Research On Laser Offset Locking Technology Based On Optical Phase-Locked Loop

Laser offset locking technology has a wide application prospect in laser frequency stabilization,laser communication,cold atom system and other fields due to its simple structure,high locking accuracy and strong anti-interference ability.Optical phase-locked loop（OPLL）is a laser offset locking method that keeps the frequency difference between tow lasers relatively stable by means of frequency and phase discrimination.It can be used to lock laser frequency with ultra-high accuracy and ultra-high stability.Optical phase-locked loops have the advantages of simple structure,large servo frequency bandwidth,wide frequency offset range and high locking accuracy,and have been widely used in atomic coherence,cold atom system,coherent beam combining and heterodyne interferometry.This paper summarizes the research work of laser offset locking technology based on OPLL,and the related technology of optical phase-locked loop is studied.The research content mainly includes the following aspects:（1）The basic working principle of optical phase-locked loop is theoretically analyzed,the small signal model of optical phase-locked loop system is established,the key parameters such as loop bandwidth,residual phase error and frequency stability of optical phase-locked loop are theoretically analyzed,and the main factors affecting the frequency and phase locking of optical phase-locked loop are studied.（2）An OPLL system with simple structure and high locking accuracy is designed to lock the laser to the reference laser to further improve its frequency stability and accuracy.A high sensitivity error signal is obtained by combining digital phase discrimination and differential operation,and the current modulation port of the laser is controlled by a high-speed analog PID controller,so as to achieve fast loop locking and reduce the propagation delay of the loop.（3）An experimental setup based on optical phase-locked loops was built,and an experiment was carried out to lock the tunable external cavity diode laser to an optical frequency comb.Experimental results show that when the external frequency reference with frequency stability of 10^-13,the beat fluctuation is within±0.3Hz after the system is locked.When the offset frequency is 50MHz,the Allan variance of 1s and 1000s integration time is 1.5×10^-9and 8.5×10^-13,respectively,and the standard deviation is 0.05Hz.After the system is locked,the line width of beat is compressed from 500k Hz to 2k Hz.The study shows that sub-Hz laser frequency control can be achieved by using the optical phase-locked loop system designed and it can significantly improve the frequency stability of ECDL.