## 5 Experimental Results and Analysis  
According to the bidirectional transceiving coaxial atmospheric laser propagation link shown in Figure 4, the experimental link was built to carry out the bidirectional coaxial atmospheric laser propagation experiment. The atmospheric turbulence emulation pool was used to simulate the atmosphere as the propagation channel. The temperature gradient during the experiment was 10 \({}^{\circ}\)C, that is, when the temperature of the atmospheric turbulence emulation tank box increased by 10 \({}^{\circ}\)C, the two terminals of the bidirectional atmospheric laser propagation link simultaneously collected the power of a group of received spot signals and stored them in the computer through the acquisition card. Figure 5 shows the variation curve of the correlation coefficient of the spot signals received by terminals A and B in the propagation link with the temperature difference \(\Delta T\) of the atmospheric turbulence emulation pool. We can see that when the atmospheric turbulence emulation pool was closed, that is, there was no atmospheric turbulence, the correlation coefficient of \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Transmitter**} \\ \hline \multicolumn{2}{c}{Aperture of optical antenna} & the effective aperture: 80 mm \\ \hline \multirow{2}{*}{Laser} & wavelength: 1550 nm \\  & output power: 0-20 mW \\ \hline \multicolumn{2}{c}{SMF circulator} & isolation parameter: 40 dB \\ \hline \multicolumn{2}{c}{**Atmospheric channel**} \\ \hline \multicolumn{2}{c}{Coherence length} & 1–40 cm \\ \hline \multicolumn{2}{c}{Intensity frequency range} & 100 Hz \\ \hline \multicolumn{2}{c}{Characteristic velocity} & \(>\)0.1 m/s \\ \hline \multicolumn{2}{c}{Turbulence intensity stability} & 15\% \\ \hline \multicolumn{2}{c}{**Receiver**} \\ \hline \multicolumn{2}{c}{Detector} & wavelength range: 185 nm–25 μm \\  & Power range: 100 PW–200 W \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters of the bidirectional coaxial atmospheric laser propagation system. Figure 4: Indoor bidirectional transceiving coaxial atmospheric laser propagation system. the channel was 0.023. That is to say, the instantaneous fading of the forward and inverse propagation atmospheric turbulence channels had no correlation at this time. Since the atmospheric channel and channel state were not involved at this time, it is mainly the influence of the experimental device on the spot signal received by the two terminals of the beam spot bidirectional coaxial atmospheric laser propagation link, especially the influence of random noise on the spot signal, after opening the atmospheric turbulence emulation pool. As the internal temperature of the atmospheric turbulence pool increased with a gradient of 10, the turbulence effect gradually became more obvious. At this time, the correlation coefficients of the forward and inverse propagation propagation channels of the bidirectional coaxial atmospheric laser propagation link gradually increased. The forward and inverse propagation channel correlation coefficients increased rapidly before the temperature difference reached about \(\Delta T=50\)\({}^{\circ}\)C, and the correlation coefficient of the forward and inverse propagation channels was above 0.7. As the temperature difference continued to increase, the effect became more obvious, and the correlation coefficient of the forward and inverse propagation channels gradually increased to above 0.9 and became stable. From Figure 6, one can see that the correlation coefficient of the forward and inverse propagation channels had a less obvious downward trend, and this was because as the temperature difference continued to increase, the turbulence effect in the atmospheric turbulence emulation pool gradually became stronger turbulence. At this time, the influence of atmospheric turbulence on the spot signal in the bidirectional coaxial atmospheric laser propagation link gradually increased, and the light intensity scintillation effect was more obvious.