# Free Space Optical Communication Networking Technology Based on a Laser Relay Station
## 3 Experiment and Analysis  
Figure 11 shows the test diagram of the point-to-multipoint free-space optical communication system. We mainly verified that the data at the transmitting end were sent to the receiving end in different directions through the repeater. Considering the production cost and the convenience of the test, we created a simple optical communication system that can send and receive data in six directions. We completed the basic prototype of the optical communication network group technology and verified it. The system mainly included a laser transmitter, a communication relay and three laser receivers. The hardware circuit mainly included a laser transmitting circuit, a laser receiving circuit and a system control circuit. A continuous laser diode was selected as the emission light source in the laser emission circuit. Considering the safety of human eyes during the experiment, the emission power of the laser diode was 200 mW, and the wavelength was 940 nm. The voltage-controlled current source chip IC-HK of iC Haus Company was used to drive the laser diode. The maximum switching frequency of 155 MH was achieved to meet the communication requirements. LSSAPD9-500 was selected as the photodetector of the laser receiving circuit. The APD had high reliability and low dark current, with a response band of 400-1100 nm and a high response to 940 nm laser, which could improve the reliability of the communication system. In the relay system, LD and APD shared a focus lens. They fitted together and were in the focus position of the lens, saving hardware resources and realizing the function of integrated transceiver. The system control circuit sampled FPGA and MCU as the main control chip. FPGA was responsible for data modulation, data demodulation, data address allocation, etc. MCU was responsible for communication debugging, displaying the sent data or received data on the liquid crystal display (LCD). The system displayed the data on the LCD in the form of two-dimensional images. The FPGA was EP4 CE55F23C6N from Altera Company, and the MCU was STM32F103 ZET6 from Italy France Semiconductor Company. In the system, the test diagram of which is shown in Figure 11, we sent three 250 \(\times\) 250-pixel color pictures with 16-bit color. The picture contents were pens, pavilions, and cartoon characters. Without moving the transmitter transmission angle, we needed to send the picture with pen contents to receiver A, the picture with pavilion contents to receiver B, and the picture with cartoon characters to receiver C. The three receivers were in different directions. We fixed the repeater on the optical platform. The transmitter was 25 cm away from the repeater, receiver A was 27 cm away from the repeater, receiver B was 30 cm away from the repeater, and receiver C was 26 cm away from the repeater. The communication distance was limited by the transmission power of the LD. Increasing the transmission function power increased the communication distance. Considering the safety of the experiment, we chose an LD with smaller transmission power and a shorter communication distance to complete the experiment. In order to verify the point-to-multipoint optical communication system, we sent three pictures to three different receiving targets through the relay system. The experimental results show that the picture content received by receiver A was a pen, the picture content received by receiver B was a pavilion, and the picture content received by receiver C was a cartoon character. Therefore, the system realized point-to-multipoint optical communication. At the same time, compared with the point-to-point optical communication system, it was more flexible and efficient. In order to verify the communication quality, we used an oscilloscope to detect the signal waveforms of the transmitting end and the three receiving ends, respectively. The three pictures were sent out serially in pixel units. One pixel in the picture was composed of 16-bit data.