Polarization diversity circuit for a silicon optical switch using silica waveguides integrated with photonic crystal thin film waveplates

We propose a compact polarization diversity optical circuit using silica waveguides and photonic crystal waveplates. By setting these circuits at the front and rear of the silicon optical devices, the polarization dependence of the silicon devices can be suppressed. Photonic crystals can be produced artificially using nanolithography, so that the retardation and orientation of the photonic crystal waveplate can be locally varied on a single chip. This enables to dramatically reduce the size of the polarization diversity circuit, which consists of a 1x2 multimode interference (MMI) coupler, two arm waveguides with quarter-waveplates (QWPs), a 2x2 MMI coupler, and output waveguides with half-waveplates (HWPs). The input light, including the transverse electric (TE) and transverse magnetic (TM) modes, is split by the 1x2 MMI coupler. The optical axes of the two QWPs, spaced 125 μm apart, are set to be orthogonal to each other, so that the phases of the TE modes in the two arm waveguides differ by 90 degrees, and those of the TM modes differ by -90 degrees. The TE mode and the TM mode are separated at the outputs of the 2x2 MMI coupler, and the polarization of the light at one of the outputs is aligned to that at the other output by the HWP. In this paper, we designed a 4x8 polarization diversity circuit for a 4x4 silicon optical switch.