Arbitrary frequency difference locking method for carrier in intersatellite coherent laser communication

The problem of carrier frequency drift in intersatellite coherent laser communication, mainly caused by the Doppler frequency shift and the frequency shift of the laser itself, can cause the circumferential rotation of the constellation and an increase in the bit error rate (BER). Based on the principles of laser internal modulation and in-phase and quadrature (IQ) orthogonal modulation, a carrier arbitrary frequency difference locking method was designed to overcome the problem of carrier frequency drift. When the light signal modulation format is binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK), this method can lock the carrier frequency difference at any point within a certain range (BPSK: ± 240 MHz & QPSK: ± 120 MHz). When the Doppler frequency shift was not applied to the light signal, the standard deviations of frequency locking error of the BPSK signal and the QPSK signal were 26.1 kHz and 29.2 kHz, respectively. When a frequency shift with a speed of ±5.2 MHz/s was applied to the light signal, and the modulation format of the signal is QPSK, the standard deviation of frequency locking error was 149.4 kHz. By using this method, the tracking of the local oscillator carrier frequency to the light signal carrier frequency can be achieved. Therefore, more bandwidth in subsequent demodulation algorithms can be used to overcome the random phase noise of the lasers, ensuring a low BER for intersatellite coherent laser communication.