Research On Inter-satellite Homodyne Coherent Laser Communication Detection Technology Based On AOFS Tuning Method

The demand for high-speed and long-distance inter-satellite information transmission is increasing day by day owing to the rapid development of modern application technology.Because the transmission rate of radio frequency communication has become the bottleneck in high-speed transmission of the spatial information,there is an urgent requirement for a new mode of communication that can increase the spatial information transmission rate and that can multiply the spatial information transmission capacity.Coherent optical communication can effectively help to satisfy the aforementioned requirements,and it will be developed for new spatial information transmission in the future.Homodyne coherent detection,which is an important part of coherent optical communication,reduces the basedband signal to “zero” intermediate frequency by tracking the signal light using a local oscillator,demodulates the transmitted signal at the receiving end,and completes the spatial information transmission.In practical applications,the loop cannot simultaneously achieve “wide” range frequency acquisition and “high” precision phase tracking because of the effects of the phase-locked loop mechanism and external factors.Furthermore,the detection sensitivity of the receiver declines during high-speed communication,degrading the overall communication performance.The manner in which a phase-locked system can be implemented to satisfy the inter-satellite link requirements is the first problem to be solved by coherent detection.In this study,to resolve the aforementioned technical difficulties,a technology for detecting inter-satellite homodyne coherent laser communication based on acousto-optic frequency shifter tuning is studied.First,inter-satellite laser communication is considered to be the application background;next,the direct detection system and the coherent detection system that are used in laser communication are compared,and the sensitivity influence factors in different detection systems are analyzed.When compared to the “direct detection” system and the “all-digital coherent” system,the “analog coherent” system is relatively mature,simple,and stable,and it does not require a high-power digital chip,indicating low power consumption.This system is the ideal detection mode for long-range and high-speed inter-satellite laser communication.In a comparative study about types of communication,modulation modes,types of optical phase-locked oscillators,and types of optical voltage-controlled oscillators,the “Binary Phase Shift Keying” modulation mode at the transmitter and the homodyne coherent demodulation at the receiver can be determined by combining their advantages and disadvantages.The COSTAS loop in the external tuning mode is used as the research background for obtaining the phase-locked loop.Further,a COSTAS optical phase-locked model is established based on acousto–optic frequency shifter tuning.After analyzing the role of the phase-locked loop in an inter-satellite coherent link and summarizing the specific requirements of the link with respect to the phase-locked loop,the design constraints are established.Subsequently,the working mechanism of the COSTAS optical phase-locked loop is studied based on acousto–optic frequency shifter tuning under the established design constraints.Based on the basic principles of acousto–optic frequency shifter tuning and the basic principles of the COSTAS optical phase-locked loop carrier recovery,a mathematical model is established to obtain the frequency difference control equation.According to the working process of each subsystem,the loop discriminator,loop filter,and optical-voltage-controlled oscillator models are perfected.In addition,the acquisition and tracking characteristics of the acousto–optic frequency shifter tuning mode are analyzed.Further,a characteristic equation is established to analyze the factors that affect the bit error rate of the COSTAS optical phase-locked loop based on acousto–optic frequency shifter tuning.The key parameters of the phase-locked loop system are simulated,and the optimization direction is provided.For traditional “multiplier-based rotation detection” and “XOR-square-law correlation detection,” the two “phase-detection” methods are easy to lose at the zero-crossing point control,and the “phase-detection” gain is low.Combined with the COSTAS loop phase-locking characteristics in the acousto–optic frequency shifter tuning mode,research on high-speed and wide-range precision optical phase-locked loop discriminator in the application background is emphasized.In research pertaining to the 90° optical mixer submodule,the mixing efficiency and phase characteristics are analyzed.Further,the mixing efficiency equation and phase delay model are obtained,and the relation between the mixing efficiency and signal-to-noise ratio as well as that between the mixing efficiency and phase delay are derived.In research on the balanced detector submodule,noise analysis,common mode rejection ratio analysis,and signal-to-noise ratio analysis were successively performed.Additionally,a model of the relation between input optical power and received noise is established,and the relation between the received optical power and noise under fixed relative intensity noise as well as that between the optical power mismatch and received relative intensity noise are established.The optimum curve is obtained.In research about the multiplier,a precise “phase-detecting” device is designed based on the delayed XOR;subsequently,the associated model is established,and characteristic analysis,parameter determination,software simulation,and performance optimization are conducted.The results denote that this method achieves high frequency-to-noise ratio “frequency” and “phase” detection,resulting in high-rate,high-precision,and wide-range “phase-detection” conditions for the receiver.In the research on high-rate and wide-range precision optical phase-locked loop discriminator,when combined with the design of the delayed XOR,the mechanism of the receiver sensitivity degradation because of phase-detection error is studied,and the bit error rate,received optical power,and phase-detection error are derived.These studies provide a theoretical basis and optimization direction for improving the system detection sensitivity.By considering the characteristics of the large control range and fast dynamic characteristics of the system,in this study,we adopt the multi-stage compound loop control technology based on acousto–optic frequency shifter tuning,combine the dynamic control parameters of each loop,and establish a fuzzy control model,improved I-type control model,and improved II-type control model,which correspond to the temperature-tracking loop,piezoelectricity-tracking loop,and acousto–optic frequency shifter precise tracking loop,respectively.Further,the design circuit of the multi-stage compound optical phase-locked loop,including the principle diagrams of the driving units,amplifying units,frequency-detection units,delayed XOR units,and main parameters of the selected devices,is provided.The inter-satellite Doppler frequency shift and laser phase noise are effectively suppressed.After the completion of the above research on the mechanism and key link design,a series of optical phase-locked loop performance test experiments are performed based on the acousto–optic frequency shifter tuning technology.The performance test results of the acousto–optic frequency shifter,loop discriminator,phase-locked loop,and receiver communication denote that the scheme is feasible and effective.The detection performance of the receiver reaches the expected objective and satisfies the application conditions of the inter-satellite laser link.This study can serve as a technical reference and theoretical support for conducting research on homodyne coherent laser communication.