EP1433022A2 - Non-linear optical stacks - Google Patents

Abstract

A non-linear optical device such as a frequency changer, parametric amplifier or mixer comprises a stack of grating layers (1, 2, 3 )of non-linear optical material. Each grating layer comprises at least one grating (5) and is bonded in spaced relation to adjacent grating layers (4), preferably by a glass consisting essentially of germanium, arsenic, selenium and a relatively high proportion of tellurium. As described, the gratings (5) in a layer are spaced and formed by periodic electrical poling. The periodicity in adjacent layers may be the same or different. The layers may be of material useful in the infra-red. The layers may be formed of GaAs, ZnSe, ZnGeP2, lithium niobate, potassium, titanyl phosphate of rubidium titanyl arsenate.

Description

Non-Linear Optical Stacks

The present invention relates to optical devices comprising a stack of two or more layers of non-linear optical material.

Non-linear optical materials are useful for the production of new wavelengths via conversion of existing laser wavelengths (e.g. M Fejer, Physics Today, May 1994)). Common applications of non-linear optical materials include frequency-doubling and optical parametric oscillators. In such devices, fundamental (input) and generated (output) optical waves propagate together through the material and energy is transferred from the fundamental to the generated waves (forward-conversion). The reverse process, in which energy is transferred from the generated waves to the fundamental wave(s), can also occur. This is known as back-conversion.

The direction and efficiency of the non-linear process is determined by the phase relationship between the various interacting waves. In general, the phase relationship changes as the waves propagate through the non-linear optical material, resulting in alternating sections of forward- and back- conversion. The maximum useful length of material is the length of one such section and is known as the coherence length for the material. This is the distance over which the interacting waves retain a suitable phase relationship for forward conversion. If the waves propagate beyond this distance, energy is transferred back from the generated waves to the fundamental wave. In general, the coherence length is too short to be useful unless a technique known as phase-matching can be employed.

The most common form of phase matching requires non-linear optical materials that are birefringent crystals. The direction of propagation and polarisation of the various waves is chosen so that a suitable phase relationship for forward conversion is maintained throughout the crystal and the coherence length is longer than the crystal.

However, the phase-matching constraint defines the direction of propagation, which may not be the direction for which the non-linear interaction is strongest. Also, the various beams may 'walk-off and become spatially separated within the crystal. It is not always possible to satisfy the phase-matching condition at all wavelengths for which the crystal is transparent. Furthermore, the number of non-linear materials suitable for use in parts of the infrared spectrum which are also birefringent is rather limited.

An alternative form of phase matching is known as quasi-phase matching. In this technique the non-linear optical material is divided into sections exactly one coherence length (or an odd multiple of coherence lengths) long, either by producing striated crystals during growth, or by appropriate poling of a crystal, or by assembly of multiple layers with different crystal orientations. Within each section a useful nonlinear optical interaction can occur as the waves get progressively out of phase. If the material axis is reversed in the following section then the interaction can continue. By reversing the axis in successive sections in this manner, an arbitrary interaction length can be achieved (J McMullen, J Appl Phys, vol 46 no 7, p3076-3081). Quasi-phase matching allows the use of non-linear optical materials that cannot be phase-matched using birefringence. This is of particular importance in optical wavebands for which there are few good birefringent non-linear optical materials available.

Striated crystals and poled crystals tend to be useful only for low aperture devices, because of their manner of production. However, the assembly method has potential for providing larger aperture devices useful in high power applications where it is necessary to minimise the possibility of optical damage to the device, although a primary difficulty in assembling different sections into a stack structure lies in producing a quasi-phase matched structure of sufficient quality to be useful. There are major problems both in providing high quality optical joins between adjacent sections, and in producing or maintaining accurate control of section lengths and distances between sections, and such problems can be acute in this type of assembly, which typically comprises from tens to over a hundred non-linear layers. The assembly of sections of non-linear optical material is generally addressed in our copending UK Patent Application No. GB 0123740.3, which relates generally to a non-linear optical device comprising a stack of at least two layers of non-linear optical material, adjacent said non-linear layers being joined by a glass layer. In embodiments thereof, the intended optical propagation direction, or at least a component thereof, is along the stacking direction, transverse to the layers. Such assemblies closely resemble a periodically poled crystal formed from a block of non-linear optical material, in which the block is effectively divided into a series of parallel laminar regions, alternate regions having different poling directions, the intended optical propagation direction, or at least a component thereof, being transverse to the planes of the laminar regions. The alternating poling pattern is commonly termed a grating, with the grating direction being that along which the alternating pattern is encountered. The laminar regions thus provide a one- dimensional grating, but clearly it would be possible to have 2 or 3 dimensional grating structures where an alternating pattern is encountered along 2 or 3 dimensions.

As discussed for example in US Patent No. 6,064,512 (Byer) it is at present not possible to produce useful poling patterns in thick layers of lithium niobate, one of the most common non-linear materials, because the necessary electric field rapidly approaches the dielectric breakdown value, and even where poling is successful the domain pattern fidelity degrades in thick plates.

