Research On Probability Distribution For Efficiency Of Spatial Light Coupling Into A Single-mode Fiber In The Presence Of Atmospheric Turbulence

In the 21 st century information is playing a more and more significant role. Demand for data rate increases from Mbps to Gbps. The data rate of microwave communication cannot reach several Gbps because of its low frequency which hardly meet users’ demand. In this situation, a new communication method with high data rate, large capacity and high reliability is needed more than ever before, especially in satellite communication area.Laser communication has become a part of people’s daily life with the development of optical fiber and can reach several Gbps easily. Compared with microwave communication, laser communication does have advantages in data rate, information capacity, security and so on. Nowadays, free-space optical communication is stepping into several areas, such as satellite-to-ground communication. However, data rate of spatial optical communication is not high enough as it was expected limited by electronic components at the beginning. With the development of Erbium-doped fiber amplifier(EDFA), weak signal light can be amplified at the receiving system in laser communication and sensitivity and data rate are highly improved.In order to be amplified by EDFA, spatial light should be coupled into a single-mode fiber first. Therefore, coupling efficiency is a key parameter to evaluate the performance of the receiving system. In the presence of atmospheric turbulence, average efficiency of spatial light coupling into a single-mode optical fiber is widely used to estimate signal to noise ratio(SNR) and bit error rate(BER) in free-space optical communication. However, it is inaccurate because coupling efficiency changes into a random variable and floats near its means at a wide range. Besides, the nonlinear relationship between coupling efficiency and BER or SNR makes it more complicated to estimate their means. Thus, a new method should be taken into consideration. The research contents of this paper are as follows:Based on the definition of coupling efficiency and probability theory, a statistical model of coupling efficiency is built in the presence of atmospheric turbulence and an analysis expression of its probability distribution function(PDF) is derived. In order to test the model, the PDF is simulated as functions of turbulence intensity and compensation terms and results are as they are expected.SNR and BER of the receiving system in satellite-to-ground laser communication link are analyzed in the presence of atmospheric turbulence using the statistical model. Furthermore, it is also analyzed how compensation terms affects the average SNR and BER.An outdoor laser communication experiment with 11.8km long distance has been done. The model is compared with the distribution of coupling efficiency summarized using experimental data. Then, the model is revised in order to fit the actual distribution.This paper is mainly about analyzing coupling efficiency affected by atmospheric turbulence in a new way. Experimental data confirms the model. Meanwhile, average SNR and BER are estimated more accurately using the PDF of coupling efficiency, which is significant to estimate the performance and improve communications quality in satellites-to-ground laser communication.