## 4 Experimental Setup and Results  
As shown in Figure 9a, the hardware of the 10 Gbps QPSK CFSO communication experimental system consisted of three main components: an optical transmitter, an atmospheric channel transmission link, and an optical receiver front-end. A PLUTO-2 phase-type spatial light modulator (SLM) manufactured by HOLOYE was used in this experiment to simulate the atmospheric turbulence channel, which had a resolution of 1920 \(\times\) 1080 pixels [27; 28]. The PLUTO-2 SLM was converted from grayscale to phase level, which was reflected in the wavefront phase of light, and thus simulated the atmospheric turbulence effect. Since current commercially available SLMs are sensitive to polarization, polarization multiplexing was difficult to implement in our experiments. However, polarization impairments caused by atmospheric turbulence are rather small and can generally be compensated for through channel equalization, and the proposed scheme gain was independent of polarization. The control computer in this experiment was connected to the SLM via a drive circuit. The control computer could generate the grayscale map corresponding to the set refractive index structure constant of the atmosphere, \(C_{n}^{2}\), and load the grayscale map onto the liquid crystal (LC) display of the SLM. In this experiment, 55 random phase masks were generated and converted into corresponding grayscale maps based on the atmospheric refractive index structure constants (6.5 \(\times\) 10\({}^{-14}\) m\({}^{-2/3}\), 5 \(\times\) 10\({}^{-15}\) m\({}^{-2/3}\), and 6 \(\times\) 10\({}^{-16}\) m\({}^{-2/3}\), respectively) corresponding to strong, medium, and weak turbulence, as shown in Figure 9b-d. The actual laboratory system scenario diagram is shown in Figure 10. The same DSP code as in the simulation was used to generate the data source at the front end of the optical transmitter. The I and Q signals corresponding to the real and imaginary parts, respectively, were sent to the high-speed arbitrary waveform generator, AWG8190A, with a maximum sampling rate of 12 GSa/s to generate baseband electrical signals. The up-light modulation of the signal was completed with the IQ modulator. A laser with a measurement linewidth of 50 kHz was integrated into the IQ modulator. The output signal light from the modulator was emitted through the telescope into free space. The light-transmitting power was controlled via adjusting the output power of the VOA to vary within a certain range. Figure 9: Experimental setup for a 10 Gbps QPSK CFSO transmission system. ECL: external cavity laser; PC: polarization controller; C: collimator; M: mirror; SLM: space light modulator. (**a**) QPSK CFSO communication experiment system diagram. (**b**) Grayscale map of strong-turbulence channel. (**c**) Grayscale map of medium-turbulence channel. (**d**) Grayscale map of weak-turbulence channel. After encountering polarization controller PC\({}_{1}\) and collimator C\({}_{1}\), the signal beam was propagated in free space with the same polarization direction as the function direction of the SLM. The signal beam reached the receiver side after being reflected by the SLM and mirror M, and passed through collimator C\({}_{2}\). At the receiver side, polarization PC\({}_{2}\) controlling was performed at first. Then, the signal light was fed into a standard single-mode fiber (SSMF), which was subsequently fed into a coherent receiver and oscilloscope for subsequent storage and data processing. The LO laser operated at a wavelength of 193.400 THz and a linewidth of 50 kHz. The FO between the transmitting laser and the LO laser was approximately 300 MHz. The laser beam wavelength was 1550 nm. Collimators C1-C2 were identical and had a focal length of 11.29 mm. The entire indoor transmission link distance was 900 mm. According to the Fresnel scaling theory, this experimental setup can be equated to an outdoor long-distance experiment [29]. In addition, the clock sources of the AWG and the oscilloscope were synchronized in this experiment.