EP1436656A2

EP1436656A2 - Bonding technique for optical components using an intermediate glass layer

Info

Publication number: EP1436656A2
Application number: EP02767686A
Authority: EP
Inventors: Keith Loder QinetiQ Limited LEWIS; Paul David QinetiQ Limited MASON; Euan James QinetiQ Limited MCBREARTY; David Arthur QinetiQ Limited ORCHARD
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2001-10-03
Filing date: 2002-10-02
Publication date: 2004-07-14
Also published as: WO2003029856A2; AU2002331985A1; WO2003029856A3; JP2005506264A; GB0123731D0

Abstract

Opposed surfaces of two optical components are joined by providing at least one of the surfaces with a thin layer (preferably no more than 100 microns thick) of bonding glass having a glass transition temperature Tg substantially lower than the glass devitrification temperature Tcl, placing the surfaces together with only the coating or coatings therebetween, and heating the assembled components under pressure to a temperature Tb which lies between Tg and Tcl and is sufficiently high to soften the glass and bond the components together. Preferably the bonding glass consists essentially of Ge, As, Se and a relatively high proportion of Te. This is particularly useful for components which are thermally sensitive and/or useful in the infra-red, or where there is difficulty in obtaining a component with an acceptable optical surface.

Description

Optical Bonding Technique

The present invention relates to optical components bonded together with index matching glass coatings. Our copending UK Patent Applications Nos. GB 0123740.3 and GB 0123742.9 relate specifically to non-linear optical devices in which components are bonded together using index matching glass coatings, and our copending UK Patent Application No. GB 0123744.5 relates to the provision of antireflection layers produced from index matching glass coatings.

The present invention facilitates the construction of bonded multi-component optical devices with low optical loss interfaces. 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.

A number of tried and tested techniques have been developed to avoid light losses resulting from the optical interfaces in composite optical devices, including the provision of a direct interface, i.e. where the surfaces are in direct optical and mechanical contact, the interposition of an adhesive layer between the components, and, where possible, diffusion bonding.

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.

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 glasses, 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.

In several types of window material, two glass layers are joined by a plastics layer under conditions of pressure and heat, and fire resistant glass has a boron containing layer sandwiched between two glass layers. Again, however, the technique depends on being able to produce joins without gaps or spaces between each optical component and the intermediate layer, and this type of bonding tends to be employed in constructions where optical performance is not critical, such as in window for buildings and vehicle windscreens. 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.

In many applications, if an intermediate bonding layer is to be employed, it is desirable to be able to provide such a layer with a thickness which not only is extremely low but which also can be accurately controlled. This is not particularly easy using the adhesive or other intermediate materials conventionally used in the art.

The diffusion-bonding approach mentioned above 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 lμm can result in a void between layers with a diameter of 1mm (D Bollmann et al, Jpn J Appl Phys, vol 35, pp3807- 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 Electrochem Soc, vol 144, no 4, pp 1439-1441).

In the present invention, at least one optical component to be bonded to another is coated with a thin layer of a glassy material. 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 t_o 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.

In the accompanying drawings:

Figures 1 to 3 are exemplary differential thermal analysis plots illustrating glass phase transitions; and

Figures 4 and 5 shows constructions of zero order waveplates made using the method of the present invention.

Figure 1 shows a differential thermal analysis plot for the material Ge₁₅As₁₅Se₂₉Te₄ι_; 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 1 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 2 shows a differential thermal analysis plot for the material Ge₁₅Asι₅Se₁₇Te ₃ 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 1 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 3 shows a differential thermal analysis plot for the material Gei₉AsπSe₁ Te_53j 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 2, 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.

The present invention provides a 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 components 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.

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 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 change in the material of one component so as to produce a component with 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.

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 1 does not conform to this criterion and so is not a preferred material. The glasses of Figures 2 and 3 are preferred materials according to this criterion.

