Coding to relax laser linewidth requirements and improve receiver sensitivity for coherent optical BPSK communications

BPSK (binary phase-shift keying) modulation with heterodyne demodulation is used in conjunction with convolutional codes to illustrate the feasibility of using coding to relax the laser linewidth requirements and improve the receiver sensitivity. The Viterbi algorithm is used, and the performance of the phase-locked loop in the carrier recovery circuit is studied. The results show that the relaxing factor of the laser linewidth can be larger than 12 at 10/sup -9/ bit error rate when a (2, 1) convolutional code with constraint length 11 is used. When the linewidth is fixed, this code can improve the receiver sensitivity by more than 9 dB.