser passed through an automated rotatable 1/2- wave plate and linear polariser, allowing fine control of the pulse energy to be achieved. Pulse energies ranging between 0.5 to 10 μJ, measured after passing through the slit and before focusing, could be used in the formation of optical waveguides. Excess pulse energies such as 2.5 μJ after the slit were found to produce lossy gratings in the QX glass that scatter significant levels of the 980 nm pump radiation. Similarly, relatively low pulse energies such as 0.5 μJ produces a weak grating in the QX glass. A pulse energy of 1.6 μJ, measured after the slit, was used to write the waveguide laser. Note other bulk material would require different pulse energies for optimum writing performance which could be readily determined by the skilled addressee.

[ 00330 ] A computer controlled XYZ stage (Aerotech FA-130) was used to scan the samples in a direction. S perpendicular to the direction of beam propagation k, at a speed of 25 um/s. The pulse train was modulated to yield a period of approximately 500nm to form a first order grating (m - 1) at an effective refractive index («^) of approximately 1 ,5, determined via the Bragg condition

m . A - rieff-. A

[ 00331 J where tn is the grating order, λ is the desired target resonance wavelength, and A is the grating period. The modulation frequency is therefore approximately 50Hz.

[ 00332 ] To generate the periodic refractive index modulation required for a waveguide-Bragg grating the laser output was square-wave modulated in intensity using an external frequency source to interrupt the regenerative amplifier Pockels cell signal. First order grating structures of period « 500 nm were produced giving an approximate laser modulation frequency of 50 Hz. The laser power was selected to create devices with low propagation loss (at both the pump and signal wavelengths) whilst maintaining a periodic grating refractive index contrast to create suitable gratings. Typically the modulation mark-space ratio was 50:50 and the modulation intensity ratio was 100% and it is noted that adjusting these ratios could be used to control the refractive index contrast in the waveguide-Bragg grating. This slow translation velocity and modulated laser method of waveguide-Bragg grating manufacture is different to previously utilized methods that have relied on a two step grating and waveguide manufacturing process or a poiπt-by-point Bragg grating waveguide manufacturing process, that produced a segmented waveguiding-grating region.

[ 00333 ] The bulk glass used in this laser study was a custom melt of Erbium and Ytterbium in a phosphate glass host. The linear waveguide-Bragg grating occupied the complete length of the waveguiding region and was approximately 20 to 25 mm long. The waveguide laser was written at a depth greater than 0.2mm into the glass sample, The waveguide and individual grating periods could be visualized using transmission differential interference contrast microscopy. After waveguide-Bragg grating manufacture the end facets of the glass were ground back by 150 μm and then polished. The grinding back of the end facets is to remove the end-portions of the waveguide-grating feature which may have experienced edge effects during the writing process.

[ 00334 ] The output fibers from two WDMs were butt-coupled to the waveguide end facets with a little index matching gel in the interstitial gap. A 976 nm and a 980 nm laser diode were used to pump the waveguide-laser from opposite ends (for example using optical fibres aligned with either or both ends of the waveguide eg. by pigtailing the optical fibres to the waveguide) and the DFB structure naturally outputted its laser light from both end facets. One of the C band WDM outputs was connected to an optical spectrum analyzer (OSA) for spectrum gathering while the other was connected to a wavemeter for output power and frequency stability measurements. The optic fibres enable introduction of pump laser radiation in either a single or doubled end pumped geometries. A schematic of the waveguide laser written in the bulk glass sample is shown in Figure 20.

