Technology Of Frequency Capture And Tracking Applied In Optical Phase-locked Loop

The homodyne coherent optical communication technology,which has the most promising potential and broad application prospect in free space optical communication,not only has the general merits of optical communication,but also has other incomparable advantages,such as high detection sensitivity,strong resistance to background light interference,various modulation modes,and so on.As a key technology in coherent communications,optical phase-locked loops(OPLL)have also been rapidly developed,and the OPLL's frequency capture and tracking technology enables it to acquire a wider range of laser frequency capture bandwidths and better tracking performance.So,the study of this technology is of great significance.Based on the analysis and simulation of the structure and principle of various types of optical phase-locked loops,for the compensation of Doppler frequency shift,this paper analyzes and compares the structure,principle and characteristic of several typical schemes for capturing and tracking laser frequency-locked loops.A digital frequency discriminator laser frequency capture and tracking scheme is proposed,and its concrete structure is also determined.The composition of each part in the structure and their respective functions are described,and their working principle and implementation process in the optical phase-locked loop are analyzed.And a mathematical model is established to simulate the process of completing the frequency capture and tracking.For each unit in the structure,including frequency division unit,counting unit and control unit,device selection and related hardware circuit has been designed.Finally,the verification experiment was designed and the frequency capture tracking function was verified experimentally,and the experimental results were analyzed.The verification experiment results show that the digital discriminator designed in this paper successfully reduces the frequency difference signal from 120.9MHz and stabilizes at 473.7kHz.The results show that the digital frequency discriminator designed in this paper can enable the optical phase-locked loop to capture and track the signal optical frequency when the frequency difference between the local oscillator and the signal light is below 650 MHz,and finally realizes the phase lock,and the control accuracy of the tunable laser's outputfrequency is 9.92 kHz.This paper verifies the credibility of this digital frequency-determined laser frequency acquisition tracking scheme,and provides technical support for optical phaselocked capture of a wider range of bandwidth and better tracking performance.