The latter problem also occurs in the production of striated crystals, where a domain pattern is established in a crystal surface which is then etched, and subsequent crystal growth thereon propagates the pattern. The production of striated crystals is also expensive and time-consuming.

Neither poling nor striated crystal growth provide the opportunity to form a three- dimensional domain pattern in a single crystal plate or layer.

Solutions to these problems suggested in US Patent No. 6,064,512 (Byer) include, inter alia, the diffusion bonding of a stack of two-dimensional patterned plates. Depending on the patterning in individual plates and their relative alignments, it is possible to produce a thick structure with a one, two or three-dimensional grating pattern. M J Missey et al in Optics Letters, vol 23, No 9, 1^st May 1998, 664-666, also describes using diffusion bonding of stacks of periodically poled lithium niobate (PPLN). In the methods disclosed in both of these articles, the domain gratings of the sheets are aligned, giving rise to structures with an extended aperture dimension in the direction of the sheet thickness. Because of the low optical damage threshold of common non-linear materials, the ability to pump over a relatively large aperture should offer improved power handling capabilities. However, recent work on optical parametric oscillators and generators using one-dimensional grating structures, particularly those which extend significantly transversely to the intended optical propagation direction, has shown that when they are pumped over their full aperture, generated optical beams suffer large levels of beam divergence and spectral broadening. As discussed for example in M Missey et al, Optics Letters 25, No 4, p248, February 2000, which particularly describes an arrangement where the gratings have different periods, it has been found that this effect can be significantly reduced if the material is formed with a plurality of discrete gratings in one dimension, each separated by a thin region of unpoled material, i.e. a thin crystal layer consisting of a plurality of spaced (parallel) gratings.

When flood illuminated by a pump beam each grating can be considered as a discrete (relatively low aperture) channel of gain providing non-linear conversion. The use of individual gain channels prevents the build up of off-axis beams as the gain in these beams as they propagate across poled/unpoled boundaries is significantly reduced. The presence of off-axis beams normally results in higher-order modes and poor beam quality from single-grating poled samples. Also, when the gratings have the same period, optical coupling can occur between neighbouring channels, ensuring that the generated beams are coherent. The multiple beams generated can then be coherently combined by various techniques to form a single beam of lower divergence than that achievable from a single grating poled sample.

Single sheets with multiple gratings can be relatively easily formed. Probably the simplest way of forming such a structure in a thin crystal layer is by poling, with the non-grating material being constituted by regions of unpoled material; however, other approaches, for example using crystal growth methods, or assembly of a sheet of spaced gratings with laterally interposed bonding glass layers (as in our copending UK Patent Application No GB 0123731.2) are not ruled out in the practice of the present invention. The aperture in one direction in a single sheet construction is determined by the thickness of the sheet. In the case of PPLN, single sheet thickness is limited to 1 mm. When a large effective aperture is required in this direction it will again be necessary to subdivide this dimension into channels to avoid the production of off-axis beams to any significant extent, and, in the case of gratings of equal period, to provide the additional benefit of useful optical coupling between the channels.

Grating structures with two-dimensional channelling are discussed in detail by S M Russell et al in IEE J Quantum Electronics, vol 37 no 7, July 2001, 877-887. However, there remains the problem of actually making such a construction.

In a first aspect, the present invention provides a non-linear optical device comprising a stack of grating layers of non-linear optical material, each grating layer comprising at least one grating, adjacent grating layers being bonded together in spaced relation.

While a simple stack may comprise just two grating layers, preferably there are at least three such layers, more preferably at least 5, and even more preferably at least 10.

In devices according to the present invention, the optical loss at the various optical interfaces, primarily due to Fresnel reflection, is desirably kept to a minimum in order to ensure long-range optical coupling between the various grating layers.

In the arrangements described in US Patent No. 6,064,512 (Byer), bonding is effected by optical cement, direct optical contacting, or diffusion bonding (also in the article by Missey). However, it is known that the presence of gaps or spaces between faces which should be in optical contact can arise from such sources as imperfections in the surfaces to be joined, or the presence of contamination at the surfaces, particularly particulate contamination, and leads inter alia to optical losses due to reflection (because of index mismatch) and scattering.

Direct interfacing requires very accurate physical preparation of the surfaces of the components, and extremely high standards of cleanliness. For this reason, while this technique is used in practice, it is difficult and best avoided if possible. The diffusion-bonding approach is difficult to realise due to a number of practical considerations. The individual layers must be highly polished and optically flat and the surfaces of the layers must be free of particulate contamination. It has been observed that a particle as small as 1 μm can result in a void between layers with a diameter of 1 mm (D Bollmann et al, Jpn J Appl Phys, vol 35, pp 3807-3809). Such voids are a source of optical loss, which can build up significantly over a stack of many layers. The temperature and pressure needed to diffusion bond the layers may also degrade the optical properties of the material (D Zheng et al, J El^'ectrochem Soc, vol 144, no 4, pp 1439-1441).

