Design of a Phase Sensor Applied in the Optical Phase-Locked Loop Based on a High-Speed Coherent Laser Communication System

The background of our research is, designing a precise phase detection sensor for high-speed coherent homodyne systems, and combining it with optical phase-locked loops (OPLLs) to achieve coherent demodulation and carrier recovery. For existing phase sensors, there are problems, such as low phase gain, long dead-time, and difficulty in distinguishing directions. Therefore, we proposed a multilevel-loop compound control phase sensor based on exclusive-OR gates. The sensing system consists of three loops. The tunable range of the temperature loop is 30 GHz. Its tuning rate value is 50 MHz/s. In addition, the inner ring consists of a piezoelectric loop and an acousto-optic frequency shifter loop. According to demand, we established a relevant model, and analyzed and calculated the parameters. The reliability and validity of this sensor are substantiated after setting up the experimental environment. The results show that the lock-in range is 30 GHz, the bandwidth is 1.5 MHz, and the real-time performance of the PLL is improved. In a communication test with a rate of 5 Gbps, the realized bit error ratio is 1.55 x 10-8 when the optical power of the signal is -40.4 dBm. Therefore, this sensor system improves detection sensitivity. At the end of this paper, we propose some ideas to improve sensitivity, which will be the focus of our next phase.