Polarization compensation in silicon-on-insulator arrayed waveguide grating devices

Silicon-on-insulator (SOI) arrayed waveguide grating (AWG) demultiplexers have been successfully demonstrated in recent years, and now provide a cost effective and manufacturable alternative to glass planar waveguide technology. SOI waveguides in general support many higher order modes and exhibit a strong polarization birefringence. Fortunately single mode ridge waveguides with zero birefringence, essential for practical AWG devices, can be fabricated through the careful choice of ridge width and height. However, these design constraints work well only for AWGs that are relatively large. As the AWG size is reduced our experimental and theoretical work demonstrates that it becomes increasingly difficult to suppress higher order modes and birefringence using ridge dimension alone. In part, it simply becomes difficult to meet the required fabrication tolerances when the ridge dimension approaches the order of a micron. We show that a novel polarization compensator scheme similar to that previously reported for a grating based demultiplexer in InP1 and consisting of simple shallow etched regions in the combiner sections of an Sol AWG, can eliminate the polarization sensitivity of the device by reducing the initial polarization dispersion of 2.22 nm to 0.04 nm. By combining the polarization compensator with mode filtering using appropriate array waveguide curvature, the shape of the array waveguides is no longer constrained. This allows the size of an AWG device to be scaled down to very small dimensions (e.g. less than a millimeter) and also permits the use of simple fabrication techniques such as wet etching. Our results were obtained on AWG devices based on 1.5 mum thick Si-on-insulator waveguides with a typical waveguide array area of a few square millimeters.