Bonding of components with an intermediate layer of a material such as an adhesive can serve to reduce or remove light loss and so increase device efficiency. For example, Canada Balsam, which has a refractive index very similar to that of conventional silica based optical glasses, was used for many years as an adhesive for mounting microscope slides and for joining lens components. Many modern optical components are of materials with indices much greater than conventional glass, for example semiconductor electro-optic devices and lenses, and this commonly occurs with components for use in the infrared region, for example. The refractive indices of conventional adhesive compositions fail to match these greatly increased indices, so that reflection at the interfaces is again increased, and it has often been necessary to resort to direct interfacing. Each intermediate layer of adhesive or other bonding material produces two interfaces, and if the joint is sufficiently bad it is even possible that optical performance could deteriorate beyond that of two directly coupled components.

Our copending UK Patent Application No. GB 0123731.2 relates generally to method of joining opposed surfaces of two optical components, the method comprising the steps of providing at least one said surface with a thin layer of bonding glass having a glass transition temperature Tg substantially lower than the glass devitrification temperature Tel, placing the said surfaces together with only the coating or coatings therebetween to form an assembly, and heating the assembly under pressure to a temperature Tb which lies between Tg and Tel and is sufficiently high to soften the glass and bond the components together. Preferably both surfaces to be bonded are coated with the bonding glass. The bonding glass is preferably substantially index matched to the layers that it joins. This method may be used in forming the non-linear optical devices of the present invention.

Our copending UK Patent Application No. GB 0123740.3 relates generally to a non- linear optical device comprising a stack of at least two layers of non-linear optical material, adjacent said non-linear layers being joined by a glass layer. By suitably arranging the layers of non-linear material a grating may be produced.

Preferably in devices according to the present invention, adjacent grating layers are bonded by at least one glass layer. Bonding may be effected by depositing a thin glass coating on one or both surfaces to be joined, and bringing the surfaces together with the coating or coatings therebetween and the applying heat and pressure.

In some cases, particularly where a relatively large spacing between layers is required, an optically inert intermediate layer, for example a layer of the unpoled non-linear material is provided between the grating layers to be bonded, being bonded to each adjacent surface by a glass layer. Again, the bond between a surface of the intermediate layer and a respective adjacent grating layer surface may be achieved by coating one or both surfaces with a thin glass coating prior to the use of heat and pressure.

To facilitate optical coupling between the different grating layers the glass preferably has a refractive index approximating the index of the non-linear optical material. Because the optical coupling is lateral to the propagation direction the index matching requirement is somewhat relaxed in relation to that for bonding a stack extending along the propagation direction. The refractive index of the glass preferably lies within 30%, more preferably within 10%, and even more preferably within 5%, of the index of the non-linear material, and is ideally substantially equal thereto. Should the bond be effected between non-linear materials of different refractive index, the refractive index of the glass should ideally take a value equal to the square root of the product of the two indices, and otherwise has preferred ranges as immediately above but measured relative to this square root value. Preferably each grating layer comprises a plurality of gratings, preferably one- dimensional. Preferably the gratings belonging to an individual grating layer are substantially identical. Preferably they have parallel grating directions.

In one preferred embodiment, e.g. for generating a single wavelength output, the gratings in all of the grating layers are substantially identical, and preferably they are substantially parallel from layer to layer. In the embodiments to be described, the optical propagation direction is parallel to the direction of all the one-dimensional grating directions. The effect is therefore merely to increase the aperture of the overall arrangement by providing an array of low aperture gratings distributed in two dimensions across the input optical beam and extending in the incident light direction. However, the skilled person will realise that it would be possible to achieve the same effect by providing grating layers which comprise gratings of different periodicity, provided that their inclinations to the incident light direction are such that the grating periodicity in the optical propagation direction is identical for each grating.

It is also known to use gratings having different periodicities relative to the optical propagation direction (for example gratings of differing absolute periodicity and/or gratings with different inclinations to the propagation direction, or combinations thereof), particularly when it is desired to process a number of different optical wavelengths simultaneously. Thus the invention encompasses constructions where the gratings in each layer and/or in neighbouring layers are not identical, for example by having differing periodicities, and or where the gratings are not substantially parallel from layer to layer.

Preferably each grating layer is formed by appropriately electrically poling a single thin layer of non-linear optical material. However, alternative methods of producing a grating layer could be adopted, for example by adapting the striated crystal growth method or by lateral bonding of the individual gratings and intermediate regions, or by a combination thereof. It would even be possible to produce an individual grating in the manner outlined in our copending UK Patent Application No. GB 0123740.3 mentioned above. Nevertheless, poling is at present the easiest and most practical method of making a single grating layer, particularly in view of the relative and absolute dimensions commonly involved, and it normally presents relatively little difficulty in forming individual grating layers of the necessary width and length. Where difficulty is encountered, it would be possible to assemble a plurality of devices according to the first aspect of the invention to provide the requisite length and/or width. Such assembly preferably includes the step of bonding adjacent stacks together, preferably using an optical glass layer, preferably of the same material as is used to join the layers of an individual stack. Generally, the preferred manner of joining stacks together is similar to the preferred manner of joining different layers of an individual stack together, as described above, and may or may not include the interposition of an intermediate layer of optically inert material such as a layer of unpoled non-linear optical material.