Preferably the bonding glass has only one glass transition temperature before the devitrification temperature is reached, making the glass of Figure 3 more preferable than that of Figure 2.

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 further glass transition temperature or the devitrification temperature. On this criterion the glass of Figure 3 is again more preferable to that of Figure 2, although both conform to the wider criteria.

The refractive index of the bonding glass is preferably close to tliose of the two components to be joined (i.e. of the material providing the surfaces to be joined) Ideally it has a value Rg substantially equal to the square root of the product of the two indices of the materials to be joined, which may or may not be equal. This reduces undesired reflections at the interfaces with the bonding glass layer. Thus, the bonding glass has a refractive index which is preferably within 20% of Rg (i.e. Rg +/- 20%), more preferably within 15%, even more preferably within 10%, and most preferably within 5%>.

The glass is preferably an inorganic glass for example a chalcogenide glass or amorphous arsenic sulphide. 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. These glasses have also been successfully coated onto silicon (n=3.43) and GaAs (n=3.28) substrates, with layer thicknesses of from 0.1 microns to greater than 3 microns, using a RF sputtering technique.

The bonding glass may comprise Ge, As, Se and Te, and one range of preferred glasses has the general formula Ge(_x-a)As_aSe(ioo-x-b)Teb, 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). The glasses of Figures 1 to 3 conform to this formula, and generally have refractive indices between 3.0 and 3.5 at 2.1 microns, and slightly lower values at 9.3 microns.

In the above formula, b is preferably at least 41. Some glasses conforming to the above formula show good or very good thermal characteristics for the purposes of the present invention. Furthermore, some compositions which have been investigated have a refractive index which is very close to that of gallium arsenide, particularly those in the range where 30<=x<=40; 10<=a<=25; 50<b<=70, and (100-x-b)>0; and other compositions in these ranges have indices close to that of silicon. For example, the compositions Geι₅As₂sSe₁₄Te₄₆, Ge₂oAs oSe₁₄Te₄₆, Geι₅Asι₅Se₅Te₆₅, and the composition of Figure 2 have good to very good thermal characteristics and indices of 3.267, 3.200, 3.246 and 3.447 respectively at 2.1 microns.

The use of glassy materials, and in particular chalcogenide glasses, for sealing a ceramic package for a semiconductor device has been disclosed in US 4954874 (Miura), although there is no mention of any optical aspect. More pertinently, the use of such materials to form low loss bonds in simple optical structures has been demonstrated elsewhere, see for example US 4072782 (Hopkins, Kramer and Hoffman), and Hopkins et al, J Appl Phys., Vol 49, 3133-3139 (1978).

US 4072782 deals with the problem of protecting an erosion or water sensitive infrared window material such as zinc selenide with a more robust outer widow such as of zinc sulphide. To this end it suggests the use of a bond between the two windows provided by an As-S-Se glass, the bond being formed largely from a thin separately interposed wafer of the glass material. As particularly described, bonding is effected by vacuum hot-pressing a 20 mil (500 micron) thick wafer of the glass between two pieces of transparent window material, each of which has previously been coated by vacuum evaporation with a 10 micron layer of the As-S-Se glass. The process takes about 2 hours and results in most of the glass being squeezed out to leave a "relatively uniform" intermediate layer of 2 to 5 microns.

Although thin coatings of the intermediate glass can be deposited with a precise required thickness, it is believed that this precision in thickness is destroyed or greatly reduced in such a process where the majority of the intermediate material is then extruded from between the windows to be joined. Furthermore, the window materials specified in US 4072782 are polycrystalline in form. For the window use envisaged in that disclosure neither of these considerations may be significant. Nevertheless, many other optical constructions involve generally monocrystalline substrates and/or require precise control in the thickness and uniformity of any intermediate glass layer, and the present invention can facilitate the formation of constructions where the substrates to be joined are substantially monocrystalline.

