Research On Loop Filtering Technology Of Optical Phase Locked Loop

Spaces laser communication system has a wide application prospect because of its outstanding characteristics of high communication rate,good confidentiality and strong anti-interference ability.From the signal modulation/demodulation mode,space optical communication is divided into intensity modulation/direct detection and phase modulation/coherent detection.Coherent detection has the characteristics of high detection sensitivity,large information capacity,good wavelength selectivity and various modulation formats so that it has become a research hotspot in recent years.In terms of signal processing,coherent communication is divided into digital coherent communication and analog coherent communication.Generally,digital coherent communication is applied to optical fiber communications,but the high power consumption and the requirement for using high-speed analog-to-digital conversion devices(A/D conversion)limit its application in spaces communication scenes.Therefore,Analog coherent optical communication system in homodyne mode has gained widespread attention in the field of spaces communication applications.Optical phase-locked loop system,as an important part of coherent optical communication system,synchronize the incident light with the local laser carrier,minimize the phase error,and then demodulate the baseband signal,which is composed of optical voltage-controlled oscillator,phase detector,loop filter,etc.The performance of the phase-locked loop system directly affects the performance of the optical communication system.The main factors affecting the performance of the phase-locked loop are Doppler frequency shift,laser line width,phase noise,detector shot noise,etc.The above errors can be effectively reduced through phase-locked selection,key component optimization,and compound loop filtering design to improve the performance of the phase-locked loop.Based on the research background of coherent communication technology,this paper conducts research on the technology of optical phase-locked loops.Through comparison and analysis of various typical phase-locked loops' implementation forms,easiness,and technical requirements,it is determined that the Costa ring is the research object.This paper derives the principle of phase detection based on the combination of 90 ° optical mixer and photoelectric balance detector,studies the function,composition and working principle of loop filter and explain the composition,characteristics and implementation form of optical voltage controlled oscillator.To provide support for subsequent bandwidth design,the relationship between loop noise characteristics and loop bandwidth has been derived,and the simulation analysis of related content has been completed.By building an optical phase-locked loop model,including optical phase detector model,loop filtering model,light voltage controlled oscillation model,piezoelectric ceramic tuning and acousto-optic tuning compound to achieve a wide range,high precision loop model,this paper completes loop parameter design and simulation analysis.Then select key components,design part of the hardware circuit design and set up the experimental system to test the performance of the phase-locked loop.The results show that the phase-lock function can be completed by the composite loop,and coherent communication can be guaranteed to proceed.The maximum phase-locking range is 30 GHz,the loop bandwidth is 1.7MHz,and the maximum phase-locking error is 5.1 °.Research results can promote the development of coherent optical communication technology.