Typical non-linear materials for use in the invention are lithium niobate (LiNbO₃ or LN), potassium titanyl phosphate (KTiOPO₄ or KTP), rubidium titanyl arsenate (RbTiOAsO₄ or RTA), gallium arsenide GaAs, zinc selenide ZnSe, and zinc germanium phosphide ZnGeP₂.

The first three of these materials are suitable for use in the visible, near and mid infrared wavelengths, and have relatively low refractive indices. For such materials, a suitable bonding glass would be arsenic trisulphide, with a refractive index n=2.4 and a softening temperature of about 210°C.

The other three materials find more use in the mid to far infra-red regions, and have relatively high refractive indices. For such materials a suitable bonding glass would be a chalcogenide glass, and infrared transmitting glasses based on the Ge-As-Se-Te (GAST) system have been prepared with refractive indices in the range n=3.00 to 3.45. One range of preferred glasses has the general formula Ge_(X-a)As_aSe₍₁oo._x- Te_b where 25<x<55 (preferably 25<x<40); 10<a<25; 40<b<70, and (100-x-b)>0 (see our copending UK Patent Application No. GB 0123743.7). Useful compositions Ge₁₅As₂₅Se₁₄Te₄₆, Ge₂oAs₂₀Se₁₄Te₄₆, and Ge₁₅As₁₅Se₅Te₆₅, and the glasses of Figures 4 to 6 also conform to this formula. More preferably, particularly for example where the substrate is of GaAs or silicon, 30<=x<=40 and 50<b<=70. In these ranges there exist glasses having good to very good thermal characteristics with indices closely matching those of GaAs and ZnGeP .

Either or both ends of the stack of the device according to the invention may be supplemented by a protective layer. Techniques for improving the optical damage threshold characteristics of optical crystals have been described. See, for example, DeMaria et al, "Apparatus for improving the optical intensity induced damage limit of optical quality crystals", US Patent No. 5,680,412 October 1997. These involve the attachment of thick pieces (several mm) of optically transparent material to the end- faces of an optical crystal. Materials are usually selected that offer improved optical damage and thermal characteristics over those of the bulk optical crystal and in some cases offer improved surface finishes for subsequent dielectric coating.

The principal requirement for the material is to have a refractive index similar to that of the bulk non-linear optical crystal in order to minimise reflection loss at the material/crystal interfaces. An example of such a selection might be the attachment of GaAs (n=3.3) to ZnGeP crystals (n=3.1) to produce improved non-linear optical crystals in the mid-infrared. Attachment techniques demonstrated to date are based on direct optical contacting. This requires careful surface preparation of both the bulk crystal, which may in itself be difficult, and the end-face material.

Preferably, a device according to the present invention includes a protective window which is bonded to an optical input and/or output face of the stack via a glass layer. The glass layer may be chosen and provided according to the criteria laid down above when discussing bonding the non-linear optical layers. The attached material can take the form of a plane layer, e.g. a disc, or a Brewster cut prism which further avoids the need for an AR coating.

Alternatively, or additionally, an optical input and/or output of the stack (with or without a protective layer) may be provided with a dielectric or anti-reflection coating. This may be a conventional dielectric coating, or it may be formed of a glass coating (preferably but not necessarily of the same material as is used to bond the non-linear optical layers together) in which is formed a "moth-eye" structure for example by embossing or etching, as more particularly described in our copending UK Patent Application No. GB 0123744.5. As explained therein, the glass coating overcomes difficulties associated with the adherence of conventional dielectric anti- reflection coatings on certain substrates, and also with the difficulty of preparation of the optical surfaces of some materials, since it can surround surface defects and contamination, and, if necessary, can itself be polished relatively easily. Again, the glass layer may be chosen and provided according to the criteria laid down above when discussing bonding the non-linear layers.

The top and/or bottom of the stack may be provided with an additional layer also bonded thereto by a glass layer. When pumping the stack it is necessary to ensure that the pump beam diameter is sufficiently small in the layer thickness direction to avoid clipping the edge(s) of the outermost layers, since otherwise multiple reflections (total internal reflection) can give sufficient gain to support off-axis modes, leading to poor beam quality and spectral broadening. Where the additional layer(s) has a refractive index close to that of the non-linear material of the grating layers, for example being of the same material but unpoled, the aperture of the stack is effectively somewhat increased although the dimension of the gain region is unaltered. Slightly larger pump beams can then be used without fear of the detrimental effects just mentioned.