By contrast embodiments of the present invention use a thin layer of a glass having a relatively low glass transition point Tg, preferably an inorganic glass, deposited on the surface of at least one of two optical components to be joined, and there is no further interposition of, or any use at all of, a relatively thick wafer of the same glass, nor is there any substantial extrusion of the glass when the components are joined. This enables the components to be joined with a very precise and controllable spacing.

The thickness of the coating applied to one or both of the surfaces to be joined will be dictated at least in part by the characteristics of the surfaces and their manner of preparation. It is extremely difficult to prepare some surfaces with a high degree of optical precision (surface quality and surface form — while most commonly the surfaces to be joined will be optically flat, other shaped surfaces fall within the invention), or the conditions are such that optical cleanness cannot be guaranteed. Under such conditions a somewhat thicker layer may be required to ensure that surface defects however caused are effectively surrounded by the bonding glass and do not contribute excessively to light scattering or separation of closely adjacent regions of the substrate surfaces.

Preferably the thickness of the coating on one or both surfaces is less than 100 microns. Commonly the thickness lies in the range 0.1 to 20 microns, more preferably 1 to 10 microns, and is most preferably no more than 5 microns. The maximum thickness of the glass in the bonded device is preferably 20 microns, more preferably 10 microns, even more preferably 6 microns and most preferably no more than 4 microns. However, layers of greater thickness can be provided for reasons mentioned immediately above.

The layer or layers of bonding glass may be deposited by any standard method, such as RF sputtering, flash evaporation, solvent evaporation or spin coating.

Preferably the bonding step is carried out under a controlled atmosphere. This may involve an increased or reduced pressure, or a near vacuum, and the atmosphere may be inert or active.

The methods of the first aspect are capable of forming a strong, low optical-loss bond between the two components. By appropriate choice of materials and conditions it can be arranged that during bonding the glass engulf any particulate contamination on either surface minimising the impact of scattering loss, and it can also soften sufficiently to flow into any surface indentations thus making this technique particularly useful for bonding to crystalline materials that are inherently difficult to polish, such as ZnGeP .

By the term "optical component" is intended not only a simple monolithic substrate providing a surface on which the low glass transition point glass is to be deposited, but also more complex components providing such a surface.

The surfaces of the components that are bonded may be planar or non-planar, for example curved in one or both dimensions, but clearly it is necessary for them to be substantially complementary in shape. A component may be laminar, such as a planar or curved parallel-sided sheet of optical material. Alternatively it may be non- laminar, i.e. without parallel major surfaces, for example a lens, with one or no planar surfaces.

A component may be or comprise a passive optical component such as a layer which resists optical radiation damage, or a lens, prism, mirror or amplitude diffraction grating, or a Brewster window, or a window with an antireflection coating; or it may be, or comprise an active optical component, e.g. an electro-optic component such as a light source (for example an LED or a laser diode), or a detector (such as a or photodiode) or a component comprising non-linear or lasing material, for example a parametric device. It may or may not be a semiconductor component.

The material of a component may be, for example, ZnGeP₂, silicon, gallium arsenide, germanium or lithium niobate, inter alia. In certain embodiments the component is adapted or suitable for use in the infra-red part of the spectrum, although the invention extends to components for use in the visible and/or near UN parts of the spectrum also. The bonding glass will be chosen to be transmissive in the part of the spectrum of intended use.

Two passive components, or two active components (e.g. an emitter and a detector in an optical coupler or isolator, or a passive component and an active component may be bonded together.

Inter alia, in embodiments of the invention, a lens is joined to a semiconductor LED or photodiode; and a protective window is joined to a non-linear optical layer.

A stack of components with multiple adjacent pairs so joined may be formed simultaneously by placing appropriately coated components into a stack assembly and subjecting the entire assembly to heat and pressure. Alternatively the stack may be formed sequentially by making the joints one at a time, or by any intermediate process.

Further features of the inventions will become clear upon consideration of the appended claims, to which the reader is referred, and also upon a reading of the following more detailed description of embodiments of the invention. 1. Semiconductor electro-optic component with lens.

