Birefringence compensation in silicon-on-insulator planar waveguide demultiplexers using a buried oxide layer

The high index contrast of silicon-on-insulator (SOI) enables the scaling down of planar waveguide components into the microphotonic regime, but has the unwanted consequence of inducing a large TE-TM polarization birefringence. For ridge waveguides this birefringence can be reduced to an acceptable level by using appropriate ridge dimensions [1], but with decreasing Si thickness the required fabrication tolerances quickly become too narrow to implement this solution. Components with slab waveguide regions such as echelle grating demultiplexers cannot be made polarization independent using this method. An alternate approach is to introduce a polarization compensation region in the combiner section of an AWG or echelle grating to eliminate the polarization dependent wavelength shift[2]. In its original implementation, the compensator is fabricated by changing the local waveguide thickness. The resulting birefringence correction can be sensitive to errors in etch depth, and the mode mismatch between compensator and slab waveguide sections is a source of approximately 1 dB extra insertion loss (IL) for SOI demultiplexers with Si waveguide thickness in the range from 2 to 5 m. We describe a new compensator structure for the SOI platform using a buried low index layer, in this case SiO2 sandwiched between the Si waveguide layer and another Si cap layer. Such a silicon-oxide-silicon (SOS) compensator on SOI can eliminate the TE-TM wavelength shift of an SOI AWG or echelle grating demultiplexer without introducing a significant mode mismatch between the compensator and slab waveguide sections. A demultiplexer with an SOS compensator has almost 1 dB lower insertion loss of an equivalent device with an etched compensator. The SOS compensator is easily implemented using standard oxide and a-Si or polysilicon deposition techniques. In this paper we present calculations and experimental results on the effective birefringence compensation, PDL and IL of SOS compensators in SOI waveguide demultiplexers.