Alternatively or additionally, the additional layer(s) may be of a material or a form providing a heat sinking or transmitting function, for example a diamond layer for transmitting heat to a larger sink or from an oven (in practice PPLN devices need to be operated at elevated temperatures).

A device according to the invention may be transmissive in part of the infrared spectrum. However, the invention extends to devices for use in the visible and/or UN parts of the spectrum. Devices according to the invention find application in many non-linear arrangements, including frequency changers, parametric amplifiers, and mixers.

Further features and advantages of the invention will become clear upon a reading of the appended claims, to which the reader is referred, and also upon consideration of the following more detailed description of embodiments of the invention, made with reference to the accompanying drawings, in which: Figure 1 shows in schematic form a first embodiment of a device according to the invention, with simple glass layer bonding, used with flood illumination;

Figure 2 shows in schematic form a second embodiment of a device according to the invention, with glass layer bonding incorporating an intermediate layer, again used with flood illumination;

Figure 3 shows in schematic form the embodiment of Figure 1 used with discrete optical pumping; and

Figures 4 to 6 are differential thermal analysis plots for three different chalcogenide glasses.

Figure 1 shows three grating layers 1 to 3 stacked vertically, and bonded by thin glass layers 4. As shown, the layer 1 comprises three one dimensional gratings 5 formed by electrical poling, which gratings run parallel to each other in the direction 7 of incident light and are spaced by regions of strips 6 of unpoled material. Both gratings 5 and regions 6 extend the full height of layer 1. Layers 2 and 3 are substantially identical to layer 1.

It is important to note that the number of grating layers, the number of gratings in each grating layer, and the structure of each grating (dimensions, periodicity) are schematic only, and will be determined by known considerations and the application in hand. Typically, however, a layer 1 , for example of lithium niobate, will have a thickness of 1 mm, a length (in the optical direction) of 50 mm and a width of 10 mm. Each grating will typically be 1 mm wide and be separated from adjacent gratings in the layer by 50 microns of unpoled material. The thickness of the glass layers 4 is typically 50 microns, but may be, for example, in the range 10 to 100 microns.

It is also important to note that although, as shown, the gratings are in vertical and horizontal register, this is not strictly necessary. Any grating may or may not be in register with other gratings. Furthermore, although, as shown, the gratings are all substantially identical, this is not necessary, and it is possible to form devices where the gratings of different layers have different widths and/or periodicities, for use over a wavelength range, or with a plurality of wavelengths, for example. In addition although the grating layers are shown as having substantially the same thickness, this is not strictly necessary, and different grating layers may have different thicknesses.

The device of Figure 2 closely resembles that of Figure 1, but the grating layers are bonded together by a sequence of a glass layer 8, an intermediate layer of unpoled non-linear optical material 9 and a second glass layer 10.

Figures 1 and 2 illustrate a use of the device where the incident illumination is directed broadly over the whole end face 11 of the stack. It is believed that index matched glass layers 4 are particularly helpful in increasing the optical efficiency of the device when used in this manner. Low optical loss in the layers 4 facilitates flood illumination under conditions where damage could otherwise occur at the optical interfaces. Such damage can lead to distortion of the pump beam and disruption of the neighbouring optical regions. Furthermore, choosing a glass which is index matched to the non-linear material aids cross talk between channels, if required, and also prevents or reduces distortion of the pump beam, since the boundaries between the layers of non-linear material and the intervening glass layers are then substantially optically transparent.

Figure 3 shows a modified use of the Figure 1 device where each layer 1 to 3 is individually illuminated by a respective shaped incident beam 1_\ to 7₃. Whether flood or individual illumination is used, the beams 12 leaving the three layers will commonly be combined by means known per se (where different layers have gratings of different geometries, the individual beams may be combined, or treated separately). This modified use could clearly be used with the Figure 2 device.

In making the device of Figure 1, the grating layers 1 to 3 are provided by wafers of suitably oriented non-linear optical material which have been electrically poled to produce the optical gratings 5 in the form of regions of alternating poling direction, and the unpoled strips 6. The wafers, now the grating layers 1 to 3, are then coated with low glass transition point Tg glass 4 using any standard method. Preferably the coating method provides precise thickness control during deposition and/or the thickness of the coating may be adjusted subsequently by removal of part of the coating. Examples of suitable methods include sputtering, flash evaporation, spin coating and solvent evaporation. To improve wetting of the non-linear optical material it is convenient to deposit approximately half the required thickness, e.g. 25 microns, of low glass transition point Tg glass onto both sides of each wafer, rather than depositing it all on one surface.

The stack is then assembled by placing the grating layers one on top of another in the correct orientation with the gratings of the different layers parallel. Care should be taken to prevent the incorporation of undue particulate material between the layers. However, small particles of diameter less than the thickness of the glass layer, although undesirable, will not prevent the layers from bonding since the glass will ultimately flow around and engulf them on heating. Provided that their optical loss characteristics are not too high, they will not unduly compromise the performance of the stack at high laser fluences. Imperfections such as roughening or scratches introduced by the processing can similarly be accommodated.

