## 4 Experiment and Results
### Angular Measurement Accuracy  
In order to address the high accuracy requirements of long-distance beam measurements between satellites, an angle measurement accuracy test was conducted. The power of the signal laser beam transmitted into the QD photosensitive surface was adjusted to \(-16\) dBm, the power of the local oscillation laser beam was \(-7\) dBm, the bandwidth of the phase-locked loop was \(10\) Hz, and the angular deflections of the FSM azimuth and pitch axes were \(0\). In order to make the direct detection signal power higher than its sensitivity in the subsequent comparison experiments, which were designed to yield more informative experimental results, the signal laser beam power was set to \(-16\) dBm. At this time, the accuracy of the heterodyne coherent angle measurement is shown in Figure 9. The experimental findings demonstrate that the azimuth axis angle measurement accuracy was \(2.54\)\(\upmu\)rad, and the pitch axis was \(2.85\)\(\upmu\)rad. Next, the influence of the local oscillator laser beam power on the measurement accuracy of the heterochromatic coherence angle was tested. The main noise sources in  
Figure 8: Angle measuring results. (**a**) Azimuth axis. (**b**) Pitch axis. Figure 9: Zero heterodyne coherent angle measurement accuracy results. (**a**) Azimuth axis. (**b**) Pitch axis. coherent detection systems include thermal noise, shot noise, and laser relative intensity noise. The accuracy of the angle measurement will mainly depend on the signal-to-noise ratio of the system. When the power of the local oscillator laser beam is small, the signal-to-noise ratio increases with the increase in power. However, when the local oscillation laser beam power exceeds a certain threshold, the laser relative intensity noise becomes the dominant factor, leading to a decrease in the signal-to-noise ratio with increasing power [24]. Therefore, it is essential to select the optimal power of the local oscillation laser beam by integrating the above characteristics with the specifications of the experimental device. The saturation optical power of the QD selected in the experimental system was \(-5\) dBm, and the measurement accuracy of the heterodyne coherence angle under different signal laser beam powers was tested when the local oscillation laser beam power was \(-7\) dBm, \(-10\) dBm, and \(-13\) dBm, respectively. The test results are shown in Figure 10. The experimental results indicate that when the local oscillation laser beam power was \(-7\) dBm, the heterodyne coherence angle measurement accuracy under the same signal laser beam power was relatively optimal. Therefore, \(-7\) dBm was selected as the local oscillation laser beam power of the experimental system. In the process of heterodyne coherent angle measurement, the phase detection accuracy of the phase-locked loop on the beat-frequency signal directly affects the angle measurement accuracy. The loop bandwidth is an important parameter that affects the phase detection accuracy. Therefore, the effect of the loop bandwidth on the accuracy of heterodyne coherent angle measurement was further tested. The conditioning signal laser beam power was \(-16\) dBm, the local oscillation optical power was \(-7\) dBm, and the test results are shown in Figure 11. The experimental findings revealed that when the loop bandwidth was set at \(10\) Hz, the accuracy of the heterodyne coherence angle measurement was optimal under the same power level. When the loop bandwidth gradually increased on the basis of \(10\) Hz, the random error increased accordingly. The phase tracking result of the phase-locked loop for the beat frequency signal had a large jitter, resulting in a certain degree of decrease in the angle measurement accuracy. When the loop bandwidth gradually decreased on the basis of \(10\) Hz, the system error increased accordingly. The phase-locked loop is difficult to accurately track and detect the phase of the beat frequency signal, resulting in a significant decrease in the accuracy of angle measurement.