Differential coding in polarizations with half-symbol-period timing offset for coherent PM-QPSK to improve phase-noise tolerance

We propose a phase-noise-tolerant coherent PM-QPSK system with differential coding and half-symbol-period timing-offset in polarizations. Owing to the improved phase noise tolerance, error-free operation of 10Gbaud PM-QPSK is experimentally demonstrated even using ∼30MHz-linewidth DFB laser.