Single-Channel 400-Gb/s OTDM-32 RZ/QAM Coherent Transmission Over 225 km Using an Optical Phase-Locked Loop Technique

A single-channel 400-Gb/s transmission with a polarization-multiplexed , 10-Gsymbol/s, four-optical time-division multiplexed (OTDM) 32 return-to-zero/quadrature amplitude modulation (RZ/QAM) scheme is demonstrated for the first time. By using an optical phase-locked loop technique and a clock recovery circuit, we have successfully obtained a very stable pulsed-local oscillator whose phase was locked to the transmitter, and achieved precise demultiplexing and demodulation simultaneously. As a result, 400-Gb/s data were transmitted over 225 km with a bit-error-rate performance below the forward-error correction limit. Furthermore, we also realized a 320-Gb/s to 450-km transmission with an OTDM 16 RZ/QAM scheme.