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 a rms phase error of 10 degrees. This value corresponds to a degradation of about half a dB 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 1.5microns 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.