## 3 Real-Time Measurement Scheme for Light Intensity Flicker and Fiber Coupling Efficiency  
The laser communication system consists of two parts, the transmitting end and the receiving end. The real-time measurement system of atmospheric turbulence light intensity scintillation and coupling efficiency is established at the receiving end of the ground station of the laser communication system, and the received signal light is divided into two beams of light through the beam-splitting prism. A beam of light is received by the APD, and the scintillation index is measured at the same time as the optical signal is demodulated. Another beam of light passes through the lens for beam convergence, coupling spatial light into the fiber. The coupling efficiency was measured by an optical power meter. Real-time measurements of the scintillation index and coupling efficiency were accomplished. The measurement principle is shown in Figure 2.  
The experimental design used a self-focusing lens for beam convergence with an effective aperture of 10 mm and an equivalent focal length of 50 mm. The core diameter of the single-mode fiber used for coupling was 10 \(\upmu\)m, and the numerical aperture NA was 0.11. An OPM-3216 optical power meter was selected to complete the received optical power measurement. The photodetector selected was an InGaAs APD high-gain-type avalanche photodiode, the bias voltage was set to 36-48 V, the current gain was up to 20 times, the detector target surface was 50 \(\upmu\)m, the dark current was 1 nA, which can be detected at a \(-\)60 dBm weak signal. If the phase effect of atmospheric turbulence on laser transmission is not taken into account for the time being and analyzed only from the energy point of view, the atmospheric  
Figure 1: Relationship of change between scintillation index and fiber coupling efficiency. (**a**) Link distance L = 3000 m. (**b**) Link distance L = 12,000 m.  
Figure 2: Schematic diagram of real-time measurement of scintillation index and fiber coupling efficiency. turbulence leads to the ups and downs of the laser energy intensity. The scintillation index of the light energy received by the detector is the following:  
\[\sigma_{I}^{2}=\frac{\left\langle I^{2}\right\rangle-\left\langle I\right\rangle^ {2}}{\left\langle I\right\rangle^{2}}=\frac{\left\langle I_{APD}^{2}\right\rangle -\left\langle I_{APD}\right\rangle^{2}}{\left\langle I_{APD}\right\rangle^{2}} \tag{7}\]  
where I is the received light intensity and \(\left\langle\cdot\right\rangle\) denotes the long time averaging or systematic averaging. \(I_{APD}\) is expressed as the current signal output from the detector after photoelectric conversion. In the experiment, the background light is the main factor that generates the noise signal, which affects the change of the received photocurrent, and it is necessary to use the filter with a bandwidth of 10 nm to complete the background light filtering to ensure the received signal-to-noise ratio. Experiments Based on Real-Time Measurement of Blimp Light Intensity Scintillation and Fiber Coupling EfficiencyDuring the experiment, in addition to the influence of background noise, thermal noise, quantum noise, etc., on the atmospheric turbulence scintillation index, the vibration of the platform during the descent of the airship will also affect the alignment of laser communication, causing the current value received by the APD and the coupling efficiency coupled into the single-mode optical fiber to change, affecting the communication performance of the system. When designing the communication system for airship-mounted laser communication, the coarse-fine composite tracking method is used to effectively compensate for the low-frequency signals caused by the vibration of the platform, and the median filtering method is used to remove the high-frequency signals. In order to ensure the accuracy of the experiment, the airship-airship laser communication experiment was carried out first, and \(C_{n}^{2}(h)\) was measured in real time.