[ 00335 ] When un-pumped the waveguide-Bragg grating structure that formed the waveguide- laser was probed using a swept wavelength system (JDS Uniphase) in reflection and transmission to reveal the characteristics of the grating and Figures 21 and 22 show the reflection and transmission spectra of the waveguide laser in its unpumped state. Measurements of this nature are subject to uncertainties because the probe light used to study a C-band grating written in an & doped material will be subject to a varying amount of absorption with wavelength. Therefore a measurement of reflection will be a lower bound of the reflectivity and a measurement of transmission will be a upper bound of grating rejection. Both spectra exhibit a strong peak at the target wavelength of 1537 nm. Note that the target wavelength can be varied by adjusting the modulation frequency of the incident laser pulses used to create the waveguide. [ 00336 ] The transmission and reflection data are not adjusted for the material absorption, fiber- waveguide coupling and WDM propagation losses. The form of the grating structure shows a dominant single Bragg wavelength of approximately 140 pm FWHM in reflection indicating that the waveguide-Bragg grating is of high quality with minimal birefringence. The transmission spectrum of this grating shows a sharp Bragg resonance superposed oo the broad C- band absorption profile of the material. The gradual decrease in transmission up to the Bragg resonance is indicative of the radiation modes of the grating in a geometry that has no well defined cladding (this contrasts with the usually discrete fiber-Bragg grating cladding mode structure). [ 00337 ] Figure 23 shows a spectra of the waveguide laser when pumped with greater than 100 mW of 980 and 976 ran light. Note that the launch efficiency of the waveguide laser is unknown so the pump energy deposited into the active region of the waveguide laser is also unknown. The laser spectra exhibits a strong, narrow linewidth peak at the target wavelength of 1537 nm. [ 00338 ] The apparent linewidth of the laser in Figure 23 is limited by the slit width of the OSA (10 pm). The wavelength of the laser was 1537.624 nm and could be adjusted by changing the temperature of the sample wherein the *^. and the <%. contributions to the grating parameters changed the Bragg wavelength. The output power of the laser emanating from each facet was estimated to be -7.3 dBm or 0.19 mW (measured after waveguide/fiber coupling losses) giving a total of 0.37 mW available output power. The laser was observed to operate at a highly stable single wavelength. The wavelength drift of the laser was measured over a period of 5 minutes (300 s) and was observed to be 6 pm. This drift was most likely due to variations in the temperature of the waveguide-Bragg grating/laser structure. The threshold pump power for laser action was 639 mW (combined). [ 00339 ] This example of the demonstrated operation of a directly written monolithic waveguide-laser is believed to be the first of its kind, and is significantly important to the field of laser physics as it opens the possibility for creating narrow linewidth lasers in bulk glasses without the use of external mirrors, and gratings or other wavelength selective components. The techniques for creating a laser demonstrated herein are compatible with other existing waveguide architectures such as splitters, couplers, amplifiers, and offers the potential for devices such as DBR waveguide-lasers, laser arrays, coupled lasers and more sophisticated waveguide and pump delivery technologies. This combination of device potential, genuine 3 -dimensional capability and ease of manufacturing significantly expands the gamut of devices and applications that the direct-write technique can enable.

[ 00340 ] The waveguide amplifier and waveguide laser devices and methods for fabrication of 5 those devices described herein, and/oi shown in the drawings, are presented by way of example only and are not limiting as to the scope of the invention. Unless otherwise specifically stated, individual aspects and components of the devices and/or fabrication methods may be modified, or may have been substituted therefore known equivalents, or as yet unknown substitutes such as may be developed in the future or such as may be found to be acceptable substitutes in the future. i o The devices and/or fabrication methods may also be modified for a variety of applications while remaining within the scope and spirit of the claimed invention, since the range of potential applications is great, and since it is intended that the present devices and/or fabrication methods be adaptable to many such variations.

Claims

Claims:

1. An optical waveguide device comprising: at least one optical waveguide formed within a optical material, the waveguide having a central longitudinal axis; and; at least one grating formation within the bulk optical material co-axial with the axis of at least one optical waveguide,

2. An optical device as claimed in claim 1 wherein the optical material is a bulk optical material and the at least one optical waveguide is disposed within the bulk optical material.

3. An optical device as claimed in claim 2 wherein the optical material is a bulk optical material and the at least one optical waveguide is embedded in the bulk optical material.

4. An optical device as claimed in claim 2 wherein, in use, light propagating in the at least one optical waveguide experiences minimal birefringence.

