| Space laser communication between satellites has the characteristics of large information capacity,high transmission rate and good security.It is gradually becoming a new space transmission technique.In recent years,which has made rapid development,and the space acquisition and tracking technique of laser communication link has been verified many times before launch and in orbit.However,the optical detection for tracking used by most research institutions was directly,which has limited detection sensitivity,and easy to be affected by strong dynamic disturbance and background light.This has also become an important reason to restrict its wide application.This dissertation proposes the concept of heterodyne coherent tracking,which takes advantage of high coherent detection sensitivity and good filtering performance,to improve the availability of laser communication inter satellite link.The following research is mainly carried out.1.Space laser communication is different from ground optical fiber communication,with more complex space transmission channel.The constraints of space laser communication include spatial transmission field distribution,aberration,background light,orbital motion,micro vibration,Doppler shift and so on.In this dissertation,the basic characteristics of long-distance dynamic transmission of satellite laser link are analyzed,based on the far-field optical transmission theory of Fraunhofer diffraction.The quantitative boundary constraints are obtained by typical laser link model,which makes a foundation for the research of heterodyne coherent tracking technology.2.The traditional laser link tracking detector is generally array detector or quadrant detector.The two-axis tracking error is obtained by directly detecting the target optical signal and extracting the spot centroid.This scheme can achieve stable tracking for space laser communication when the link distance is not too far and the signal-to-noise ratio is high enough.However,when applied to long-distance transmission,strong platform disturbance and strong background light irradiation,the tracking accuracy will be significantly reduced due to insufficient signal-to-noise ratio.In order to solve this problem,heterodyne coherent tracking technology is proposed in this dissertation.The intensity fluctuation of the signal coherent with the local oscillator is used as the feedback of the tracking closed-loop control system to compensate the tracking error,which takes advantage of high coherent detection sensitivity and good filtering performance.The tracking signal-to-noise ratio is still high enough when the solar background light directly,and make the availability of the laser link improved significantly.3.The nutation heterodyne coherent tracking technology by signal laser is proposed,which based on the optical fiber coupling detection method.The signal light is artificially introduced into a small range periodic motion,so the IF signal coherent with the local oscillator light will produce intensity fluctuation.Here,the corresponding relationship between the intensity fluctuation and the position of nutation signal is used to extract the tracking position error.The basic detection model of optical nutation heterodyne coherent tracking signal is constructed,and the relationship between nutation characteristic parameters and tracking signal-to-noise ratio is quantitatively analyzed.The analysis results show that the half angle range of nutation should be 1.2μrad,and the bandwidth of nutation should not be less than 1 k Hz.A nutation heterodyne coherent tracking simulation system is built to verify that heterodyne coherent tracking has a strong ability to suppress the composite micro vibration of satellite platform.The effective suppression bandwidth reaches 206 Hz,and the tracking accuracy is better than 0.056μrad(1σ),which has increased by an order of magnitude compared with traditional direct tracking with 0.5μrad(1σ).4.The dual four quadrant balance detection heterodyne coherent tracking technique is proposed,based on the space optical detection.Make the received signal mixed with the local oscillator and transmitted to two four quadrant detectors,and the two quadrant detectors correspond to the detection unit for balance detection.The four if signals obtained are regarded as the four signals on the"synthetic"four quadrant detector,the spot centroid is solved,and the tracking angle error is extracted.A typical laser terminal model is constructed,through theoretical analysis and calculation,which can achieve 48 d B tracking signal-to-noise ratio and tracking accuracy of 0.051μrad(1σ)during direct sunlight.The principle of heterodyne coherent tracking is verified,and the tracking accuracy is 0.065μrad(1σ);Then,the principle verification by laser communication terminal both in the laboratory and outside is taken,and the tracking accuracy is better than0.18μrad(1σ)。5.The verification of heterodyne coherent tracking technique for space laser communication link system is carried out,based on the principle verification.The system test platform with simulation of far-field and dynamic characteristics is constructed,which is based on the principle of Fourier transform.The simulation of micro vibration dynamics coupling simulation of satellite platform,carrier suppression single sideband modulation Doppler shift,wide spectrum spatial background stray light and so on are realized.The tracking performance of laser communication link level is verified.According to the test results,the heterodyne coherent tracking technology can keep the tracking link stable for a long time,with the tracking accuracy better than 0.16μrad(1σ),when equivalent solar light intensity with 1366W/m~2,orbital motion with 0.3°/s,micro vibration with 0～100 Hz,Doppler shift with 10 GHz.The research results provide a new practical scheme for improving the anti-interference and stable tracking performance of laser link and promoting the commercial application of satellite laser terminal. |
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