Prior Art Discussion Planar waveguides are typically produced by deposition of buffer, guiding, and cladding layers on a substrate. The guiding layer is selectively treated to define guiding cores having a greater refractive index than the surrounding buffer and cladding layers. The buffer layer is typically deposited on a silicon wafer substrate, and the deposition method is often spin or dip coating and drying. The guiding layer is advantageously a sol-gel material which is photo-sensitive due to addition of a photo-initiator species. The guiding cores are advantageously defined by photo- induced cross-linking reactions. The cladding layer is typically deposited by spin coating, dip coating, or casting.

While this approach may appear to be straight-forward and effective, problems arise in practice. These problems primarily result in high optical losses, usually because the cladding layer may not be adequately adhered to the guiding layer. This may arise because of voids or dust particles between the cladding and guiding layers.

In general, because the process involves deposition of multiple layers of material it is prone to errors at each stage, any one error having the potential to cause undesirable irregularities at the guiding core interfaces.

US5080962 describes production of an optical device in which a waveguide is formed by partial densification of an Si02 gel. This document describes internal waveguides (in Fig. 6), producing using a multi-layered body.

The invention is therefore directed towards providing a process for producing a planar waveguide in which there is less potential for defects to arise. Another object is to provide a simpler process.

SUMMARY OF THE INVENTION According to the invention, there is provided a method of producing a waveguide comprising the steps of providing a body of material and selectively treating the material to define an internal guiding core and an integral surrounding cladding within the body.

In one embodiment, the method comprises the steps of impregnating the material with a refractive index modifier, and selectively removing modifier from the material to define the core and the integral cladding.

In another embodiment, the core is defined by exposing a discrete region of the material to actinic radiation to densify a region of the material, removing modifier to a greater extent from the non-densified region, and subsequently removing modifier from part of the densified region adjacent an exposed surface of the body.

In a further embodiment, the modifier is removed from said part of the region by solvent extraction.

In one embodiment, a supercritical fluid extraction system is used to remove said modifier.

In another embodiment, the body is exposed to radiation in a manner whereby the densified region does not extend to a surface of the material on the opposite side to that exposed to the radiation.

In a further embodiment, the invention comprises the further step of performing solvent extraction over a shallow depth below the exposed surface before radiation exposure.

In one embodiment, the invention comprises the further step of baking the body after radiation exposure to ensure desired densification without over-heating during radiation exposure.

In another embodiment, the material is a sol-gel.

In a further embodiment, the sol-gel is spun onto a substrate.

In one embodiment, the substrate is a silicon wafer.

In another embodiment, the sol-gel material is a condensation product of 3- glycidoxypropyl-trimethoxysilane, dimethyl-diethoxysilane and diphenyl- dimethoxysilane.

In a further embodiment, the photoinitiator is a triarylsulphonium hexafluoroantimonate salt.

In one embodiment, the body is a film and the waveguide is a planar waveguide.

In another embodiment, the film is formed on a sacrificial substrate, and said substrate is subsequently removed.

In a further embodiment, the material is a polymer.

The invention also provides a production system comprising means for producing a waveguide in a method as defined above.

DETAILED DESCRIPTION OF THE INVENTION Brief Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Figs. 1 to 3 are diagrammatic cross-sectional elevational views of material on a substrate being formed into a planar waveguide; and Fig. 4 is a set of plot of photoinitiator absorption vs. UV wavelength plots for different photoinitiator concentrations.

Description of the Embodiments In the invention, a single homogenous body of material (for example, a polymer or a sol-gel) is treated so that an internal guiding core and an integral cladding are formed within the material. This avoids the problems associated with the prior multi-layer approach.

In one embodiment, the material is a sol-gel material and the process of our European Patent Application No. 1209492 is used to define a region having a greater refractive index by UV exposure. The contents of this patent specification are incorporated herein by reference. The greater refractive index region has an exposed

surface. However the material is subsequently treated to modify the upper part of this region so that it has a refractive index similar to that of the laterally surrounding material. This treatment involves a solvent extraction process using a supercritical fluid (SCF) extraction system. SCF is a fluid that has been heated and compressed above its critical temperature and pressure. At these conditions SCFs have densities greater than gases but comparable to those of liquids, enabling them to function as highly efficient solvents. As a result, reduced sample preparation time and increased rate of recovery are usually observed as compared to classical solvent extraction.

The SCF removes the refractive index modifier close to the exposed surface. Of course, this part of the region is most easily accessible to the solvent. Thus, the upper part of the region becomes effectively an integral cladding. Furthermore, by previous control of the UV wavelength and exposure time, the region does not extend downwardly fully to the substrate. Therefore the final guiding core is physically isolated and embedded within the material, having an integral cladding above and below.