5. An optical waveguide device comprising: a bulk optical medium comprising a signal inlet and a signal outlet and a pump inlet and a pump outlet; a signal waveguide extending from the signal inlet through the optical medium to the signal outlet, said signal waveguide being capable of transmitting light from the signal inlet to the signal outlet; and a pump waveguide extending from the pump inlet through the optical medium to the pump outlet, said pump waveguide being capable of transmitting a pump beam from the pump inlet to the pump outlet wherein the optical waveguide device is adapted to cause at least a portion of the pump beam transmitted in the pump waveguide, in use, to be coupled with the signal waveguide 6, An optical device as claimed in claim 2 wherein the signal inlet and the signal outlet are associated with a first and second extremity of the optical medium respectively.

7. An optical device as claimed in claim 2 or claim 6 wherein the signal inlet and the signal outlet are associated with a third and fourth extremity of the optical medium respectively.

8. An optical device as claimed in claim 7 wherein the signal inlet and the pump inlet are spaced from, one another.

9. An optical device as claimed in any one of claims 2 to 8 wherein the pump waveguide is adapted to be evanescently coupled with the signal waveguide. s 10. An optical device as claimed in any one of claims 2 to 9 wherein the signal and the pump waveguide each have minimal polarisation dependence for light propagating therein in use.

11. An optical device as claimed in any one of claims 2 to 9 wherein the signal and the pump waveguides each comprise a corresponding central longitudinal axis, and wherein eithero the signal waveguide, the pump waveguide or both the signal and the pump waveguide each comprise at least one grating co-axial with the corresponding central longitudinal axis over at least a portion of its length.

12. An optical device as claimed in claim 11 wherein the paring is a Bragg grating.

13. An optical device as claimed in claim 12 wherein the grating is a distributed feedbacks Bragg grating.

14. An optical device as claimed in claim 1 J wherein the grating comprises alternating adjacent regions of different refractive index co-axial with the axis of the waveguide.

15. An optical device as claimed in any one of claims 2 to 14 wherein the optical device is an optical amplifier wherein the optical medium is doped with an active ion optical device such0 that, in use, light launched in the pump inlet is coupled to the signal waveguide and interacts with a light signal propagating in the signal waveguide thereby to amplify the light signal.

16. An optical device as claimed in any one of claims 2 to 14 wherein the optical device is a waveguide laser wherein the optical medium is doped with an active ion and the signal waveguide comprises at least on optical reflector therein defining a resonator cavity. 5 17. An optical device as claimed in claim 16 wherein the optical reflector is at least one grating formation co-axial with the signal waveguide.

18. An optical device as claimed in claim 16 wherein the optical reflector is at least one distributed feedback Bragg grating formation co-axial with the signal waveguide.

19. An optical device as claimed in any one of claims 11 to 14 wherein the grating is co- axial with and symmetrical about the central longitudinal axis of the waveguide.

20. A laser comprising: an optical waveguide embedded in a bulk optical material, the waveguide having a central longitudinal waveguide axis:

5 at least one grating formation in the waveguide co-axial with the waveguide axis; and a pump means adapted to deliver a pump beam to the optical waveguide.

21. A laser as claimed in claim 20 wherein the coaxial grating is either a single distributed reflector or a pair of spaced reflectors, the coaxial grating defining an optical resonator in the waveguide, and wherein the coaxial grating is adapted for refection of a desired wavelength of i o light such that light of the desired wavelength is capable of oscillating within the resonator cavity.

22. A laser as claimed in claim 21 wherein the coaxial grating is a single distributed grating defining a resonator cavity in the waveguide, wherein the single distributed grating extends substantially the entire length of the waveguide. is 23. A laser as claimed in claim 20 wherein the grating is a Bragg grating and is adapted for either reflection oτ transmission of a desired wavelength of light according to the Bragg condition.

24. A laser as claimed in claim 20 wherein the grating is a distributed feedback Bragg grating. 0 25. A laser as claimed in claim 20 wherein the grating comprises alternating adjacent regions of different refractive index co-axial with the central longitudinal axis of the waveguide.

