INTEGRATED OPTICAL POLARISER

Abstract: The polariser is formed on a silicon rib waveguide (4) having an upper face and two side faces, and comprises a section of the silicon rib waveguide (4) with a buffer layer (6) and a light absorbing layer (7) on either the upper face of the rib or at least one side face of the rib. The buffer layer (6) has a refractive index similar to that of the waveguide (4) such that light travelling along the waveguide (4) and incident upon the respective face thereof passes into the buffer layer (6) and then into the light absorbing layer (7) so either the TM (transverse magnetic) or TE (transverse electric) mode of the light travelling along the waveguide is attenuated.

Description: INTEGRATED OPTICAL POLARISER TECHNICAL FIELD This invention relates to an integrated optical polariser for a silicon rib waveguide. BACKGROUND ART Integrated optical circuits often require the light transmitted therein to be polarised. This can be achieved using a separate polariser, e.g. in the form of an optical fibre, connected to the chip carrying the optical circuit. It would, however, be preferable for the polariser to be integrated on the chip. DISCLOSURE OF INVENTION The present invention provides an integrated optical polariser formed on a silicon rib waveguide having an upper face and two side faces, the polariser comprising a section of the silicon rib waveguide having a buffer layer and a light absorbing layer on either the upper face of the rib or at least one side face of the rib, the buffer layer having a refractive index such that light travelling along the waveguide and incident upon the respective face of the rib waveguide passes into the buffer layer and then into the light absorbing layer whereby either the TM (transverse magnetic) or TE (transverse electric) mode of the light travelling along the waveguide is attenuated. According to another aspect of the invention, there is provided a method of forming a polariser on a silicon rib waveguide having an upper face and two side faces in which a buffer layer is provided on the upper face or at least one of the side faces of a section of the rib, and a light absorbing layer provided on the buffer layer, the buffer layer having a refractive index such that light travelling along the waveguide and incident upon the respective face of the rib waveguide passes into the buffer layer and then into the light absorbing layer whereby either the TM (transverse magnetic) or TE (transverse electric) mode of the light travelling along the waveguide is attenuated. Preferred and optional features of the invention will be apparent from the subsidiary claims of the specification. BRIEF DESCRIPTION OF DRAWINGS The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a known rib waveguide formed in a silicon- on-insulator chip; Figure 2 illustrates removal of oxide cladding from a top surface of the rib shown in Figure 1 ; Figure 3 is a cross-sectional view illustrating the structure of a polariser formed on the rib; Figure 4 is a perspective view of a polariser such as that shown in Figure 3; and Figure 5 is a perspective view of another embodiment of a polariser formed on a broadened portion of a rib waveguide. BEST MODE OF CARRYING OUT THE INVENTION The polariser described herein is based on a silicon-on-insulator chip. A process for forming this type of chip is described in a paper entitled "Reduced defect density in silicon-on-insulator structures formed by oxygen implantation in two steps" by J. Morgail et al. Appl. Phys. Lett., 54, p526, 1989. This describes a process for forming a Very Large Scale Integrated (VLSI) silicon- on-insulator wafer. The silicon layer of such a wafer is then increased, e.g. by epitaxial growth, to make it suitable for forming the basis of the integrated interferometer described herein. Figure 1 shows cross-section of an optical waveguide formed on such a chip. The chip comprises a layer of silicon 1 which is separated from a silicon substrate 2 by a layer of silicon dioxide 3. The rib waveguide 4 is formed in the silicon layer 1. Figure 1 also shows an oxide cladding 5 formed over the rib waveguide 4. Further details of this form of waveguide are given in a paper entitled "Low Loss Single Mode Optical Waveguides with Large Cross-Section in Silicon-on-lnsulator" by J. Schmidtchen et al in Electronic Letters, 27, p1486, 1991 and in PCT patent specification no. WO95/08787. This form of waveguide provides a single mode, low loss (typically less than 0.2 dB/cm for the wavelength range 1.2 to 1.6 microns) waveguide typically having dimensions in the order of 3 to 5 microns which can be coupled to optical fibres and which is compatible with other integrated components. This form of waveguide can also be easily fabricated from conventional silicon-on-insulator wafers (as described in WO95/08787 referred to above) and so is relatively inexpensive to manufacture. The polariser described herein is formed on a section of a silicon rib waveguide 4. In the preferred embodiment, the oxide cladding 5 is first removed from the upper surface of the rib 4 as illustrated in Figure 2, e.g. by using buffered hydrofluoric acid. A buffer layer 6 is then deposited on the rib 4 followed by a light absorbing layer 7 as shown in Figure 3. Figure 4 shows a perspective view of such a polariser formed on a section of rib waveguide. The polariser section typically has a length of 3 mm or less and preferably 1 mm or less. Unpolarised light travelling along the rib waveguide 4 comprises TM (transverse magnetic) and TE (transverse electric) modes and the polariser shown in Figure 3 is designed to attenuate the TM mode. The angle of incidence of the TM mode with the top surface of the rib waveguide is very shallow. Thus, to cause a significant proportion of the TM mode to be transmitted through this surface rather than being totally