## 5 Experimental Results and Analysis
Therefore, the correlation coefficient of the forward and inverse propagation channels slightly reduced, that is, the reciprocity of the forward and inverse propagation channels slightly worsened. Figure 6 shows the spot measurement data of the two receiving terminals of the bidirectional free-space laser propagation link under different turbulence intensities. In the experimental data under three different intensity turbulence conditions--weak, moderate, and strong--a set of representative time-domain data change curves of the spot signal of the receiving terminal are selected. The blue line in the figure is the power of the spot signal received by terminal A of the bidirectional propagation link, and the red curve is the power of the spot signal received by terminal B of the bidirectional propagation link. Figure 7a-c shows the captured time series of the light spot signal power under the conditions of weak turbulence (\(T=80\)\({}^{\circ}\)C), moderate turbulence (\(T=150\)\({}^{\circ}\)C), and strong turbulence (\(T=220\)\({}^{\circ}\)C). It can be found that the changes to the received optical signals  
Figure 5: The correlation coefficient changes with the temperature difference of the atmospheric turbulence emulation pool. of terminals A and B over time were very similar, and the correlation coefficients of the received light spot signals were calculated to be 0.9324, 0.8839, and 0.8695, respectively, indicating that the instantaneous fading characteristics of the optical signals received by the two terminals had a certain correlation. Figure 7 shows a distribution diagram of the correlation coefficients of the optical signals received by terminals A and B and their average values under different atmospheric turbulence conditions. Using the temperature adjustable characteristics of the atmospheric  
Figure 6: Spot measurement data of two receiving terminals of a bidirectional free-space laser propagation link under different turbulence intensities: (**a**) \(T=80\)\({}^{\circ}\)C weak turbulence, (**b**) \(T=150\)\({}^{\circ}\)C moderate turbulence, (**c**) \(T=220\)\({}^{\circ}\)C strong turbulence. Figure 7: Correlation coefficient distribution and its average value. turbulence emulation pool, 100 sets of experimental data were collected under different turbulence intensities. Equation (1) is used to calculate the correlation coefficient of the received signal at both ends of the bidirectional coaxial atmospheric laser propagation link. Among them, the blue symbol is the sample data of the weak atmospheric turbulence, the black symbol is the sample data of the moderate atmospheric turbulence, and the green symbol is the sample number of the strong atmospheric turbulence. It is found that in the three sets of sample data, the correlation coefficients of the received spot signals of the two terminals were all above 0.7, and the average values were 0.8956, 0.8826, and 0.8610, respectively. Most of the correlation coefficients of the received spot signals were above or near their average line. Under different turbulence intensities, the forward and inverse propagation channels of the bidirectional coaxial atmospheric laser propagation link had good reciprocity. The intensity of atmospheric turbulence influenced the reciprocity of the forward and inverse propagation channels. The axial atmosphere laser propagation link had better reciprocity between the forward and inverse propagation channels under weak turbulence conditions. As the intensity of atmospheric turbulence increased, the channel correlation coefficient decreased slightly, that is, the reciprocity of channel deteriorated. Figure 8 shows the spot measurement data and probability density distribution diagram of the receiving terminal of the bidirectional transceiving coaxial atmospheric laser propagation link. The received power is used to characterize the strength of the instantaneous signal at the receiver. From left to right, the normalized received signal changed with time.