The stack of wafers is then placed in an oven and heated to the softening point of the glass. The atmosphere within the oven may be controlled, e.g. to remove air which would otherwise become trapped within the finished structure. While the structure is hot, pressure is applied to force out any remaining air and to encourage the glass to flow and adjacent layers to bond. The temperature and pressure are maintained for a suitable period before the oven is finally cooled. The finished structure can then be removed from the oven and used as-is, with the optical propagation direction parallel to the two-dimensional array of stripes so formed, or anti-reflection coatings may be applied to the ends to improve the efficiency of the device.

Although this particular method employs simultaneous bonding of all the grating layers of a stack, and as such is most preferred, it is equally possible to add at least some grating layers separately in a separate bonding process, or to use any intermediate variation.

Optionally, one or each of the incident and exit ends of the stack is also provided with a glass coating, which is then embossed or etched with a "moth-eye" antireflection pattern. This glass coating may be deposited before or after assembly of the stack, preferably after, and the pattern is formed therein after the stack is assembled. However, in a variation, each individual grating layer may be so provided with a "moth-eye" antireflection pattern, either before or after assembly of the stack. In a further variation, one or each of the incident and exit ends of the stack is supplemented with a layer of an optically protective material, with or without provision of an outwardly facing "moth-eye" glass coating antireflection pattern or other antireflection layer.

The glass is chosen to have a refractive index close to that of the wafer material to minimise the reflection losses. The glass must also have good optical transmission at the intended operating wavelengths.

For non-linear conversion in the visible, near infrared and mid infrared bands the wafers could be of periodically poled lithium niobate (PPLN), which has a refractive index of n=2.2, periodically poled KTP (PPKTP) or periodically poled RTA (PPRTA). Arsenic trisulphide As₂S₃ is a suitable low glass transition point Tg glass with a refractive index n=2.4 and a softening temperature of about 210°C. The maximum operating temperature of arsenic trisulphide glass is about 150°C, which is sufficiently high to allow heating of the bonded stack to prevent problems associated with photo-refractive damage.

For use in the mid-infrared waveband, e.g. with GaAs, ZnSe or ZnGeP₂ stacks, chalcogenide glasses have the required properties. Glasses in the Ge-As-Se-Te (GAST) system which satisfy the index-matching condition for GaAs have been identified, and samples of the glass have been prepared.

The choice of preferred glass for use in the invention, including its thermal properties and relative refractive index, will be further explained with reference to Figures 1 to 3, which are differential thermal analysis plots for three different chalcogenide GAST glasses.

In addition to a glass devitrification temperature Tc, corresponding to a change from a glassy phase to a melt phase or a crystalline phase, many glasses have at least one glass transition temperature Tg where a glassy phase is retained but with somewhat different properties. In particular, heating the glass through a temperature Tg to obtain a higher temperature glassy phase (the reader will appreciate that the phase change may require other conditions, and in particular the phase change may take a significant time) can provide a phase which is appreciably softer or more mobile.

Glass phase transitions may be detected by differential thermal analysis, wherein heat is supplied at a controlled rate to a sample and the temperature of the sample is plotted over time. During differential thermal analysis the temperature initially follows a generally linear plot, and phase transitions are indicated by deviations from linearity. In particular a glass transition temperature Tg may be identified by a discontinuity in the plot, generally in the form of a knee. Further transition points may be identified at higher temperatures, and at least one of these may correspond to the devitrification temperature. The latter may be identified since upon performing the reverse measurement by cooling the sample the corresponding knee is absent or at least does not occur at the same temperature.

Figure 4 shows a differential thermal analysis plot for the material Ge₁₅Asi₅Se₂ Te₄ι using the following cycle:

1. Hold at 20.00°C for 1.0 minutes

2. Heat from 20.00°C to 450.00°C at 10.00°C/minute

3. Hold for 10.0 minutes at 450.00°C

4. Cool from 450.00°C to 20.00°C at 10.00°C/minute

Two inflection points on the rising part of the curve at 120°C and 240°C are respective first and second glass (glass/glass) transition temperatures Tgl and Tg2. Steeper transitions Tel and Tc2 at 290°C and 380°C are transitions associated with crystal phases, and the lower of these temperatures, Tel, will be. the devitrification temperature since at that point the material ceases to be in a glassy phase. In Figure 4 it will be observed that the curve is not retraced upon cooling, shows no (reverse) glass/glass transition points corresponding to Tgl and Tg2, and does not return to the starting point. Thus any thermal processing of this material is likely to be associated with marked changes in the properties of the material, and these changes may be dependent on a number of factors (e.g. times, temperatures, heating rates, atmospheres) so that any change may well be difficult to reproduce reliably. As used herein, "first glass transition temperature" refers to the lowest glass transition temperature above ambient.