It is known to bond germanium lenses to infrared semiconductor gas-sensor devices such as light emitting diodes (LED's) or detectors to provide a compact, low-cost, method of improving device performance. In the case of detectors the apparent size is increased, and in the case of LED's made from high refractive index material it can have very large benefits in improving the external efficiency and in shaping the emission profile.

Currently bonding is achieved by gluing the lens to the semiconductor device, but this technique has proved unreliable and very difficult as it requires the glue thickness or air gap to be less than 0.2 microns, requiring tight control of surface flatness (better than λ/10), surface profiles and cleanliness. A technique for bonding the lenses to the semiconductor devices is required which minimises interface loss and is suitable for mass production, and the present invention is regarded as providing a significant advance therein. In the embodiment of the invention below there is described the use of low melting point (<200°C) infrared transmitting glasses to bond an optical component to a semiconductor electro-optic device.

The prospective facing planar surfaces of a germanium convex lens and a GaAs LED are each coated with a layer of Gei₅Asi₅Se₁ Tes₃ glass, about 5 microns thick, by RF sputtering as described in the section "Surface Coating". They are subsequently placed together in a desired alignment and the assembly is subjected to a controlled pressure and temperature cycle to form a strong, low optical-loss bond. The maximum temperature in the cycle is lower than 200°C, and the cycle is designed so that the glass should engulf any particulate contamination minimising the impact of scattering loss.

The lens could be replaced by an optical grating or other interference device, such as a Fresnel lens.

2. Protective optical component

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 uncoated 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 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 ZnGeP₂ 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. If necessary, once contacted in place the attached material can then be anti-reflection coated using standard dielectric coating methods.

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.

A major face of a protective lamina of GaAs (n=3.3) and a major face of a ZnGeP₂ crystal (n=3.1) are each coated with a thin layer of a low softening point glass based on the Ge-As-Se-Te system, the lamina and crystal are assembled with the coated surfaces in contact, and heat and pressure are applied to bond the assembly together without affecting the mechanical or optical properties of either the lamina or the crystal. The bonded assembly is useful as an improved non-linear optical crystal in the mid-infrared.

Large Aperture Phase Retardation Plates.

Phase retardation plates, or waveplates, are commonly used to alter the polarisation status of laser and other optical beams. They rely on the birefringent nature of certain crystalline optical materials or polymer films, and are often supplied as zero (first) or multiple order plates providing quarter or half wave retardation. Multiple order plates are made from a single birefringent plate having a thickness providing an odd integer multiple of either the quarter or half wave thickness.

Zero order wave plates are made from two birefringent plates with optic axes arranged mutually orthogonally, the plates differing in thickness by precisely the quarter or half wave thickness. Two thicker plates are used because the quarter of half wave thickness in most birefringent crystals is too thin to enable stable construction of a zero wave plate from a single plate. Compared with multiple order plates, zero order plates offer improved acceptance angles, bandwidth and operating temperature ranges, but manufacture is more complex, since two plates need to be mounted. Furthermore, the presence of two additional optical surfaces increases reflection losses.

Currently, zero order wave plates are made by optically contacting the two plates, or mounting the plates in a clamping ring with an air gap between them, or gluing them together with an optically transparent cement. Each technique has associated disadvantages. Crystalline quartz and mica are commonly used as birefringent materials for the visible and near infra-red ranges, but the choice of suitable birefringent materials for use in the mid to far infra-red is very limited.

Sapphire is commonly used in the mid infra-red (2-5.5 microns). CdS may be used for the far infra-red (6-12 microns), but needs to be used with the air gap method since no suitable optical cement is available and optical contacting is difficult. The relatively high refractive index of CdS (n=2.2 at 5 microns) gives rise to a high Fresnel reflection loss from the 4 interfaces, resulting in an overall transmission of 55% for an uncoated zero order wave plate. The latter can additionally lead to undesired etalon effects.

