Study On Circular Polarization Modulation In Wireless Optical Communication Through The Atmosphere

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.