## 2 Principle of Detection
### Design of Mie-Scattering Lidar  
The Mie-scattering lidar system used to detect atmospheric turbulence consists of a laser-emitting system, a receiving system to receive the return signals generated by the scattering effect of atmospheric molecules and aerosol particles, and the atmospheric turbulence effect, a spectroscope system to divide and filter the return signals, and a data acquisition system to amplify and acquired return signals. Figure 1 shows a schematic diagram of the Mie-scattering lidar system.  
The laser beam is emitted by an Nd:YAG laser in the transmitting system. After expanding through the beam expander, the laser beam is transmitted to the detection height through atmospheric Fresnel diffraction, atmospheric molecular extinction, and atmospheric turbulence. The backscattering return signal is received by a Schmidt-Cassegrain telescope and is transmitted through the small aperture variable diaphragm and optical fiber into the spectroscope system. The signal is then collected and processed by the data acquisition system and transmitted to the computer.  
For the emitting system, the laser used in this study is an Nd:YAG solid-state laser, which consists of an incident laser with a fundamental frequency wavelength of 1064 nm and a double frequency wavelength of 532 nm. In this study, the incident wavelength is selected as 532 nm, the corresponding single-pulse energy is 150 mJ, the energy stability is less than 3%, the power stability is less than 2%, and the repetition frequency is 10 Hz.  
For the receiving system, according to the backscattering multiplier effect of lidar light waves in the nonuniform medium, the effect is most obvious when the combined transceiver structure is adopted [28; 29]. Therefore, the coaxial structure of light intensity glinting lidar is adopted, which has the characteristics of a small blind area, convenient collimation, and a compact structure. The telescope used in this study is a Schmidt-Cassegrain version type, with a parabolic primary mirror and two hyperboloid secondary mirrors in the barrel. The telescope aperture \(D=254\) mm, and the focal length \(F=2500\) mm.  
In the spectroscope system, the laser signal transmitted from the optical fiber is collimated through a flat, convex lens and then passes through a 532 nm filter to filter the stray light. The beam splitter with R:T = 3:7 is used to divide the beam in two ways: the reflected signal passes through a filter with an optical density of 0.1 to the flat, convex lens and then converges into the photomultiplier (PMT1) and the transmitted signal passes through a filter with an optical density of 0.2 to the flat, convex lens and then converges into the PMT2. The purpose of using different optical density filters is to test the built Mie-scattering lidar system and to judge the amplification of the strong and weak signals.  
For the data acquisition system, the PMT selected in this study and the high-voltage power supply were produced by Licel in Germany and can convert weak optical signals into electrical signals. In order to facilitate the collection and display of the PMT-converted voltage signal by the data acquisition unit, an amplifier is installed to amplify the electrical signal and eliminate the signal oscillation. The data acquisition unit is a high-speed fluorescent oscilloscope, which has a bandwidth of 1 GHz, a sampling frequency of 100 M/s, and a recording length of 10,000 points and can sample the data points after 512 times that of the average return signal. Table 1 specifically lists the main parameters of atmospheric turbulence detection using Mie-scattering lidar.  
Figure 1: Schematic diagram of the Mie-scattering lidar system.