IMPROVEMENTS RELATING TO OPTICAL WAVEGUIDES

Abstract: A method of preventing the diffusion of an element out of the material of an optical waveguide substrate during thermal diffusion of a deposited pattern of dopant material into the substrate comprising the step of providing the substrate with a diffusion barrier layer after deposition of the dopant material pattern but prior to the thermal diffusion.

Description: IMPROVEMENTS RELATING TO OPTICAL WAVEGUIDES This invention relates to optical waveguides and relates more specifically to such waveguides which are formed by the thermal diffusion of a deposited waveguide pattern (e.g. channels) of a suitable dopant material into a substrate of another material in order to produce an optical waveguide having light propagation channels of relatively high refractive index near the surface of the substrate. The dopant material of waveguide devices which form the basis for a range of electro-optic modulation and switching devices typically comprises titanium which is diffused into a lithium niobate substrate. The quality of the modulation and switching devices embodying such waveguides and, hence, the performance of such devices, is influenced by (a) the ability to provide well-defined channel waveguide patterns (b) the attenuation of any optical waves constrained to propagate in such channels and (c) the proximity of any metallic or other electrodes that are absorbent at the optical frequencies involved. One commonly encountered problem in the fabrication of optical waveguides by the thermal diffusion of deposited titanium channel patterns into a lithium niobate substrate is the unwanted diffusion of the element lithium out of the substrate materials. This unwanted diffusion produces changes in the crystal stoichiometry one effect of which is to increase the extraordinary refractive index of a layer at the surface of the lithium niobate substrate. This has an adverse effect on modulation and switching devices utilising such channel waveguides since between the channels of the waveguide there now exists a planar waveguide for waves of the extraordinary polarisation. Various measures have been proposed to overcome the problem of lithium being diffused out of the lithium niobate substrate but these have not only made the diffusion process more complex but the measures are not in general controllable to an extent that permits the routine reproduction of given device characteristics. According to the present invention the diffusion of an element (e.g. lithium) out of the material of an optical waveguide substrate (e.g. lithium niobate) during thermal diffusion of a deposited pattern of dopant material into the substrate is avoided by providing the latter with a diffusion barrier layer, preferably over the entire surface of the substrate, after deposition of the dopant material pattern and prior to the thermal diffusion step. In applying the present invention to lithium niobate substrates the diffusion barrier layer may comprise silicon oxynitride which is known to act as a diffusion barrier to species of ions with radii as small as those of hydrogen ions and which is therefore suitable as a barrier to the diffusion of lithium from the lithium niobate substrate. Material of the composition SiOxNy may also be used as the diffusion barrier material and, consequently, a range of materials from SiON to Si3N4 could be utilised to prevent diffusion of lithium out of the lithium niobate substrate. However, an advantageous choice may be made from this range of diffusion barrier materials so that the diffusion barrier layer also provides an effective dielectric buffer layer to prevent substantial absorption of optical waves propagating along the waveguide channels in cases where metal electrodes are applied over the waveguide for the purpose of accessing the required electo-optic co-efficient. By way of explanation it may here be mentioned that in the Z-cut orientation of lithium niobate the highest electro-optic co-efficient, r33, is accessed by propagating light in the transverse magnetic polarisation in a waveguide wherein the applied electric field is parallel to the crystal Z-axis (the optic axis). For this polarisation, the propagating optical wave undergoes substantial absorption when electrodes are deposited directly on the waveguide surface without the intervention of any separating or buffer layer. It is therefore common practice to deposit a dielectric buffer layer between the waveguide surface and the metal electrodes to overcome this loss. In the present invention the layer of silicon nitride which acts as the diffusion barrier layer also becomes the buffer layer. This dielectric buffer layer is usually of low refractive index in order to minimise perturbation of the wave guide propagation constants. This favours the use of diffusion barrier layer material nearer to the form SiON in the range of materials above specified since it has the lower refractive index. As the barrier layer material is changed by decreasing the oxygen content, progressing ultimately to the form of Si3N4, the refractive index increases. However, the same general principles of the invention apply regardless of the material composition within this range. Another advantage that the invention provides is that the layer of diffusion barrier material (e.g. silicon oxynitride) also serves suppress surface deformation of the wave guide substrate which frequently results from the thermal diffusion process. Whilst the exact composition of the rough surface, layer often produced during diffusion is not established with certainty it is related to compound formation as the result of the diffusion of lithium out of the lithium niobate substrate, as well as being due to stress and surface tension effects associated with the diffusion process. The provision of the diffusion barrier and buffer layer also acts to suppress the substrate surface deformation referred to and thereby reduce the attenuation normally produced by such deformation. Thus, the propagation loss of waveguides fabricated according to the present invention is significantly reduced. By way of example the present invention will now be described with reference to the accompanying drawing which illustrates the steps in fabricating an optical channel wave guide in accordance with the present invention. Referring to Figure l(a) of the drawing the optical waveguide is fabricated from a substrate 1 of lithium niobate on to which is deposited, as by evaporation, sputtering, or other process, titanium which defines a pattern of channels 2. Before the titanium is diffused into the surface of the substrate 1 to produce light propagating channels of relatively high refractive index just below the substrate surface and, as shown in Figure l(b), a diffusion barrier layer 3 of material selected from the range SON to Si3N4 is deposited, as by chemical vapour deposition, or other process, over the entire surface of the substrate 1. As already mentioned the choice of diffusion barrier layer material will be made in dependence upon the need for a dielectric buffer layer of low refractive index where metal or other electrodes are applied to the substrate. The deposited titanium channel pattern 2 is then thermally diffused into the surface of the lithium niobate substrate to provide as shown in Figure 1(c) a pattern of light propagating channels 4 located just below the surface of the substrate. These channels 4 will have a high refractive index. The diffusion barrier layer prevents ions of lithium from being diffused out of the substrate material during the thermal diffusion process where the substrate is subjected to temperatures up to about 11500C. After the thermal diffusion step metal or other electrodes 5 and 6 (e.g. aluminium, or chromium and gold) may be deposited as by evaporation, sputtering, or other process, on to the surface of the substrate 1 as shown in Figure lEd). The interposition of the barrier layer 3 also serves to provide a dielectric buffer layer of low refractive index which serves to reduce absorption, and thus attenuation, of the optical signals propagating along the waveguide channels due to the proximity of the metal or other electrodes 5 and 6. The lower the refractive index of the buffer layer material the lower will be the absorption of the propagating light signals. The material SiON will therefore be preferred in order to minimise the absorption of light signals by the electrodes. As already explained the diffusion barrier/dielectric buffer layer 3 also serves to suppress the tendency of the substrate surface to roughen during the thermal diffusion step. In this way attenuation of the propagating light signals in the waveguide channels will be reduced still further. Although the present invention has been described specifically as applied to a channel waveguide having a lithium niobate substrate into which titanium is thermally diffused to provide the light propagating channels, it should be understood that the principles of the present invention whereby a diffusion barrier layer is applied to the substrate after the application of a dopant material pattern but before the thermal diffusion step may be applied to other forms of optical waveguide utilising different materials.

