Coherent Polarization Division Multiplexed QPSK Receiver with Fractionally-Spaced CMA for PMD Compensation

A digital coherent polarization division multiplexed (PDM) QPSK receiver with fractionally spaced constant modulus algorithm (CMA) is implemented in FPGA. Real-time measurements at 5Gbit/s demonstrate that polarization mode dispersion (PMD) can be effectively compensated with multiple number of taps when the received data is two-fold over-sampled. With 4 taps CMA, the 1dB OSNR penalty point extends up to differential group delay (DGD) of 5 times effective bit width (inverse of data rate at 5Gbit/s).