Figure 5 shows a differential thermal analysis plot for the material Ge₁₅As₁₅Se₁₇Te₅₃ using the following cycle:

1 Hold for 1.0 minute at 20.00°C

2 Heat from 20.00°C to 440.00°C at 10.00°C/minute 3 Cool from 440.00°C to 80.00°C at 10.00°C/minute 4 Hold for 20.0 minutes at 80.00°C 5 Heat from 80.00°C to 440.00°C at 10.00°C/minute 6 Cool from 440.00°C to 80.00°C at 10.00°C/minute 7 Hold for 20.0 minutes at 80.00°C 8 Heat from 80.00°C to 440.00°C at 10.00°C/minute 9 Cool from 440.00°C to 20.00°C at 10.00°C/minute

10. Hold for 60.0 minutes at 20.00°C

Compared with Figure 4 this plot is a relatively simple trace involving first and second glass (glass/glass) transition temperatures Tgl and Tg2 at 145°C and 260°C, and a single devitrification temperature Tel 330°C. On cooling, while the curve is not retraced, reverse glass transition points Tgl and Tg2 at 275°C and 160°C are exhibited. The trace is repeatable, as evidenced by measurements over three cycles with heating to 330°C.

Figure 6 shows a differential thermal analysis plot for the material Ge₁₉AsuSe₁₇Te₅₃ using the following cycle:

1. Hold at 20.00°C for 1.0 minutes

2. Heat from 20.00°C to 500.00°C at 10.00°C/minute 3. Hold for 10.0 minutes at 500.00°C

4. Cool from 500.00°C to 20.00°C at 10.00°C/minute

5. Hold for 60.0 minutes at 20.00°C

This plot is even simpler than that of Figure 5, showing just a single glass/glass transition temperature Tg at 170°C, and no devitrification point up to a temperature in excess of 470°C. There is a large temperature interval between Tg and the highest temperature investigated.

Although the preceding description is in terms of GAST glasses the reader will understand that similar considerations will apply to other glasses.

The conditions under which bonding is effected, including the choice of Tb and the bonding glass material, are preferably selected so that there is no undesired change in either of the non-linear optical components which are bonded together, for example by destroying or distorting them or their surfaces, or producing an irreversible phase change therein. In most cases the ideal is that the components are substantially wholly unaffected by the bonding process, or at least that subsequent to the bonding process they correspond substantially to the starting components even if some form of change has occurred in the meantime. However, it is envisaged that there may be occasions when Tb is selected to cause a desired change such as an irreversible phase in the material of one or both non-linear optical components so as to produce modified but desired properties.

The conditions under which bonding is effected are preferably selected such that there is no substantial extrusion of the glass out from between the bonded surfaces and/or so that the thickness of the thin layer or layers remains substantially constant.

Where the bonding glass exhibits a plurality of glass transition temperatures above ambient, Tb is preferably selected to lie between the first and second glass transition temperatures.

The glass is preferably an inorganic glass for example a chalcogenide glass.

The bonding glass may comprise Ge, As, Se and Te, and one range of preferred glasses has the general formula Ge(_x-a)As(_a)Se₍₁oo_-x-_b) e_(b), where 25<x<=40, 10<=a<=25, 40< b<=70 and (100-x-b)>0. The glasses of Figures 1 to 3 conform to this formula.

Preferably the bonding glass is selected such that it undergoes the bonding cycle reversibly, so that its properties at the end of the cycle are substantially identical to those at the commencement of the cycle. The glass of Figure 4 does not conform to this criterion and so is not a preferred material. The glasses of Figures 5 and 6 are preferred materials according to this criterion.

Alternatively as mentioned above the bonding glass may be amorphous arsenic trisulphide.

Preferably the bonding glass has only one glass transition temperature before the devitrification temperature is reached, making the glass of Figure 6 more preferable than that of Figure 5.

Preferably there is an interval of at least 50°C, more preferably at least 100°C, and even more preferably at least 150°C, between the (first) glass transition temperature and any other transition temperature, whether a fiirther glass transition temperature or the devitrification temperature. On this criterion the glass of Figure 6 is again more preferable to that of Figure 5, although both conform to the wider criteria.

Infrared transmitting chalcogenide glasses based on the Ge-As-Se-Te system have been prepared with refractive indices in the range n=3.00 to 3.45, which have a close index match to GaAs (n=3.28) and ZnGeP₂ (n=3.1). These glasses have also been successfully coated onto GaAs substrates, with layer thicknesses of from 0.1 microns to greater than 3 microns, using a RF sputtering technique.

Claims

I . A non-linear optical device comprising a stack of grating layers of non-linear optical material, each grating layer comprising at least one grating, adjacent grating layers being bonded together in spaced relation.

