Study On Polarization Modulation System In Wireless Laser Communication

Wireless laser communication is an important branch in optical communication. It uses a laser beam as carrier to transmit information in space, with its excellent characteristics of great information content, high level of security, without authorization license, small size equipment, flexible to build and so on. It plays an important role in intersatellite, ground stations and underwater communications.The modulation technology is one of the key technologies in wireless laser communication. At present, intensity modulation is used extensively in wireless laser communication. In this thesis, polarization modulation system is discussed. The coding theory of polarization modulation, system implementation and changes of polarization characteristics of laser beam propagating in atmosphere are studied. The main contents are as follows:(1) The coding theory of polarization modulation is discussed. Moreover, the performance of polarization modulated direct detection system is analyzed. In power efficiency, bandwidth, information capacity and bit error rate, comparisons are presented with several commonly used intensity modulation schemes, such as OOK, PPM, DPPM and DPIM. Polarization modulation has better receiving characteristics than intensity modulation in bandwidth requirement and bit error rate, although it requires higher average power and has smaller information capacity.(2) In the OptiSystem simulation environment, OOK intensity modulation and linear and circular polarization modulated direct detection system models are simulated. Their performance is analyzed and compared in the same simulation condition. The result is that dual differential receiver has better system performance. Some key factors, such as detector noise, channel attenuation and beam divergence angle, which affect system performance are also analyzed. Thus it is beneficial to design the real communication systems that can reduce main noise sources and to improve the quality of received signal.(3) The polarization characteristics of Gauss– Schell model beam is studied, based on the unified theory of coherence and polarization. The results show that the degree and state of polarization remain unchanged of completely polarized Gauss– Schell model beams propagating in turbulent atmosphere in certain assumptions. However, the degree of polarization of partially polarized beams will increase slightly and its value approaches initial value of the source plane in the condition of far field approximation. The polarization degree is independent of the atmospheric turbulence. In free space, the polarization degree will increase and remain a stable value after certain distance.The study shows that polarization modulation can suppress atmospheric interference and improve communication quality, thus it is a much superior modulation scheme than intensity modulation in wireless laser comuunication.