In more detail, referring to Fig. 1 a film 1 of an epoxy-functionalised silica sol-gel material is deposited by spin coating on a silicon wafer substrate to a depth of 50 microns. A refractive index modifier, in this embodiment from the silicon alkoxide containing phenyl groups, (diphenyl-dimethoxysilane) is added to the sol-gel before deposition on the substrate. The film 1 is then partially gelled (densified) by heating at a temperature of 100°C for 30 minutes.

As shown in Fig. 2, photo-lithography is then used to selectively further densify a region 2 of the film 1 exposed by a mask 3. The intensity and wavelength of the photo-lithographic exposure and the depth of the film 1 are such that the region 2 does not extend fully down to the substrate, as shown in Fig. 2. In this embodiment, the radiation dosage is 15 light units from a DEK 1600 Exposure System (DEK <BR> <BR> Printing Machines Ltd. , Weymouth, Dorset, U. K. ). This machine has a broadband UV, non-collimated light source. The measured intensities at 365 and 405nm are 7.6

mW/cm2 and 25.7 mW/cm2 respectively. The light unit detector provides the same dose of radiation over a prolonged period by accounting for the loss of efficiency of the UV lamp. It is believed that 15 light units is equivalent to 25 seconds exposure.

A solvent extraction process is then used to remove the refractive index modifier species to a greater extent outside of the region 2. The solvent is for example iso- propyl-alcohol. This is indicated by the absence of hatching in Fig. 3. In addition, the refractive index modifier in the upper part of the region 2 is removed using a different solvent extraction system. In this embodiment the supercritical fluid (SCF) extraction system is used. Extractions are performed in C02 at temperatures from 298. 15-318. 5K and pressures in the range 75-178 bar.

As shown in Fig. 3, the process results in a smaller densified region 4, with a greater concentration of refractive index modifier specifies.

In another embodiment there is an extraction step immediately after heating (curing) the sol-gel. This extraction step removes the sol material near the surface. Such extraction is performed by immersing in iso-propyl-alcohol for 1 minute. The waveguide area is then exposed to UV as shown in Fig. 2 with an intensity and duration of 15 light units from a DEK 1600 Exposure System (DEK Printing Machines Ltd. , Weymouth, Dorset, U. K. ), as detailed above. The film is then baked at 90°C for approximately 30 mins. This is to allow the activated photo-initiator produced by the UV radiation the opportunity to react with the epoxy moieties.

Unstrained epoxy moieties of the kind used in the example formulation are insufficiently reactive to react appreciably at room temperature. Finally, there is a second solvent extraction step using SCF as for the first embodiment to ensure that all of the cladding volume has the same refractive index. The first extraction step may be performed so that the material above the waveguide has a graded index.

Referring to Fig. 4 a plot of photoinitiator absorption vs. UV wavelength is illustrated for photoinitiator concentrations of 0.025 g/l, 0.05 g/l, and 1.0 g/l. These plots are used to choose an optimum UV source to achieve the desired depth of cross-linking, thereby setting the depth of the cladding space below the waveguide.

Looking at the curve in figure 4, we expect the highest absorption of UV radiation to occur at a wavelength around 310nm, depending on photo-initiator concentration.

When using thick films of the order of 50 microns or higher, due to the high absorption efficiency at this wavelength, UV radiation will not penetrate the entire film depth. This ensures fabrication of a'bottom'cladding region within the same film. Alternatively, a 365nm UV source can be used in conjunction with a photo- initiator that has a maximum absorption at this wavelength.

While the method has been described above using sol-gel materials, it is envisaged that a polymer may be used instead. This would be achieved by spin coating, dip- coating or casting a layer of suitable photo-crosslinkable polymer material onto a substrate. This layer would then be dried and processed in the same manner as for a sol-gel.

It is also envisaged that the waveguide need not necessarily be on a substrate. This could be achieved by depositing the material onto a sacrificial substrate and subsequently removing the substrate by a dissolution process. This provides a free- standing film which is free from stress induced by differential thermal expansion of the substrate and the material.

It will be appreciated that the invention provides an integral planar waveguide, thus avoiding the problems associated with binding of multiple layers. Also, because the process is performed with only one layer, it is simpler and more repeatable.

It is envisaged that the waveguide may not be planar. For example it may alternatively be a fibre. In this example, the fibre may be dipped for solvent

extraction around the periphery. The invention is particularly applicable to waveguide materials which do not have good inter-layer bonding properties, such as fluorinated polymers and any silicon-containing polymers. Also, as described above the material may be a polymer rather than a sol-gel.

The invention is not limited to the embodiments described but may be varied in construction and detail.