BPSK homodyne detection experiment using balanced optical phase-locked loop with quantized feedback

It is demonstrated that 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 laser in a 50-MBit/s system, quantized feedback allows a 2/sup 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 the authors observe a 2.0-dB crosstalk penalty in addition to the carrier-transmission penalty of 5.2 dB. The results imply a 30-to-50-fold increase in the ratio of linewidth to bit rate than can be used with the balanced loop, which has a structure much simpler than the decision-directed or Costas loops.>