BPSK HOMODYNE DETECTION EXPERIMENT USING BALANCED OPTICAL PHASE-LOCKED LOOP WITH QUANTIZED FEEDBACK

We demonstrate the quantized feedback can effectively cancel the data-to-phase-lock crosstalk in a balanced optical phase-locked loop for homodyne detection of pilot-carrier BPSK. Using external-cavity semiconductor lasers in a 50-MBit/s system, quantized feedback allows a 2(15) - 1 PRBS to be detected without penalty from crosstalk, and with only a 0.5-dB penalty for pilot carrier transmission. Without quantized feedback the modulation angle must be reduced substantially, and we observe a 2.0-dB crosstalk penalty in addition to the carrier-transmission penalty of 5.2 dB. Our results imply a 30-to-50-fold increase in the ratio of linewidth to bit rate that can be used with the balanced loop, which has a structure much simpler than the decision-directed or Costas loops.