Research And Implementation Of Multi-mode Coherent Optical Modulator

Space laser communication has a very broad application value in the future ultra-high-speed satellite-ground and inter-satellite wireless communication.This paper studies the signal-light modulation technology.Specifically,this paper studies how to serialize the baseband xGbps digital signal and then move it to the optical carrier?1550nm?.The most important thing is that the EVM modulation index of the optical signal should be small enough.Firstly,the principle of electro-optic modulation is introduced.Applying electric field on LiNbO₃ crystal will affect the refractive index of the optical waveguide in the crystal and produce the phase modulation of the optical signal,which is called PM modulator.Then,based on two PM modulators,an MZM modulator can be constructed.When MZM works in push-pull mode,it can synthesize AM signals vectorally.Two orthogonal MZM channels can form an ideal optical IQ vector modulator.Based on this,a software-defined multi-mode coherent light modulator is proposed,and the position of the bias point of MZM and the magnitude of the back-end input double-channel data level are obtained when the modulator realizes OOK,BPSK and QPSK respectively.The bias point of MZM drifts with the working time,temperature and external environment.If the EVM index of modulation is small enough,it must be controlled.Therefore,this paper presents an MZM bias point detection algorithm based on the first-order bias voltage derivation algorithm with average power,and based on this algorithm,the closed-loop adaptive control is implemented.In order to verify the correctness of the algorithm,the simulation of MATLAB is carried out to verify the effectiveness of the algorithm.The parameters of the adaptive feedback control are determined preliminarily in the simulation,which can guide the reasonable selection of step voltage and average number of times in the subsequent programming,so as to ensure the optimal performance of the system.Baseband high-speed serial data jitter and noise exerted on MZM will affect the modulation performance,so this paper designs a low jitter,low noise high-speed bare-parallel,series converter circuit.Firstly,the tree structure is selected,and then the detailed circuit is refined.First,the 8:1 and 2:1 tree scheme is designed.Then,a good clock network is designed for the board level circuit to ensure that the setup time and holding time of the digital circuit meet the requirements.The test board put into production shows that the bare code stream of 2.5Gbps meets the requirements of low jitter and low noise,thus solving the bottleneck of the whole machine development.After the design of the prototype engineering product of the coherent light modulator is completed,the EVM indexes of the two products under QPSK state are7.53%and 6.0%respectively,which proves their excellent modulation performance under 2.5Gbps.The prototype product has completed mechanical test,thermal vacuum,stable performance and good consistency of indicators,which proves that it can meet the requirements of space environment and has the ability to complete space missions.