## 1 Introduction
However, researchers have conducted a lot of studies on the fiber coupling efficiency problem in atmospheric laser communication systems, both theoretically and experimentally [16; 17; 18; 19]. However, the effect of light intensity scintillation caused by atmospheric turbulence on the fiber coupling efficiency still remains to be studied. In view of the theoretical and experimental defects in current research, this paper establishes the relationship between light intensity flicker and spatial coherence radius based on the principle of fiber coupling efficiency. The effect of light intensity scintillation on the fiber coupling efficiency is also analyzed using spatial coherence radius. A laser communication system was built. To ensure the accuracy of the experiment, the real-time measurement of \(C_{n}^{2}(h)\) was first carried out. Then, real-time measurement experiments were conducted on atmospheric light intensity flicker and optical fiber coupling efficiency at different transmission distances and altitudes above the ground. Analysis of the Influence of Atmospheric Light Intensity Scintillation on Optical Fiber Coupling Efficiency  
Optical signals in the transmission process will be affected by atmospheric turbulence. Turbulence-induced scintillation effects damage the phase and amplitude distortion of the optical signal to reach the receiving end of the spatial coherence, reducing the degree of matching between the focused optical field and the mode field of the single-mode fiber, reducing the coupling efficiency of the spatial light to the single-mode fiber, and affecting the quality of communication of the system. For plane wave and Kolmogorov turbulence spectra, the coupling efficiency [20] of the single-mode fiber after passing through the atmospheric turbulence channel is expressed as:  
\[\eta=8\text{a}^{2}\int_{0}^{1}\int_{0}^{1}\exp[-(\text{a}^{2}+  
a = 1.12 is the optimal value to take. The coupling efficiency depends only on the spatial coherence radius.  
Under the inclined path, the calculation of the spatial coherence radius is defined as:  
\[\rho_{c}=\left[1.46k^{2}sec\xi\int_{h_{0}}^{h}C_{n}^{2}(h)dh\right]^{-3/5} \tag{2}\]  
\(C_{n}^{2}(h)\) is a parameter that measures the strength of atmospheric turbulence. Light intensity scintillation is one of the main turbulence effects affecting atmospheric laser transmission. The intensity scintillation effect can be expressed by the scintillation index, which is a statistical measure that describes the change in intensity during light propagation and is commonly used to quantify rapid fluctuations in light intensity or scintillation due to atmospheric turbulence. When Rytov is less than 1, the value is similar to the light intensity scintillation variance \(\sigma_{I}^{2}\). For oblique range transport, the Rytov variance under the plane wave approximation is:  
\[\sigma_{I}^{2}=2.25\mu_{1}k^{7/6}(H-h_{0})^{5/6}sec^{11/6}\xi \tag{3}\]  
where \(\xi\) is the zenith angle, H is the communication height, and \(h_{0}\) is the ground station height. Then, \(\mu_{1}\) is:  
\[\mu_{1}=\int_{h_{0}}^{H}C_{n}^{2}(h)\bigg{(}\frac{h-h_{0}}{H-h_{0}}\bigg{)}^{5 /6}dh \tag{4}\]  
Then, we can deduce that \(C_{n}^{2}(h)\) is:  
\[C_{n}^{2}(h)=-\frac{5}{6}\times\frac{\sigma_{I}^{2}}{2.25k^{7/6}sec^{11/6}\xi} (h-h_{0})^{-5/6} \tag{5}\]  
The relationship between the scintillation index and the spatial coherence radius can be obtained as follows:  
\[\rho_{c}=\left[-1.2167k^{2}sec\xi\int_{h_{0}}^{h}\frac{\sigma_{I}^{2}}{2.25k^{ 7/6}sec^{11/6}\xi}(h-h_{0})^{-5/6}dh\right]^{-3/5} \tag{6}\]  
Combining the above equation with Equation (1) enables the analysis of the effect of the light intensity scintillation index on the fiber coupling efficiency. According to the above theoretical results, the receiving aperture is set to 10 cm.