2. A device according to claim 1 wherein the stack comprises at least 3 grating layers.

3. A device according to claim 1 or claim 2 wherein adjacent grating layers are bonded by at least one glass layer.

4. A device according to claim 3 wherein adjacent grating layers are bonded by a single glass layer.

5. A device according to claim 3 wherein adjacent grating layers are spaced by an intermediate layer which is bonded to each grating layer by a respective glass layer.

6. A device according to claim 5 wherein the intermediate layer is formed of the non-linear optical material.

7. A device according to any one of claims 3 to 6 wherein said glass has a refractive index within 30% of the index of the non-linear material.

8. A device according to any one of claims 3 to 7 wherein the glass is an inorganic glass.

9. A device according to claim 8 wherein the glass is a chalcogenide glass.

10. A device according to claim 9 wherein the glass is amorphous arsenic trisulphide

I I. A device according to claim 8 wherein the glass comprises Ge, As, Se and Te.

12. A device according to claim 10 wherein the glass has the general formula Ge_(x__a)As_(a)Se₍₁oo-_x-b)Te_(b), where 25<x<=40, 10<=a<=25, 40< b<=70 and (100-x-b)>0.

13. A device according to any preceding claim wherein each grating layer comprises a plurality of spaced gratings.

14. A device according to claim 13 wherein the gratings belonging to an individual grating layer are substantially identical.

15. A device according to claim 13 or claim 14 wherein the gratings belonging to an individual grating layer have parallel grating directions.

16. A device according to any preceding claim wherein the gratings in all of the grating layers have parallel grating directions.

17. A device according to any preceding claim wherein the gratings in all of the grating layers are substantially identical.

18. A device according to any one of claims 13 to 16 wherein at least one grating has a periodicity different from another grating.

19. A device according to any preceding claim wherein the non-linear optical material is selected from lithium niobate (LiNbO₃), potassium titanyl phosphate (KTiOPO₄), or rubidium titanyl arsenate (RbTiOAsO₄).

20. A device according to any preceding claim where the non-linear material is periodically poled to provide said grating(s).

21. A device according to any one of claims 1 to 18 wherein the non-linear optical material is selected from gallium arsenide GaAs, zinc selenide ZnSe, and zinc germanium phosphide ZnGeP₂.

22. A device according to any preceding claim wherein a further, non-grating, layer is bonded to the top and/or bottom of the stack and is effectively index matched to the non-linear material of said grating layers.

23. A device according to any preceding claim wherein a further layer is bonded to the top and/or bottom of the stack for providing a heat sinking function.

24. An optical device substantially as hereinbefore described with respect to Figure 1 or Figure 2 of the accompanying drawings.

25. An optical arrangement comprising a device according to any preceding claim and illumination means for directing light into the device, wherein the illumination means is arranged to flood illuminate a side of said stack.

26. An optical arrangement comprising a device according to any preceding claim and illumination means for directing light into the device, wherein the illumination means is arranged to illuminate individual said grating layers from a side of said stack.

27. An optical arrangement according to claim 25 or claim 26, and wherein the gratings in all of the grating layers have parallel grating directions, wherein the light source is arranged to direct light into the device in a direction parallel to the direction of the gratings.

28. An optical parametric amplifier, mixer or frequency changer comprising a device as claimed in any one of claims 1 to 24, or an optical arrangement according to any one of claims 25 to 27.

29. An optical assembly comprising at least two devices each according to any one of claims 1 to 24.

30. A method of making an optical device comprising the steps of providing a plurality of grating layers of non-linear optical material, forming in each said grating layer a grating structure, forming a stack structure incorporating said grating layers and bonding the layers together in a stack in spaced relation.

31. A method according to claim 30 wherein said bonding step provides a layer of optical glass between the surfaces of adjacent grating layers.

32. A method according to claim 30 wherein said bonding step includes the steps of placing an intermediate layer between adjacent grating layers, and bonding each surface of said intermediate layer to a surface of each respective said grating layer with a glass layer.

33. A method according to claim 31 or claim 32 wherein the bonding step comprises providing at least one of each pair of surfaces to be bonded with a thin layer of bonding glass having a glass transition temperature Tg substantially lower than the glass devitrification temperature Tel, placing the said surfaces together with only the coating or coatings therebetween to form an assembly, and heating the assembly under pressure to a temperature Tb which lies between Tg and Tel and is sufficiently high to soften the glass and bond the components together.

34. A method according to claim 33 wherein both surfaces to be bonded are coated with the bonding glass.

35. A method according to any one of claims 31 to 34 wherein bonding is effected under conditions such that there is no substantial extrusion of the glass out from between the bonded surfaces.

36. A method according to any one of claims 31 to 34 wherein bonding is effected under conditions such that the thickness of the glass coating or coatings remains substantially constant.

37. A method according to any one of claims 30 to 36 wherein all said non-linear layers of the stack are bonded together in a single step.

38. A method according to any one of claims 29 to 37 wherein said grating structures are formed by electrical poling.

39. A method of making an optical device substantially as hereinbefore described with respect to Figure 1 or Figure 2 of the accompanying drawings.