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Atmospheric turbulence has a greater influence on the performance of the atmospheric laser communication system increasing the bit error rate (BER) and reducing the signal to noise ratio (SNR). If the bidirectional free space laser transmission link has channel reciprocity, the change of optical signal intensity at the two terminal is correlated, and channel state information (CSI) can be obtained at the transmitter, which saves channel space and improves real-time detection of instantaneous CSI and communication channel capacity. In this paper, under the condition of weak-fluctuation, according to the Rytov approximation theory, the relationship between the correlation of optical signal of receiving terminal and transmission path of bidirectional free space laser transmission link is deduced and analyzed, and then the coaxial atmospheric laser transmission link of bidirectional transmitting and receiving is built for field experiment. Experimental results show that the intensity of optical signal at the receiving terminalof bidirectional free space laser transmission link is correlated. The correlation coefficient is related to the location of transmission path. The probability density distribution of the optical signal collected by the experiment obeys the terminal and approximation theory, the relationship between the correlation of optical signal of receiving terminal and transmission path of bidirectional free space laser transmission link is deduced and analyzed, and then the coaxial is correlated. The correlation coefficient is related to the location of transmission path. The probability density distribution of the optical signal collected by the experiment obeys the lognormal distribution, and the real-time change trend of the intensity of the speckle signal at the both receivers is same. Therefore, the atmospheric channel of the bidirectional free space laser transmission link is reciprocal. The conclusion of this paper is of great significance for realizing high-rate and low bit error rate transmission in atmospheric channel.
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