Polarization-insensitive MMI-coupled ring resonators in silicon-on-insulator using cladding stress engineering

We review the design considerations and experimental results of a novel design of polarization insensitive ring resonators in silicon-on-insulator (SOI) ridge waveguides. The polarization insensitive coupling is achieved using a multi-mode-interference (MMI) coupler. Cladding stress-induced birefringence is used to correct the round trip phase difference between the TE and TM polarizations. Experimental demonstration is presented for such ring resonators fabricated on SOI with 1.5 cm Si waveguide core layer. For resonators with radius R = 200 mu m, polarization insensitive operation is achieved in both the resonance wavelength and linewidth over a similar to 4 nm wavelength range using a 7.5 mu m x 84 mu m MMI coupler and 0.8 mu m thick oxide cladding with -250 MPa stress, with the resonance wavelength shifts between TE and TM polarizations less than 3 mu m. The quality factor Q of similar to 15,000 and free-spectral range (FSR) of 0.46 nm is measured. For resonators with a smaller radius of 50 mu m, similar FRS of 1.3 nm and extinction of 13 dB are observed for TE and TM, although the resonance wavelengths are shifted, in agreement with the theoretical prediction. By choosing the proper combination of the cladding stress and thickness, zero-birefringence condition can be achieved for resonators of different cavity lengths.