Frequency and Phase Estimation for Coherent QPSK Transmission With Unlocked DFB Lasers

This letter presents a hardware-efficient frequency estimator and an advanced phase estimation algorithm capable of tracking the phase noise of a 10-GBaud optical quadrature phase-shift-keying transmission system with standard distributed-feedback lasers in the presence of a frequency mismatch up to 1.2 GHz. This algorithm allows us to implement a digital coherent receiver without an analog frequency control circuit.