Study of multistage composite loop control based on optical phase-locked loop technology

The Doppler shift of the satellite-to-satellite coherent communication system reaches gigahertz magnitude. Owing to the influence of the linewidth and phase noise of the tuned laser, the optical phase-locked loop (OPLL) system is required to be higher. We have realized the function of the optical phase loop. According to the characteristics of the Costas phase-locked loop, we combine temperature tuning, piezoelectric (PZT) ceramic tuning, and acousto-optic frequency shifter (AOFS) tuning. This method realizes the frequency control of the local oscillating light through the compound control of the inner and outer rings. Finally, we set up an experimental test system to test the loop performance. The results show that this system can achieve a lock range of up to 4 GHz, and the phase-locked loop (PLL) bandwidth is 1.7 MHz. Furthermore, the Doppler frequency shift compensation between the signal light and the local oscillating light, the laser linewidth suppression, and the phase fast locking are also realized. A residual phase error of 5.1° lays the foundation for the design of the OPLL of the coherent optical communication system.