Stable QPSK Demodulation Using a Digital Optical Phase-Locked Loop

We demonstrated stable error-free demodulation of 20-Gb/s Quaternary Phase-Shift-Keying (QPSK) for 80-km fiber transmission with a Digital Optical Phase Locked-Loop (DOPLL). We used a second order Active Loop Filter (ALF), which gave better stability than the first order ALF commonly used in conventional OPLLs, and optimized the LF constants. We theoretically compared the system stability with the second order ALF to that with the first order ALF and showed that the former had better phase margins under the assumption that they had the same loop bandwidth. This result indicates that the second order ALF is suitable for operation under noise attributed to the digital processing.