Design and Fabrication of Polarization-Independent Micro-Ring Resonators

Origin of polarization sensitivity of photonic wire waveguides (PWWs) is analysed and the effective refractive indices of two different polarization states are calculated by the three-dimensional full-vector beam propagation
method. We find that PWWs are polarization insensitive if the distribution of its refractive index is uniform and the cross section is square. An MRR based on such a polarization-insensitive PWW is fabricated on an 8-inch silicon-on-insulator wafer using 248-nm deep ultraviolet lithography and reactive ion etching. The quasi-TE mode is resonant at 1542.25nm and 1558.90nm, and the quasi-TM mode is resonant at 1542.12nm and 1558.94nm. The corresponding polarization shift is 0.13nm at the shorter wavelength and 0.04nm at the longer wavelength. Thus the fabricated device is polarization independent. The extinction ratio is larger than 10dB. The 3dB bandwidth is about 2.5nm and the Qvalue is about 620 at 1558.9nm.