By application of the method of the present invention it is possible to provide a zero order wave plate using either a single birefringent plate or two birefringent plates.

Figure 4 schematically illustrates the formation of a zero order wave plate by coating a surface of each of two birefringent plates 1, 2 with a respective glass layer 3, 4 (Figure 4a) and subsequently bonding the plates together with the application of pressure and heat (Figure 4b). The thickness of the resulting glass layer 5 is not critical, but should be sufficient to engulf any particulate contamination on the plate surfaces. Its refractive index should be chosen to match the index of the plates 1, 2 according to criteria discussed above. The thermal properties of the glass, and the bonding temperature and pressure will need to be chosen bearing in mind the properties of the plates 1, 2. The method used for applying the glass coatings may be any of those mentioned above, and it would be possible to use a coating on only one of the components to be joined, although this is not preferred.

In the construction of Figure 4, there are still four optical interfaces, and it is necessary to provide two plates, and to ensure that they are accurately orientated in the bonded product.

Figure 5 shows an alternative form of zero order wave plate which can be achieved by application of the method of the present invention. Here, the plate 1 of Figure 4 is replaced by a non-birefringent substrate 6, Figure 5a, and after bonding of the substrate 6 and plate 2, Figure 5b, the plate 2 is polished down to a layer 7 of the desired quarter or half wave optical thickness, Figure 6c. Preferably the refractive index of the substrate 6 is chosen to closely match that of the plate 2, and the index of the glass should approximate the square root of the product of the indices of the substrate 6 and plate 2 as described above.

This design not only reduces the amount of costly material processing that is required, but it also reduces any requirement for mutual orientation of two plates.

In the assemblies shown in Figures 4 and 5, the birefringent plates could be of CdS, with a refractive index of 2.2, in which case a suitable glass for bonding would be arsenic trisulphide. The latter is readily available commercially, has a low softening temperature of about 210°C (so that bonding can take place below 250°C), good optical transparency and a refractive index of 2.4. In Figure 5 the non-birefringent substrate 6 could then be of zinc selenide (refractive index 2.4).

By preparing zero order wave plates in this way, they can be made mechanically robust and have a large aperture. There are no air gaps, nor is a clamping ring required. With the construction of Figure 5, the substrate can have any desired thickness.

Claims

1. A 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.

2. A method according to claim 1 wherein the thickness of the bonding glass layer on one or both surfaces is no more than 100 microns.

3. A method according to claim 1 or claim 2 wherein the joining is effected under conditions such that there is no substantial extrusion of the glass out from between the bonded surfaces.

4. A method according to claim 1 or claim 2 wherein the joining is effected under conditions such that the thickness of the thin layer or layers remains substantially unchanged.

5. A method according to any preceding claim wherein the bonding glass exhibits a plurality of glass transition temperatures above ambient, and Tb is selected to lie between the first and second glass transition temperatures.

6. A method according to any one of claims 1 to 4 wherein the bonding glass has only one glass transition temperature before the devitrification temperature is reached.

7. A method according to any preceding claim wherein the bonding glass is an inorganic glass.

8. A method according to claim 6 wherein the bonding glass is a chalcogenide glass.

9. A method according to claim 7 wherein the bonding glass comprises Ge, As, Te and Se.

10. A method according to claim 9 wherein the bonding glass has the general formula 25<x<=40; 10<=a<=25; 40<b<=70, and (100-x-b)>0.

11. A method according to claim 7 wherein the bonding glass is amorphous arsenic sulphide.

12. A method according to any one of claims ^"8 to 11 wherein at least one substrate comprises a material selected from ZnGeP GaAs, Si, ZnSe, ZnS and lithium niobate.

13. A method according to any preceding claim wherein the bonding glass is selected such that it undergoes the bonding cycle reversibly.

