| Free space optical communication (FSO) is a kind of wireless communication system which employs laser light as carrier to transmit information in free space. Compared with traditional RF wireless communication, FSO has wideband, good security, immunity to electromagnetic interference, lower power consumption, small size and large transmission capacity, etc. Therefore, FSO is a very promising technology in many application fields, including interstellar communication, satellite-ground communication and ground communications. The research of FSO has been carried out for many years, which focuses on the research of the laser source, modulation format, atmospheric channel, ATP (acquisition, tracking, and pointing) system and receiving technology.However, light propagates through earth’s atmosphere may be severely affected, and atmospheric turbulence is the main reason for the deterioration of FSO channel. Atmospheric turbulence will produce a range of effects on laser beam, resulting in reduced quality of communication, or even the loss of communication in extreme cases. Therefore, the study of atmospheric channel model is very important. Due the degradation of FSO channel, improving the receiver sensitivity is also crucial to improve the overall performance of FSO system. Thus, investigating receiving technology and modulation format is necessary to improve sensitivity. In addition, compared with fiber channel, FSO channel is more suitable for space mode multiplexing to increase transmission capacity. This paper studies the several key technologies of FSO system, including FSO channel, receiving technology and spatial multiplexing method. The main research works and innovations of this dissertation are as follows:1) The model of atmospheric channel. Based on Kolmogorov’s theory of turbulence, atmospheric phase screen method is used to establish a channel model. Laser transmission in turbulence is simulated numerically. Spatial light modulator (SLM) has been used to build an indoor experimental platform, which could be used to emulate atmospheric turbulence with different strength. Experiment is conducted to study the effects of turbulence on the laser beam, including the far-field intensity distribution and receiving power.2) The study of receiver technology. The performance of spatial diversity reception is analyzed. Three common combining algorithms are implemented and simulated to compare performance under BPSK and QPSK transmission, respectively. The performance of coherent detection and digital signal processing (DSP) algorithm is investigated. The performance of coherent free space optical communication system is studied experimentally. The receiver sensitivity of various modulation formats is analyzed and compared.3) Orbital angular momentum (OAM) and its applications in FSO system are investigated. Spatial multiplexing technique based OAM is introduced. With the utility of spatial light modulator, experiment is conducted to convert common Gaussian beam into OAM of arbitrary order by using corresponding greyscale images. |
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