Research On Fast Acquisition Technology Of Optical Phase Locked Loop In Space Coherent Laser Communication

Space laser communication has the advantages of high speed,good security,low power consumption,and good anti-interference.Among them,homodyne coherent laser communication is widely used due to its strong anti-interference ability and high detection sensitivity.As an important part of coherent laser communication,the optical phase-locked loop is mainly used to synchronize the frequency and phase of the signal light and the local oscillator light.Different from optical fiber communication,space laser communication is affected by environmental changes and the movement of communication terminals,resulting in a frequency difference and phase difference between signal light and local oscillator light,which affects communication reception and demodulation.Limited to the loop mechanism,there is a contradiction between large-scale acquisition and high-precision phase-locking in an optical phase-locked loop.At the same time,achieving fast and large-scale frequency acquisition and stable phase-locking become the key to establishing a spatial coherent laser communication link.Based on the background of high-speed inter-satellite laser communication and the application requirements of long-distance,high-rate spatial coherent laser communication,this paper has carried out research on optical phase-locked loops and its fast capture technology.First,the optical phase-locked loop optical discriminator,loop filter,optical voltage-controlled oscillator and other components and the working principle of the optical phase-locked loop are studied,and the performance characteristics and applications of various optical phase-locked loops are compared and analyzed.In this paper,a Costas ring with high sensitivity and low line width requirements is selected,and a wide-range fast capture optical phase-locked loop system is designed.Secondly,based on the characteristics of space homodyne coherent laser communication,the principle of optical phase-locked loop acquisition is analyzed,and the effect of Doppler frequency shift on intersatellite laser communication is studied.A fast acquisition scheme based on frequency scanning was established,and an optimal control strategy for a multi-stage compound loop was proposed to solve the contradiction between the large-scale acquisition of the loop and the phase lock with high precision.An optical phase detector based on a delayed XOR gate was designed to meet the frequency and phase identification requirements during large-scale capture;a fine tuning loop based on an acousto-optic frequency shifter was designed to achieve high-precision phase locking;and a local oscillator laser was designed The coarse tuning ring suppresses temperature drift through laser temperature control,achieves a wide range of low-frequency scanning,and achieves fast scanning through voltage tuning.By establishing a compound loop control model,the loop parameters and compensation functions are optimized.Finally,an experimental platform was built to test the performance of the designed optical phase-locked loop system.Through in-depth research on the working mechanism of optical phase-locked loop,a multi-stage composite loop control method for optical phase-locked loop is proposed,which resolves the contradiction between the loop capture range and phase-locked accuracy.A wide-range fast capture optical phase-locked loop system based on acousto-optic frequency shifter and local oscillator laser is designed.The experimental verification shows that the designed system has 10 GHz frequency difference capture performance(including Doppler frequency shift,temperature drift,and frequency change of laser characteristics,etc.),the maximum capture time is less than 10 s,and it can effectively compensate the Doppler of ± 8GHz Frequency shift.The experimental measurement system's bit error rate is better than 1E-7,which verifies that the designed system has wideband,fast acquisition and tracking capabilities.