## 4 Experimental Setup  
The experimental system was a bidirectional transceiving coaxial atmospheric laser propagation system, as shown in Figure 4. First, the laser \(l_{1}\) of terminal A outputs a continuous wave of 1550 nm from the port \(a^{\prime}\) into the single-mode fiber (SMF) circulator \(c_{1}\), and then it outputs it through the port \(b^{\prime}\). After the transmissive collimating optical antenna \(a_{1}\) expands the beam, it compresses the beam divergence and transmits it into the atmospheric turbulence channel to reach the link terminal B. The transmissive collimating optical antenna \(a_{2}\) of terminal B receives, reduces the beam, and shapes it into parallel light. The signal enters from port \(a^{\prime}\) of the SMF circulator and outputs from port \(b^{\prime\prime}\). The detector \(d_{2}\) detects the light spot signal, and the acquisition card \(k_{2}\) collects the light spot signal. Then the computer \(p_{2}\) stores and processes the image information. At the same time, the bidirectional transceiving coaxial atmospheric laser propagation system terminal B emits the same laser beam. After the port \(c^{\prime\prime}\) enters the SMF circulator \(c_{2}\), the port outputs, through the transmissive collimating optical antenna \(a_{2}\), the beam is expanded, the beam divergence angle is compressed, and then it is transmitted to the atmospheric turbulence emulation pool channel for propagation. Then, the transmissive collimated optical antenna \(a_{1}\) enters the SMF circulator \(c_{1}\) of terminal A through port \(b^{\prime}\), and finally outputs from port \(c^{\prime}\). The light spot signal is detected by the detector \(d_{1}\), the light spot signal is collected by the acquisition card \(k_{1}\), and the image information is stored and processed by the computer \(p_{1}\). It should be noted that the equipment, parameters and experimental conditions of the bidirectional propagation link have always been strictly consistent during the experiment. The sampling programs of the computers \(p_{1}\) and \(p_{2}\) go through the synchronization server set in advance to ensure that terminals A and B sample the power of the signal synchronously, so as to ensure the reasonableness of the measurement data to the greatest extent. During the data collection process, every 10 groups of spot signals are collected, and 1 group of local spot signals are collected to ensure the reliability of the measurement data. For example, at terminal A, the signal that is emitted by laser \(l_{1}\) and passed through SMF circulator \(c_{1}\), and finally detected by optical power meter \(d_{1}\), is the local spot signal of terminal A. Note that when measuring the local light spot signal, the prepared tool should be used to cut off the bidirectional atmospheric laser propagation link to remove the influence of the received light spot signal on the local optical signal. The sampling frequency of the detector was 1 kHz, the acquisition  
Figure 3: Variation of large gas coherence length with temperature difference.  
time was 60 s, and 60,000 frames of grayscale images of the light spot were collected for each experiment.  
A high-performance continuous wave laser source was used, which works at 1550 nm and is a semiconductor laser. The isolation parameter of the SMF circulator was 40 dB. In addition, the main parameters in the experiment are shown in Table 1.