Minimum required power for carrier recovery at optical frequencies

The purpose of this study is to evaluate the minimum optical power required to achieve carrier recovery with arms phase error of 10°. This value corresponds to a degradation of about half a decibel for a coherent communication system using BPSK modulation. Two basic carrier recovery configurations were selected for this study. One uses an injection-locked oscillator; the other utilizes a phase-locked loop. The local source for both circuits is a laser providing 4 mW at 1.5 μm and having a full 3-dB linewidth of 20 MHz. The results show that the carrier recovery requirement can be achieved with a phase-locked loop receiving an unmodulated signal of -50 dBm. The minimum required power increases to a least -26 dBm for an injection-locked oscillator. A reduction of the sources linewidth by a given factor will decrease the minimum required power by the same amount.