14. A method according to any preceding claim wherein the bonding glass is selected such that there is an interval of at least 50°C, between the first glass transition temperature and the next higher transition temperature, whether a further glass transition temperature or the devitrification temperature.

15. A method according to any preceding claim wherein the bonding glass is selected to have a refractive index which lies in the range 0.8Rg to 1.2 Rg, where Rg is the square root of the product of the refractive indices of the two components to be joined.

16. A method according to any preceding claim wherein the substrates are substantially monocrystalline.

17. A method according to any preceding claim wherein the thickness of the bonding glass layer on one or both surfaces lies in the range 0.1 to 20 microns.

18. A method according to any preceding claim wherein the thickness of the glass in the bonded device is no more than 20 microns.

19. A method according to any preceding claim wherein the or each layer of bonding glass is/are deposited by a method selected from RF sputtering, flash evaporation, solvent evaporation and spin coating.

20. A method according to any preceding claim wherein the bonding step is carried out under a controlled atmosphere.

21. A method according to any preceding claim wherein the said surfaces are planar.

22. A method according to any one of claims 1 to 20 wherein said surfaces are non-planar and complementary in shape.

^"23. A method according to any preceding claim wherein at least one said component is selected from a layer which resists optical radiation damage; a lens; a prism; a mirror; an amplitude diffraction grating; a Brewster window; and a window with an antireflection coating.

24. A method according to any preceding claim wherein at least one said component is selected from an electro-optic component; a component comprising non-linear material; and a component comprising lasing material.

25. A method according to any preceding claim wherein at least one said component comprises semiconductor material.

26. A method according to claim 25 wherein said semiconductor material is selected from ZnGeP ; silicon; gallium arsenide; and germanium.

27. A method according to any preceding claim wherein at least one said component comprises lithium niobate.

28. A method according to any preceding claim wherein one said component is a lens and the other said component is a semiconductor LED or photodiode.

29. A method according to any preceding claim wherein one said component is prone to optical damage and the other said component is relatively immune to such damage.

30. A method according to any preceding claim wherein a stack of appropriately coated said components is assembled and subjected to heat and pressure to join all the components together simultaneously.

31. A method according to any one of claims 1 to 22 wherein at least one said component comprises a plate of birefringent material.

32. A method according to claim 31 wherein the other said component comprises a plate of birefringent material.

33. A method according to claim 31 wherein the other component comprises a substrate of non-birefringent material.

34. A method according to claim^"! 3 and further comprising the step of reducing the thickness of said plate of birefringent material after the component have been bonded together.

35. A method according to any preceding claim and including the further step of joining a further component to a surface of one of said two components by repeating the method according to any one of claims 1 to 29.

36. A method according to any preceding claim wherein a said layer of bonding glass is provided on both said components.

37. A method according to any preceding claim wherein an external face of a said optical component is provided with a dielectric or anti-reflection coating.

38. An optical device in the form of a zero order wave plate comprising two birefringent plate components joined together by a layer of optical glass.

39. An optical device in the form of a zero order wave plate comprising a single birefringent plate component bonded to a non-birefringent substrate component by a layer of optical glass.

40. An optical device comprising a semiconductor electro-optic component bonded to a lens, grating or interference component by a layer of optical glass.

41. An optical device comprising an optical component bonded to a protective optical component by a layer of optical glass.

42. An optical device according to any one of claims 38 to 41 wherein the optical glass is of inorganic material.

43. An optical device according to claim 42 wherein the inorganic material is a chalcogenide glass or arsenic trisulphide.

44. An optical device according to any one of claims 38 to 43 wherein the components and the optical glass are transmissive in at least part of the infra-red region.

45. An optical device according to claim 44 wherein the two components and the optical glass are transmissive in at least part of the mid or far infra-red region.

46. An optical device according to any one of claims 38 to 45 wherein the optical glass is closely refractive index matched to the two components.

47. A method of joining adjacent surfaces of two optical components according to claim 1 and substantially as hereinbefore described.