Research On Key Technologies Of Sensor-Less Adaptive Optics For Coherent Free-Space Optical Communication

Free space optics communication(FSOC)technology is a wireless communication method with the laser as information carrier and the atmosphere as transmission channel.It has the advantages of large data capacity,high communication rate,good confidentiality,flexible network setup and no need for frequency band licenses.The conventional FSOC adopts the intensity modulation/direct detection(IM/DD)method,but the current development direction of the optical communication field is the coherent FSOC(CFSOC)with higher detection sensitivity,flexible modulation method and good channel selectivity.However,its application is significantly limited by atmospheric turbulence,owing to the intensity scintillation,beam drift,and arrival angle fluctuation,and will lead to wavefront phase distortion,beam quality degradation,and phase mismatch at the coherent receiver,thereby reducing the communication quality and stability.Adaptive optics(AO)technology has been utilized to compensate for wavefront aberrations caused by atmospheric turbulence,and the Shack-Hartmann wavefront sensor(SH-WFS)is the most widely used in conventional AO systems.However,owing to the inherent shortcomings of its working principle,its measurement accuracy is reduced under strong atmospheric turbulence or low optical power,thereby directly affecting the system performance.Therefore,the flexible and low-cost sensor-less adaptive optics(SLAO)technology has been proposed in recent years.There are two significant features in SLAO: one is the use of high-speed cameras to replace SH-WFS,the other is no need of wavefront measurement and reconstruction.The control signal of the wavefront corrector is algorithmically calculated by the optimization algorithm to dierectly optimize system performance indicators based on the received images caputred by the high-speed camera.Therefore,this thesis mainly focuses on the highspeed and high-reliability imaging system and the wavefront distortion optimization algorithms,which are the two key technologies of SLAO in CFSOC system.The specific research work is as follows:1.The research background,significance and research status of CFSOC system and SLAO technology at home and abroad are expounded.The technical framework,development trend,existing problems and solutions of SLAO in CFSOC system are analyzed.2.The theory of coherent optical communication and the Kolmogorov turbulence model are studied,and the mathematical process of the negative impact of atmospheric turbulence on the mixing efficiency(ME)and bit error rate(BER)of CFSOC systems is analyzed.The technical advantages of SLAO in CFSOC system compared with traditional AO are analyzed,and two key technologies of SLAO are further refined in principle: high-speed and high-reliability imaging system and multi-dimensional largescale optimization algorithm.3.The high-speed and high-reliability imaging system,which is one of the key technologies of SLAO,has been studied.The analysis of imaging requirements,the selection of CMOS sensors,and the design of low-noise electronics hardware and software have been completed.When receiving the high-speed image data,the receiving phase may mismatch due to channel matching differences,inconsistent transmission paths and other reasons.Therefore,the source-synchronous high-speed data receiving algorithm is proposed to ensure the correct reception of SLAO highspeed images.Then a real-time window monitor method is presented against the image data transmission acquisition errors again caused by the drift of temperature and voltage or other external reasons when the SLAO continuous operation in complex environments.Without effects to image data transmission,this method can ensure the high reliability of the SLAO high-speed imaging system for a long-time data receiving.4.The multidimensional optimization algorithm,another key technology of SLAO,is studied.The relationship between the performance indicators of the CFSOC system and the optimization algorithm fitness in SLAO is established based on the Zernike polynomials.The conventional SPGD is introduced,however,its convergence speed is low and easy to fall into local extremes,which limits its practical applications.Thereafter,a hybrid algorithm called Adam SPGD is proposed that combines Adam with SPGD to more effectively compensate the wavefront aberration in CFSOC.Theoretical analysis,numerical simulation and experimental results show that the proposed Adam SPGD can increase the convergence speed by ?50%,and significantly improve the parameter robustness and dynamic performance of the SLAO system,effectively suppress the negative effects of different intensities of atmospheric turbulence on ME and BER,and improve communication quality.This chapter also proposes the Nadam SPGD algorithm,whose performance is also significantly better than the traditional SPGD algorithm.5.The Swarm Intelligence(SI)algorithms without calculating gradients are introduced in SLAO to provide a new idea for solving the problems of atmospheric turbulence distortion correction.The technical characteristics of three SI algorithms,PSO,AFS and CS,and their advantages and disadvantages in SLAO systems are analyzed.Thereafter,a novel hybrid swarm intelligence algorithm(HSIL)that takes full advantages of PSO,AFS,and the Lévy flight process is proposed to more effectively compensate the wavefront aberrations.Theoretical analysis and numerical simulation experiments demonstrate that the proposed algorithm outperforms conventional algorithms,and has improved in all aspects of convergence speed,correction accuracy,performance stability,parameter robustness and global optimization capability.The negative influence of various intensities of atmospheric turbulence on ME and BER in CFSOC can be effectively suppressed.The excellent performance makes HSIL suitable for SLAO to improve the quality of optical communications.This paper has carried out key technical research and related design work in CFSOC theory,SLAO modeling,high-speed and high-reliability imaging system,wavefront distortion optimization algorithms.It has certain guiding significance and reference value for the design of SLAO in CFSOC systems.