| The development of space laser communication has changed the way of human information transmission,making communication more timely and efficient.At present,the United States,Japan,Europe,and China have all carried out space laser communication bidirectional link tests of satellite-ground link,inter-satellite link,and deep space link.Pointing,acquisition and tracking(PAT)technology of the laser communication terminal is the core of all space laser communication system applications.At present,the beacon-free optical communication system has become the mainstream trend in development.In beacon-free optical communication system,the detected optical signal for fine tracking is a modulated optical signal.To improve the quality of beacon-free optical communication,it is necessary to measure the spot position of modulated optical signal with high quality.This thesis investigates the position measurement technology of modulated optical signal based on four-quadrant detector.Firstly,the principle of how four-quadrant detector to work is expounded,and mathematical modeling of the spot detection principle of four-quadrant detector is carried out.Secondly,the index of how to evaluate the spot position detection performance is given,and the influence of all kinds of factors on the spot detection performance is analyzed and verified by MATLAB simulation.Finally,the four-quadrant detector output signal preprocessing circuits is designed,and simulation experiments are carried out on the two preprocessing circuits by using the circuit simulation software TINA,and then combined with the amplitude detection method of modulated optical signal,the measurement accuracy of the spot position coordinates of the detected light signal after passing the four-quadrant detector and preprocessing circuit is greatly improved.The main contents of this thesis are as follows:(1)Various factors affecting the spot position detection performance of fourquadrant detector are analyzed.Based on the principle of spot position detection of four-quadrant detector,the influences of the dark area width of detector,the spot size of detection light,the background light proportion and the noise current ratio on spot position detection performance are analyzed one by one,and simulation experiments is carried out by MATLAB.On this basis,the methods to improve the spot position detection performance are given.According to the methods of improving the spot position detection performance,the performance of background light noise,dark current noise,and thermal noise contained in the detection circuit,and interference noise caused by stray light from one’s own side is analyzed.(2)A three-stage low-pass filter preprocessing circuit and a three-stage band-pass filter preprocessing circuit have been designed.OPA657,produced by Texas Instruments,is selected as the operational amplifier of transimpedance amplifier circuit,secondary amplifier circuit,and filter circuit.The structure and parameters of the threestage circuit which can suppress most of background light noise,dark current noise,thermal noise,and stray light noise are given.Simulated by TINA,the output response of the modulated optical signal with noise after going through two preprocessing circuits is shown.The time-domain output signal of the modulated optical signal after most of the noise has been filtered by the preprocessing circuit is shown.(3)The amplitude detection method of the modulated optical signal based on cross-correlation algorithm is studied.The time domain output signal after the fourquadrant detector and pre-processing circuit is detected by MATLAB with the intercorrelation algorithm,and the spot position coordinate value is further obtained.When the noise interference is small,the amplitude detection method of cross-correlation algorithm is applied to detect the output signal of the low-pass preprocessing circuit,which can obtain the spot position coordinate with higher accuracy.When the noise interference is large,the amplitude detection method of cross-correlation algorithm is applied to detect the output signal of the band-pass filter preprocessing circuit,which can obtain the spot position coordinate with higher accuracy. |
