## 3 Underwater Communication Experiment and Discussion  
To verify the performance of the constructed UWOC system, an underwater laser communication test platform was established in a laboratory environment, as shown in Figure 7a. The platform primarily consisted of a transmitter, a receiver, and an underwater channel. The underwater channel was simulated by a 2 m water tank, and the communication distance was extended to 10 m by employing reflective mirrors. A specific proportion of sea salt was added to pure water until the salinity reached 35 ppt to simulate clear seawater. The experimental water temperature was maintained at room temperature. The operating current and attenuation intensity were adjusted so that the output optical power was 23.3 mW, and the received optical power was measured to be 17.15 mW transmitted through a 2 m water tank. The attenuation coefficient of the water body, c = 0.153 m\({}^{-1}\), was calculated from the Beer-Lambert law, described in Equation (6) as  
\[\tau_{c}=\exp^{-cZ}, \tag{6}\]  
where \(\tau_{c}\) is the ratio of transmitted optical power to received optical power; \(c\) represents the total loss coefficient; and \(Z\) denotes the transmission distance. The water quality was tested and categorized as class I, with a total loss factor of 0.151 m\({}^{-1}\)[51]. The experiments were conducted in water of this quality. The transmitter of the system consists of a host computer, a development board, a laser, and a collimated beam spreading optical antenna, as shown in Figure 7b. The host computer acts as a signal source and generates the data to be transmitted. The data are then transferred for processing to the development board, which consists of an FPGA core board, a driver circuit, and a DAC. The FPGA performs the critical signal processing tasks, while the DAC converts the processed digital signals into analog signals, which are then used by the driver circuit to light up the laser. The laser beam is then expanded and collimated by an optical antenna and transmitted through the underwater channel. The FPGA adopts ZYNQ7020 from Xilinx (San Jose, CA, USA), and the power supply comes from the 12 V voltage of the power adapter, which is transformed into 5 V voltage by the DC-DC chip (model JW5060) to supply power to the system, or the 5 V of the USB serial port at the PS end is directly connected to the PC to supply power to the system. To ensure that the analog signals generated by the DAC meet the laser's driving requirements,  
Figure 6: Structure diagram of receiver demodulation. a dedicated laser driver circuit was designed. This circuit maintains the input signal's bandwidth while increasing the output current to a maximum of 3 A. The modulation rate can reach up to 150 MHz. This design provides the laser with the necessary band-width and current level for the drive signal, thereby guaranteeing the stability of laser operation. The 10-bit DAC is realized by using the 3 PD5651E, which has a sampling rate of 125 Msps, and the OSRAM PLPT9 450 LB_E laser (OSRAM, Munich, Germany) is selected as the light source, which has an optical power of 5 W and a typical wavelength of 447 nm to match the underwater low-absorption window. Considering the absorption and scattering properties of seawater, too large a beam divergence angle will lead to the problem of energy dispersion and a significant reduction of the communication distance; a set of optical antennas is designed in this study. These antennas consist of a fast-axis collimator, a slow-axis collimator, and a Galilean beam expander. This combination effectively collimates and expands the laser beam, reducing the divergence angles in the fast and slow axes from 0.855 rad and 0.157 rad to 0.315 mrad and 0.180 mrad, respectively. This improvement in beam collimation enhances the system's performance by minimizing energy dispersion. The receiver, as shown in Figure 7c, first uses optical devices for optical signal processing.