26. A laser as claimed in claim 20 wherein the grating is a first-order Bragg grating (w=l)for a desired wavelength λ according to the Bragg condition m . λ = n^. Λ , where n^ is the effective refractive index of the grating and Λ is the grating period. 5 27. A laser as claimed in claim 25 wherein the spacing between the alternating adjacent regions of different refractive index co-axial with the central longitudinal axis of the waveguide is configured according to the Bragg condition m . λ = n^. Λ for a desired wavelength λ where m is the order of the grating , n_ejgr is the effective refractive index of the grating and A is the grating period.

28. A laser as claimed in claim 27 wherein the positional error in the spacing between the alternating adj acent legions of different refractive index co-axial with the central longitudinal axis of the waveguide is less than 12% of the optimal spacing determined by the grating period A. s 29. An optical device as claimed in any one of claims 20 to 28 wherein the grating is coaxial with and symmetrical about the central longitudinal axis of the waveguide,

30. A system for fabrication of an optical device comprising: a pulsed laser source; o a pulse control means for modifying the pulses from the laser source; a pulse focussing system for directing pulses from the laser source to a fabrication location in space; a sample stage for holding and positioning a sample in which the optical device is to be fabricated; and 5 a stage controller means for controlling the location of the sample stage with respect to the fabrication location.

31 , The system as claimed in claim 30 wherein the pulse control means is capable of synchronising the positioning of the sample stage with respect to the pulse control means of the pulse laser source. 0 32. The system as claimed in claim 31 wherein the pulse control means is capable of configuring of the pulses of the laser system with respect to the positioning of the sample stage with respect to: configuring the pulse energy of the pulses, configuring the repetition rate of the pulses, configuring the duty cycle of the pulses; configuring the path the sample stage takes during fabrication; configuring the number of times the sample stages traverses a path or a5 portion thereof, configuring the speed or velocity of the sample stage for desired portions of the waveguide structure.

33. The system as claimed in claim 31 wherein the pulse control means is capable of configuring the laser pulses for the formation of a grating formation co-axial with a waveguide formation in the optical device.

34. The system as claimed in claim 31 wherein the pulse control means is capable of configuring the laser pulses for simultaneously forming a grating formation co-axial and a waveguide formation in the optical device

35. A method of forming an optical device comprising the steps of: placing a bulk optical material within which the device is to be fabricated onto a translation stage; configuring the pulses of a pulsed laser system and the positioning of the sample stage for the writing of a desired waveguide device structure within the bulk optical material; providing a pulsed laser signal to the optical material at a fabrication location within the optical material; and translating the sample on the sample stage in accordance with requirements, wherein the position of the sample stage and the pulsed laser signal are synchronised wherein the ^' laser pulses modify the refractive index of the optical material in the fabrication location to form at least one waveguide within the optical material to form the optical device.

36. A method of forming an optical device as claimed in claim 31 wherein the configuring of the pulses of the laser system comprises one or more of; configuring the pulse energy of the pulses, configuring the repetition rate of the pulses, configuring the duty cycle of the pulses.

37. A method of forming an optical device as claimed in claim 31 wherein the configuring of the positioning of the sample stage comprises one or more of: configuring the path the sample Stage takes during fabrication; configuring the number of times the sample stages traverses a path or a portion thereof, configuring the speed or velocity of the sample stage for desired portions of the waveguide structure,

38. A method of forming an optical device as claimed in claim 31 wherein the step of configuring the pulses of a pulsed laser system and the positioning of the sample stage for the writing of a desired waveguide device structure within the bulk optical material is adaptable for the writing of a grating formation co-axial with the central longitudinal axis of a waveguide formed within the optical material

39. A method as claimed in claim 38 wherein the step of configuring the pulses of apulsed laser system and the positioning of the sample stage for the writing of a desired waveguide device structure within the bulk optical material is adaptable for the simultaneous writing of a grating formation co-axial with the central longitudinal axis of a waveguide formed within the optical material