# Laser Transmission Characteristics of Seawater for Underwater Wireless Optical Communication
## 1 Introduction  
As more and more underwater human activities develop, such as environmental monitoring, underwater scientific data collection, marine archaeology, offshore oil field exploration, port security, and marine military, the demand for underwater communication is rapidly increasing [1]. Wired communication systems, such as fiber optic systems, can provide real-time data transfer underwater but have a limited scope in practical applications because of high cost and operational difficulty. Thus, underwater wireless links emerge to meet the actual needs. Acoustic communication technology is commonly used for underwater wireless communication, offering a communication range of over several kilometers underwater with a low bit error rate (BER). However, it typically provides low data transmission rates around tens of kbps, which are insufficient for high-bandwidth applications such as image and real-time video transmission [2]. In contrast, underwater wireless optical communication meets the demands of short- to medium-distance applications and could offer a significantly higher transmission rate [3]. Due to the lower absorption of the blue-green laser within 450-550 nm underwater [4], laser diodes (LDs) or light-emitting diodes (LEDs) could be used in underwater optical communication (UWOC) systems to achieve in the order of tens of Mbps data transfer rate [5; 6] and exceeding Gbps in short or medium-long experimental tanks [7; 8]. In recent years, numerous studies have focused on underwater laser communication regarding channel modeling [9; 10; 11; 12; 13; 14] and system design and development [15; 16; 17; 18; 19; 20; 21], among which channel modeling is crucial for understanding how underwater laser light is transmitted and optimizing system design. The BER is a vital parameter of the UWOC system and is relevant to the signal-to-noise ratio (SNR) [22]. Therefore, when designing an underwater laser communication system, the key challenge is maximizing the SNR, which is proportional to the optical power received by the receiver. The received power after a certain distance can be expressed as [23]:  
\[P_{R}(t,d,r)=P_{T}(t)G_{T}P_{Lw}(t,d,r)G_{R} \tag{1}\]  
where \(P_{R}(t,d,r)\) is the received power, which is a function of the transmission time \(t\), the offset relative to the emitted laser beam center \(d\), and the transmission distance \(r\), \(P_{T}(t)\) is the transmitting power, \(G_{T}\) is the transmitter gain determined by the beam aperture and divergence, \(P_{Lw}(t,d,r)\) is the channel loss due to absorption and scattering, and \(G_{R}\) is the receiver gain determined by the receiver aperture and field of view (FOV). In addition to the absorption by the water medium, underwater channel loss \(P_{Lw}(t,d,r)\) is caused by spatial and temporal dispersion, which is attributed to the scattering of water molecules and particles as well as the natural divergence of the emitting laser. Due to multiple scattering events, spatial dispersion temporal dispersion, and furtherly constrain the signal bandwidth of underwater laser channels. In brief, the underwater signal intensity is determined by the optical properties of the water bodies. There are two main methods for channel modeling: approximate analytical method [24] and numerical method [25]. The approximate analytical method gives a closed form established under certain approximate conditions, resulting in a lack of accuracy and further restriction of the application scope. In contrast, the Monte Carlo simulation method is a more practical way since it can handle various complex nonlinear problems without simplifying or making any assumptions. The main thread of Monte Carlo ray tracing [26; 27; 28; 29; 30]is recording photon trajectories by simulating the photon's interactions with the optically active substance in water based on sampling from the probability density functions (PDFs) associated with the water's inherent optical properties (IOPs).