internally reflected therefrom, it is necessary to provide a layer thereon having a refractive index as close as possible to that of the silicon waveguide. Silicon has a high refractive index, around 3.5, so the buffer layer 6 deposited thereon should, preferably, also have a high refractive index. If the buffer layer 6 is formed of a material having a refractive index of at least 2, an appreciable proportion of the TM mode passes into the buffer layer and hence into the light absorbing layer 7 deposited thereon to cause attenuation of the TM mode. The buffer layer 6 may, for instance, be formed of zinc sellenide which has a refractive index of about 2.5, or a similar dielectric material. The light absorbing layer 7 is preferably a metallic layer, e.g. of aluminium. The metallic layer 7 causes attentuation of the light received from the waveguide. The use of a high refractive index buffer layer 6 is critical in forming a polariser for applications which demand a high level of performance, e.g. having a high extinction ratio, for example, at least 40 dB. The native oxide layer 5 may also be used as the buffer layer, although the performance of the polariser is limited as the refractive index step between the silicon (having a refractive index of about 3.5) and the oxide layer (having a refractive index of about 1.5) is relatively large. An extinction ratio (between the TM and TE modes) of about 10 - 12 dB over a length of about 3 mm can be achieved with such a structure (which may be satisfactory in some circumstances). The thickness of the oxide layer 5 is also difficult to control as it can grow too thick and thus reduce the extinction ratio even further and is not therefore ideally suited to a reproducible production process, although in some circumstances it may be satisfactory. For these reasons, the oxide layer is preferably removed and replaced by a buffer layer 6 of higher refractive index as described above. The buffer layer must be formed of a material having a suitable refractive index, and should not be an oxide (otherwise the native silicon dioxide layer is likely to re-form). The higher the refractive index of the material selected, the less likely its temperature dependence will affect the polarisation extinction ratio. For a buffer layer with a high refractive index, appreciable attenuation of the TM mode occurs over a wider range of buffer thicknesses, maximum attenuation occurring at a greater thickness the higher the refractive index. Figure 5 shows a modified form of polariser. In this embodiment, the rib waveguide 4 has tapered regions 4C either side of the polariser section and the polariser is formed on a section 4A of the waveguide which is wider than the sections 4B of waveguide leading thereto and therefrom. The polariser section 4A is typically at least 8 microns wide and preferably 10 microns wide. This embodiment has a number of advantages. Firstly, it allows greater variation in the accuracy of alignment of a mask used in the removal of the oxide layer 5, so increases the tolerances permitted during the manufacturing process. Secondly, the affect of the buffer layer 6 and metallic layer 7 is increased as they now extend over a majority of the wave travelling along the waveguide. In a 4 micron wide rib waveguide an appreciable portion of the wave travels in regions of the silicon layer 1 lying beyond the lateral extent of the rib 4 and so is not affected by the layers applied to the top surface of the rib whereas in a 10 micron wide rib waveguide, the majority of the wave travels in the rib and in the region of the silicon layer 1 directly beneath the rib 4 and so is affected by the layers applied to the top of the rib 4. The tapered regions 4C typically have a length of around 1 mm. The buffer layer 6 typically has a thickness in the range 20 to 500 Angstrom and preferably in the range 80 to 220 Angstrom depending on the refractive index of the buffer layer. A thickness of around 170 Angstrom may, for example, be used for a buffer layer having a refractive index greater than 2.0 whereas a buffer layer of silicon dioxide, which has a refractive index of about 1.5, may only have a thickness of about 30 Angstrom. The metallic layer 7 typically has a thickness of 1 - 2 microns but may be considerably thinner, e.g. 500 Angstrom or less, so long as it functions to absorb light. As shown in Figure 4, the buffer layer and metallic layer 7 may extend into the tapered regions 4C. It will be appreciated that the buffer layer 6 and metallic layer 7 may be provided on a side face of the rib 4, or on both side faces, to attenuate the TE mode. However, it is generally easier to provide these layers on the upper surface of the rib 4 as described above. If the buffer layer and metallic layer were provided over both top and side surfaces of the rib 4, they would cause attenuation of both the TE and TM mode so the device would not act as a polariser. As mentioned above, a significant feature of the polariser described herein is that it is integrated with the rib waveguide. It can therefore form part of an integrated optica 1 circuit formed on a silicon-on-insulator chip with all the advantages this entails. The difficulties of using a separate off-chip polariser are thus avoided. As discussed above, a buffer layer of high refractive index is preferably required to provide a high performance polaπser in view of the shallow angle of incidence of the TM mode with the upper surface of the rib waveguide. However, it should be noted that the angle of incidence can also be increased by increasing the height of the rib waveguide 4. As mentioned above, the rib waveguide typically has a width of about 4 microns and a height of about 4 microns (measured from the top of the silicon layer 1 ). However, larger rib waveguides, e.g. having a height of up to 10 microns, may be used in some circumstances.

