SOI based ultracompact polarization insensitive filter for PDM signal processing.

We propose and fabricate a scheme that combines a two-dimensional (2D) grating coupler and a delay interferometer (DI) in a loop configuration. According to the polarization states, the 2D grating coupler first splits the input signals into two paths, which copropagate in the loop and share a common DI, and then combines the two paths together to the same input port. The proposed device is polarization insensitive and can be used for polarization division multiplexed (PDM) signal processing. For demonstration, the PDM non return-to-zero differential-phase-shift-keying signals can be demodulated successfully. The bit error ratio measurements show an error free operation, reflecting the good performance and the practicability of the proposed device.