# Experimental Study on Phase Scintillation of Optical Transmission in Atmospheric Turbulence
## 3 Experimental Measurements
The center position of each spot is calculated by means of the center-of-mass formula. Comparing the spot position with the reference position yields the local slope of the wavefront at each sub-aperture;
4. Finally, based on the wavefront slope, the complete wavefront phase is reconstructed using a specific wavefront reconstruction algorithm. The wavefront phase is visualized using computer software. From January 2024 to July 2024, three experimental laser transmission measurement links of different distances were established in Xi'an, Shaanxi Province, and multiple measurement experiments were conducted. The three links are as follows. For link 1, the transmitter is located on the 11 th floor of Teaching Building 6 at the Jinhua Campus of Xi'an University of Technology, and the receiver is located on the 8 th floor of Science Building No. 2 on the same campus. The link length is 0.42 km. The terrain along the link is mainly trees and buildings, with no obstructions in between. Link 1 (0.42 km) is mainly used to study the effects of near-ground turbulence and to simulate short-range scenarios such as urban communication. It is more reproducible, as the turbulence is mainly near-ground turbulence with fewer influencing factors. For link 2, the transmitter is located on the rooftop of the Xinya Garden Community in Beilin District, Xi'an, and the receiver is located on the eighth floor of Science Building No. 2 at the Jinhua Campus of Xi'an University of Technology. The link length is 2 km. The terrain along the link is mainly streets and buildings, with no obstructions in between. Link 2 (2 km) represents more typical medium-distance transmission conditions for suburban or field medium-distance laser communication scenarios. The reproducibility is moderate, and the increase in propagation distance introduces more random turbulence effects. But, it is still mainly affected by the near-surface turbulence layer, and the experimental stability is high. Figure 4: Operating schematic of Shack–Hartmann wavefront sensor. For link 3, the transmitter is located in Bailuyuan, Xi'an, and the receiver is located on the 12 th floor of Teaching Building 6 at the Jinhua Campus of Xi'an University of Technology. The link length is 10.2 km. The terrain along the link is mainly trees, streets, and buildings, with no obstructions in between. Link 3 (10.2 km) is suitable for military long-range communications or high-altitude platform optical system scenarios. Repeatability is lower, due to the significant cumulative effect of long-distance propagation superimposed on the influence of high-altitude turbulence layers, as well as higher requirements for weather conditions and sensor accuracy. Tables 1-3 provide the weather conditions for the experiments conducted on the three links. T1-T23 represent the 23 measurement samples. Figure 5: Wavefront phase (left) and slope (right) in link 1 under different weather conditions. Figure 6: \(Cont\). As shown in Figures 5-7, in link 1, the wavefront phase diagrams under different weather conditions exhibit a convex parabolic shape in all weather conditions, except for Figure 5a, where the snowy condition shows a concave shape. This is primarily due to the scattering and absorption effects of snowflakes, leading to significant wavefront phase distortion and concave features. In Figure 5a under heavy rain and Figure 5f under snowy conditions, the edge regions of the wavefront phase diagrams show irregular undulations with irregular curvature, indicating more complex distortion. In contrast, the wavefront phase curvature under sunny (Figure 5a) and cloudy (Figure 5b) conditions is relatively smooth with no abrupt changes, with defocus aberration being the primary distortion type. The slope vector diagrams reveal that the vectors generally radiate outward from the center, with variations in density and direction. Under sunny (Figure 5a) and cloudy (Figure 5b) conditions, the slope vectors show no significant disturbances or irregularities, indicating fewer high-order aberrations.