## 2 Experimental Details
### System Design  
Figure 2 shows the overall structure of the proposed FSO system based on a common optical path. Each FSO terminal transmits and receives laser beams simultaneously in the common optical path. Data transmission and beacon functions were incorporated together to increase efficiency for SWaP without additional optical beacon subsystems. The InGaAs-based distributed feedback laser was selected as a light source for OGT and OAT, at wavelengths of 1590 nm (L-band) and 1530 nm (C-band), respectively. This was implemented to minimize possible interferences in the common optical path. If a common  
wavelength is shared in the TRx terminals, signal loss of at least 75% is expected due to the inevitably required 1:1 beam splitter (BS). The direct modulation method was utilized for non-return-to-zero on-off keying intensity modulation. To obtain sufficient optical gain, a L-band erbium-doped fiber amplifier (EDFA) with 21 dB gain and C-band EDFA with 23 dB gain were included in the OGT and OAT, respectively. To avoid unwanted light, anti-reflection coated band-pass filters (BF) were adapted in the first lens of optics inside the OGT and OAT. The customized plate type wavelength division multiplexing (WDM) filter prevents unwanted light from shining onto the avalanche photodiode (APD) and QPD. It is based on special coatings that distinguish transmitting and receiving beams. The WDM filter in the OAT reflects/transmits 1530/1590 nm light, and the WDM filter in OGT reflects/transmits 1590/1530 nm light. WDM filter also helps the received beam to safely enter the APD and QPD via a 7:3 BS. An additional BF is placed in front of the APD and QPD to remove the background noise. APD is employed for highly sensitive optical signal detection, and QPD provides precise location information to the PAT system. The output current of the APD was fed to a transimpedance amplifier (TIA) for low noise signal amplification. The microcontroller unit (MCU) controls the PTU and the electrical motor stage (EMS) by using the data produced by the QPD and the APD. The EMS is used to control the divergence angle of the transmission beam by controlling the light source position in the FSO terminal [40]. When the light source position is 0 mm in EMS, the divergence angle is the largest. As the location of the light source moves from 0 to 10 mm, the divergence angle decreases. The EMS can increase the scanning efficiency by increasing the detection probability for the target FSO terminal by initially adopting a large divergence angle [40]. Because the large divergence angle faces an extra power loss problem [40; 41], an adaptive beam narrowing strategy was employed in the PAT system to compensate for extra power loss and to improve link robustness. The PAT system was operated in two modes. First, coarse tracking was carried out for scanning counter FSO terminals by adopting a large divergence angle. When the QPD in each FSO terminal began to produce the output signal, the PAT system started fine tracking. When the output of the QPD exceeded the threshold value, the LOS link was established and APD-based data communication starts. After the LOS link was established, an adaptive beam narrowing strategy could prevent link performance degradation in long distance adalink. The maximum angular velocity of the PTU was 1.0472 rad/s, and the angular resolution of the PTU was equally set to 0.025\({}^{\circ}\) for pan and tilt. Figure 1: System concept of free space optical (FSO) communication between ground control station (GCS) and unmanned aerial vehicle (UAV). Figure 3 shows the FSO terminal mounted on the PTU, its internal structure, custom-designed QPD main board, and QPD sensor board for precision PAT. The FSO terminals in the OAT and OGT are lighter than 8 and 12 kg, respectively. The OGT has a 100 mm aperture, and the total length of its optics is approximately 350 mm. The OAT has a 70 mm aperture, and the total length of its optics is approximately 250 mm.