Design of a delayed XOR phase detector for an optical phase-locked loop toward high-speed coherent laser communication.

Abstract

Optical phase-locked loops are an effective detection method in high-speed and long-distance laser communication. Although this method can detect weak signal light and maintain a small bit error rate, it is difficult to perform because identifying the phase difference between the signal light and the local oscillator accurately has always been a technical challenge. Thus, a series of studies is conducted to address this issue. First, a delayed exclusive or gate (XOR) phase detector with multi-level loop compound control is proposed. Then, a 50ps delay line and relative signal-to-noise ratio control at 15dB are produced through theoretical derivation and simulation. Thereafter, a phase discrimination module is designed on a 15 cm×5 cm printed circuit board board. Finally, the experiment platform is built for verification. Experimental results show that the phase discrimination range is -1.1 to 1.1GHz, and the gain is 0.82mV/MHz. Three times the standard deviation, that is, 0.064V, is observed between the test and theoretical values. The accuracy of phase detection is better than 0.07V, which meets the design standards. A coherent carrier recovery test system is established. The delayed XOR gate has good performance in this system. When the communication rate is 5Gbps, the system realizes a bit error rate of 1.55×10-8 when the optical power of the signal is -40.4 dBm. When the communication rate is increased to 10Gbps, the detection sensitivity drops to -39.5 dBm and still shows good performance in high-speed communications. This work provides a reference for future high-speed coherent homodyne detection in space. Ideas for the next phase of this study are presented at the end of this paper.