Claims: CLAIMS 1. An integrated optical polariser formed on a silicon rib waveguide having an upper face and two side faces, the polariser comprising a section of the silicon rib waveguide having a buffer layer and a light absorbing layer on either the upper face of the rib or at least one side face of the rib, the buffer layer having a refractive index such that light travelling along the waveguide and incident upon the respective face of the rib waveguide passes into the buffer layer and then into the light absorbing layer whereby either the TM (transverse magnetic) or TE (transverse electric) mode of the light travelling along the waveguide is attenuated. 2. An integrated optical polariser as claimed in Claim 1 in which the buffer layer has a refractive index of 2 or more. 3. An integrated optical polariser as claimed in Claim 2 in which the buffer layer comprises zinc sellenide. 4. An integrated optical polariser as claimed in Claim 1 , 2 or 3 in which the buffer layer has a thickness in the range 20 to 500, and preferably in the range 80 to 220 Angstrom. 5. An integrated optical polariser as claimed in any preceding claim in which the light absorbing layer comprises a metallic layer. 6. An integrated optical polariser as claimed in claim 5 in which the metallic layer is formed of aluminium. 7. An integrated optical polariser as claimed in any preceding claim having a length of 3 mm or less and preferably 1 mm or less. 8. An integrated optical polariser as claimed in any preceding claim in which the rib waveguide width and height dimensions of up to 10 microns and preferably in the range of 3 to 5 microns. 9. An integrated optical polariser as claimed in any preceding claim in which the polaπser is formed on a section of the rib wider than the sections of rib leading thereto and therefrom. 10. An integrated optical polariser as claimed in Claim 9 in which the rib waveguide tapers between the wider section on which the polariser is formed and the sections leading thereto and therefrom. 11. An integrated optical polariser as claimed in Claim 8 or in Claims 9 and 10 in which the polaπser is formed on a section of the rib at least 8 microns, and preferably 10 microns wide. 12. A method of forming a polariser on a silicon rib waveguide having an upper face and two side faces in which a buffer layer is provided on the upper face or at least one of the side faces of a section of the rib, and a light absorbing layer provided on the buffer layer, the buffer layer having a refractive index such that light travelling along the waveguide and incident upon the respective face of the rib waveguide passes into the buffer layer and then into the light absorbing layer whereby either the TM (transverse magnetic) or TE (transverse electric) mode of the light travelling along the waveguide is attenuated. 13. A method as claimed in Claim 12 in which an oxide layer is removed from the upper face or at least one of the side faces of a section of the rib and a buffer layer having a refractive index of 2 or more deposited thereon. 14. A method as claimed in Claim 12 in which the oxide layer is removed using hydrofluoric acid. 15. A method as claimed in Claim 13 or 14 in which the buffer layer comprises zinc sellenide.

Inventors: MCKENZIE JAMES STUART, MOREY JOHANNA CLAIRE, SHAW MATTHEW PETER, MCKENZIE, JAMES, STUART, MOREY, JOHANNA, CLAIRE, SHAW, MATTHEW, PETER
Applicants: BOOKHAM TECHNOLOGY LTD, BOOKHAM TECHNOLOGY LIMITED
Application year: 1998
Main classification: G02B6
IPC codes: G02B6