# High-Precision Tracking of Free-Space Optical Communication System on Mobile Platforms
## 3 Prototype of FSO Communication System for Mobile Platform  
According to the system design introduced in Section 2, Figures 5 and 6 show the prototype of the FSO communication system for mobile platforms. Table 1 shows the link budget of the system. When the communication distance is 2 KM, the beam divergence angle is 1 mrad, and the receiving diameter is 60 mm, the geometric loss caused by transmission is 36 dB. After considering the atmospheric transmission loss and the transmit-receive coupling loss, the system also has a link margin of 12 dB, which fully meets the needs of kilometer-level communication. Figure 4: (**a**) Design of primary mirror system. (**b**) Design of tubular folding optical path. Figure 3: Transceiver block diagram of precise targeting loads module. Table 2 shows the prototype specifications. Due to the design of the common aperture transceiver, the wavelength of receiving light is different from the transmitting light for the same laser communication device in order to avoid receiving data sent by itself. At the same time, in order to ensure normal communication between devices at both ends, the transmitting wavelength of the device at the opposite end is the same as the receiving wavelength of the device at the local end. The transmitted optical power through high-power EDFA is optimal at +33 dBm (this value can be changed by configuration EDFA up to +35 dBm), and the received optical power is at a minimum of \(-\)30 dBm. The precision sight structure has a tracking range of \(\pm\)26.2 mrad, consistent with the field of view of FSM. The coarse sight structure has a tracking range of \(\pm\)785.4 mrad, consistent with the field of view of the electro-optical pod. The divergence angle of the laser emitted by the system is 0.5 mrad. The R-C telescope system has a primary mirror diameter of 60 mm, a primary mirror multiple of 10, and a secondary mirror diameter of 12 mm. The field of view of the \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Transmitted optical power & +33 dBm \\ Transmitting antenna coupling loss & \(-\)5 dB \\ Loss of atmospheric transmission & \(-\)2 dB \\ Geometric loss & \(-\)36 dB \\ Receiving antenna coupling loss & \(-\)6 dB \\ EDFA previa filler loss & \(-\)2 dB \\ Received optical power & \(-\)30 dBm \\ System margin & 12 dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Link budget of system. Figure 5: (**a**) Prototype of FSO communication system: front view. (**b**) Prototype of FSO communication: interior view. Figure 6: (**a**) Prototype of precise targeting load module: front view. (**b**) Prototype of precise targeting loads module: top view. (**c**) Prototype of tubular folding optical path: front view. camera is 3 mrad, and the diameter of the spot on the CCD target surface is 100 um. The angular resolution and the angular acceleration of FSM are 2.4 urad and 10000 rad/s\({}^{2}\), and the mirror reflectivity is greater than 99.5%. Therefore, FSM can change the direction of the optical path with low loss and quickly compensate for optical axis offset. The design of the tubular folding optical path increases the difficulty of coaxial alignment, so we need to carry out coaxial calibration of the system after prototype assembly is completed. Firstly, the assembly error will lead to the tube light path not being coaxial and the situation that the electro-optical pod is not coaxial with a precise targeting load. To solve this problem, we use a parallel light tube, a rotating platform, and a gasket to carry out the first step of coaxial adjustment. Secondly, the receiving end is not coaxial with the tube optical path. In the calibration and debugging phase, we adjust the beam angle by controlling FSM2 to ensure that the tube optical path is coaxial with the receiving end. This angle information is written into the parameter package as the calibration origin of the FSM2, so that the receiving end will be aligned once the system is turned on.