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

A digital coherent polarization-division-multiplexed quadrature phase-shift keying receiver with a fractionally spaced constant modulus algorithm (CMA) is implemented in a field-programmable gated array. Real-time measurements at 5 Gb/s demonstrate that polarization-mode dispersion can be effectively compensated with multiple numbers of taps when the received data is two-fold over-sampled. With a four-tap CMA, the 1-dB optical signal-to-noise ratio penalty point extends up to differential group delay of five times effective bit width (inverse of data rate at 5 Gb/s).