Claims: CLAIMS 1. A method of preventing the diffusion of an element out of the material of an optical waveguide substrate during thermal diffusion of a deposited pattern of dopant material into the substrate comprising the step of providing the substrate with a diffusion barrier layer after deposition of the dopant material pattern but prior to the thermal diffusion. 2. A method as claimed in claim 1, in which the substrate comprises lithium niobate and the element lithium is prevented from being diffused out of the substrate. 3. A method as claimed in claim 1 or claim 2, in which the dopant material is titanium. 4. A method as claimed in claim 2, in which the diffusion barrier layer comprises silicon oxynitride. 5. A method as claimed in claim 2, in which the diffusion barrier material is selected from a range of materials SiON to Si3N4 so that the diffusion barrier layer also provides an effective dielectric buffer layer to prevent substantial absorption of optical waves propagating along the waveguide channels where metal electrodes are applied over the waveguide for the purpose of accessing the required electro-optic coefficient. 6. A method as claimed in any preceding claim, in which the diffusion barrier layer is applied to the entire surface of substrate. 7. A method as claimed in any preceding claim, in which metal and other conductive electrodes are applied over the barrier diffusion layer after the thermal diffusion step. 8. A method as claimed in claim 1, in which a channel pattern of titanium is deposited on the lithium niobate substrate as by evaporation or sputtering, in which a diffusion barrier layer selected from the range of materials SiON to Si3N4 is deposited, as by chemical vapour deposition, over the entire surface of the substrate, in which the deposited titanium channel pattern is thermally diffused into the surface of the substrate by subjecting the substrate to temperatures up to about 11500C, and in which metal electrodes are deposited on to opposed surfaces of the substrate including that into which the titanium has been diffused. 9. An electro-optical waveguide structure including a substrate provided with a diffusion barrier layer in accordance with claim 1 or claim 2.

Inventors: BENNION, IAN, STEWART, WILLIAM JAMES
Applicants: PLESSEY OVERSEAS LIMITED
Application year: 1985
Main classification: G02B6
IPC codes: G02B6