【作者】 杨鹏；

【导师】 艾华；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 光学工程， 2012， 博士

【摘要】 无线激光通信具可靠性高、容量大、保密性好等特点,在通信和军事方面具有很高的应用价值。针对现有无线光通信系统可靠性低,同时为降低大气湍流和通信终端相对运动对通信系统性能的影响,本文利用偏振调制技术改善系统的性能,提出了采用圆偏振光编码调制方式,设计了相应的激光通信系统,对偏振光在大气传输过程中的性能和激光通信系统性能进行了分析,并开展了简单验证实验,取得了一定的研究成果。主要内容具体如下：首先在偏振光学的基础上的,对偏振移位键控的编码原理进行了分析,提出采用以圆偏振态进行编码的圆偏振键控编码技术(CPolSK)。激光的圆偏振特性在大气传输过程中变化很小,且圆偏振态具有旋转对称的特性,使系统的性能不受两通信终端相对运动的影响,降低了技术的实现难度,具有很高的可靠性,特别适用于作为移动平台的通信终端。CPolSK系统是利用两个圆偏振态进行组合编码,单通道传递信息。基于圆偏振态在大气传输中的优势,同时为提高通信量,提出圆偏振复用编码原理(CPolDM),不仅可保证通信的可靠性,又成倍地提高了系统容量。同时,从原理上推导了线偏振态和圆偏振态之间的转换过程,为圆偏振调制的编码解码提供理论基础。其次,根据大气激光通信系统的要求,提出了两种圆偏振调制的具体实现方案,同时结合两种调制原理,设计了圆偏振调制激光通信的实验验证系统。根据通信系统的功能模块,分别设计了发射系统、接收系统和光学天线。其中包括激光器选取、光束准直整形系统的设计以及调制解调电路,并根据系统要求设计了发射接收光学天线等光机系统。该通信系统可验证两种圆偏振编码方式,且结构简单,实现方便,非常适用于作为星载通信终端。第三,论述了激光在传输过程中受到大气散射、折射、湍流等各种效应的影响,重点分析了大气信道对光信号的偏振度和偏振态造成的影响,推导了大气散射对偏振光的退偏效应。以部分相干电磁厄米-高斯光束为模型,推导了部分相干激光的偏振特性在大气湍流传输过程中的变化规律,并推导出了激光在传输过程中的偏振不变性,可用于圆偏振调制系统的性能优化。根据圆偏振调制的特性,通过把大气等效为一个线性延迟器,来分析测量圆偏振光在大气中总的双折射量,不仅可测定圆偏振态的改变量,还可以估算系统的功率的衰减,评判系统的功率是否满足要求。最后,为验证圆偏振编码原理,对圆偏振激光通信系统的性能进行分析。首先通过对几种调制方式的各项性能进行对比,结果表明圆偏振调制虽然平均功率高,但带宽需求小且误码率低,可通过偏振复用技术提高容量,是一种性能优良的调制方式。并通过软件模拟仿真了圆偏振调制通信系统,通过仿真结果对比,证明差分接收圆偏振调制系统具有优良的性能。其后,分析了实际的圆偏振通信系统,进行了简单的圆偏振编码通信实验,实现了数据的发送与接收,证明了圆偏振编码的可行性和通信系统设计的合理性,尤其是在湍流扰动情况下,CPolSK具有较低的误码率。为完善圆偏振通信系统系统,分析了光学元件对通信性能的影响,并提出采用相干接收技术等措施来提高系统的接收灵敏度、误码性能以及抗大气扰动的能力。更多还原

【Abstract】 Wireless laser communication has advantages of low power consumption. high reliability, high level of security, high data-rata capacity and so on, so it has a high application value in communication and military aspects. In order to improve the reliability of the optical communication system, and reduce the impact of atmosphere disturbance and communication terminal relative motion to the performance of communication system, a new polarization modulation method is introduced which uses circular polarization to express the information. The corresponding laser communication system is designed, the performance of the polarized light propagating in atmosphere and the communication system is analyzed. The simple verification experiment is carried out, obtained some achievements in the dissertation. The main research contents are as follows.First, based on the polarization optics and the polarization shift keying, the circular polarization shift keying (CPolSK) modulation is proposed which uses the two kinds of circular polarization to denote the data states of the information. The circular polarization modulation has high reliability since the polarization properties of laser change relatively small while propagating through the atmosphere. Because of the rotational symmetry of circular polarization, the system performance is unaffected by the relative motion of the two terminals, and is especially suitable for mobile communications terminal, and can work well. The CPolSK system is the use of the two circular polarization states for the combination of coding and single-channel transmission of information. Based on the advantages of circular polarization in the atmospheric transmission, and to improve information capacity, we again propose the circular polarization multiplexing (CPolDM) modulation method. The CPolDM not only ensures the reliability of the communication, but also doubly increases information capacity of system. The conversion process between linear polarization and circular polarization provides a theoretical basis for the circular polarization modulation encoding and decoding.Secondly, according to the requirements of laser communication in the atmospheric, combined with two kinds of circular polarization modulation method, communication system of circular polarization modulation is designed. According to the function of communication system, the transmitter, receiver and optical antenna are designed. The communication system includes laser alignment straight shaping system, the circuit of modulation and demodulation, the opto-mechanical system of transmitting and receiving optical antenna. Two modulation schemes can be verified in the same system. The laser communication system is very simple and convenient, which could be used in space borne communication terminal.Thirdly, this dissertation introduces the infection to laser communications by the atmospheric absorption, refraction, scattering and turbulence, which particularly analyses variances of the state of polarization (SOP) and degree of polarization (DOP) of signal light during laser beam propagating in the atmosphere, and discusses the depolarization caused by the atmospheric scattering. Based on the specific analysis of each parameter which changes in the SOP of laser beam, we introduce various measures to improve the performance and reliability of the system. Base on the Electromagnetic Hermite-Gaussian(EHG) beam model, the change process of polarization properties of partially coherent light laser propagating in the turbulent atmosphere is deduced, and derived the condition for circular polarization to keep polarization property unchanged, which can be used for optimizing the performance of circular polarization modulation system. According to the circular polarization modulation characteristics, the atmosphere is regarded as a linear delay retardant. Based on the analysis of the birefringence of circular polarization in the atmosphere, it not only can be used to test circular polarization state change quantity, but also estimate the power attenuation and the availability of the FSO system.Finally, in order to verify the performance of circular polarization modulation, the performance of the communication system is analysed. Compared circular polarization modulation with several modulations, the results show that circular polarization modulation has the lowest bandwidth request and the smallest BER, but with the highest average power. Polarization modulation can also improve the channel capacity by polarization multiplexing, so it is an excellent modulation scheme. Through simulating and analyzing the polarization modulation system in the software, it shows that the dual differential receiver polarization modulation system has the best performance. Subsequently, base on the analysis of the actual circular polarization communication system, which is verified simply experiments, completing the transmitting and receiving of data to prove the feasibility of the circular polarization modulation and the rationality of the system design, especially CPolSK has very low BER in the turbulence. To improve the performance of the communication systems, the impact on the communication performance of optical components is analyzed in the paper, and the technology of coherent detection is adopted, which effectively can improve the receiving sensitivity, BER performance and the ability of anti-atmospheric disturbance.更多还原