Fig. 1. Optical wireless communication APT system diagram^[5]

Fig. 2. Typical wireless laser communication APT system diagram^[1]

Fig. 3. Schematic diagram of experimental azimuth^[33]

Fig. 4. Composite axis pointing system^[42-43]

Fig. 5. Suppress the error before and after the angle increment^[43]. (a) Angle increment before error suppression; (b) Angle increment after error supperssion

Fig. 6. Alignment response curve^[43]

Fig. 7. The relationship between input voltage and angle^[43]. (a) Control voltage and angle in θ_x direction; (b) Control voltage and angle in θ_z direction

Fig. 8. Structure of beam detection system^[5]

Fig. 9. Four kinds of situations of light beam detection^[5]. (a) α=0, p=0; (b) α≠0, p=0; (c) α=0, p≠0; (d) α≠0, p≠0

Fig. 10. Transceiver integrated UAV relay APT system^[45]

Fig. 11. The signal waveform of the oscilloscope at the receiving^[45]

Fig. 12. Coordinate position distribution of spot center^[45]

Fig. 13. 1.3 km far-field experimental system assembly structure diagram^[5]

Fig. 14. Spot position coordinates (2016-05-25 22:46~2016-05-26 22:00, rainy, 13 ℃~18 ℃)^[5]. (a) Azimuth direction; (b) Pitching direction

Fig. 15. Schematic diagram of beam tracking system^[47]

Fig. 16. Tracking curve of beam (2017-12-05 18:00~2017-12-06 6:00, cloudy, −1 ℃~9 ℃)^[47](a) Azimuth direction; (b) Pitching direction

Fig. 17. Statistical results of maintaining the beam position (2017-12-05 18:00~2017-12-06 6:00, cloudy, −1°~9°)^[47] (a) Azimuth direction; (b) Pitching direction

Fig. 18. Assembly drawing of beacon free optical APT system of 10.2 km experiment^[43]

Fig. 19. Spot center coordinates curve (2018-09-30 21:00~2018-10-01 0:00, sunny, 17 ℃)^[48]. (a) Azimuth direction; (b) Pitching direction

Fig. 20. Spot center coordinates curve (2018.10.1 21:00~2018.10.2. 0:00, cloudy, 12 ℃)^[48]. (a) Azimuth direction; (b) Pitching direction

Fig. 21. Spot center coordinates curve (2018-10-02 21:00~2018-10-02 0:00, cloudy, 14 ℃)^[48]. (a) Azimuth direction; (b) Pitching direction

Fig. 22. Spot center fitting curve^[48].(a) Change curve of spot center position; (b) Temperature and humidity curve

Fig. 23. 100 km field experiment scene^[48]. (a) Receiving terminal; (b) Transmitting terminal

Fig. 24. Spot center coordinate change curve (The first experiment)^[48]. (a) Spot center coordinates in horizontal direction; (b) Spot center coordinates in pitch direction (2019-08-18 23:00~2019-08-19 02:00, sunny, 14 ℃)

Fig. 25. Spot center coordinate change curve (The second experiment)^[48]. (a) Spot center coordinates in horizontal; (b) Spot center coordinates in pitch direction (2019-08-20 23:00~2019-08-20 02: 00, cloudy and rainy, 9 ℃)

Fig. 26. Wireless optical communication IM/DD system with fast alignment of two-dimensional mirror^[40]

Fig. 27. Experiment of wireless optical communication for 10.3 km^[40]

Fig. 28. Beam tracing curve (2021-07-24 23:00~2021-07-25 6:00)^[40]. (a) Pitching direction; (b) Azimuth direction

Fig. 29. Power spectrum density estimate^[40]. (a) X position; (b) Y position

Fig. 30. Spot tracking curve and PSD^[40]. (a) Curve of the beam tracking; (b) Curve of the beam tracking in X and Y directions; (c) X PSD; (d) Y PSD

Fig. 31. Receive and transmit signal waveforms^[40]. (a) Transmitting signal; (b) Receiving signal