Planar optical waveguides are known. See, for instance, C. H. Henry et al., Journal of Lightwave Technology, Vol. 7(10), p. 1530 (1989), which reviews planar waveguides fabricated on a Si substrate by a technique that comprises chemical vapor deposition. See also M. Kawachi, Optical and Quantum Electronics, Vol. 22, p. 391 (1990), which reviews waveguides fabricated on Si by a technique that comprises flame hydrolysis. Both of the above references are incorporated herein by reference.
Planar waveguides are indispensable features of many Integrated Optical Circuits (IOCs), typically serving to provide interconnection. However, planar waveguides can also provide filtering and other functions, and an advantageous way of making, for instance, waveguide filters and reflectors comprises embedding a Bragg grating into a planar waveguide. See, for instance, C. H. Henry et al., Journal of Lightwave Technology, Vol. 7(9), p. 1379 (1989), and Vol. 8(5), p. 748 (1990), both incorporated herein by reference. The former discloses a Bragg filter comprising a planar waveguide with a Si.sub.3 N.sub.4 core and pure silica cladding, and the latter a waveguide with a phosphorus-doped silica core and a pure silica cladding. In the former case the grating was etched into the SiO.sub.2 upper cladding layer, and in the latter by depositing and patterning a 30 nm Si.sub.3 N.sub.4 film on the SiO.sub.2 lower cladding layer.
Prior art planar waveguide Bragg reflectors and filters (collectively "Bragg devices") typically are quite polarization dependent. This is an obvious disadvantage in applications (e.g., optical fiber communications) in which the polarization of the radiation can vary randomly. This application discloses Bragg devices whose response can be substantially polarization-independent.