# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Underwater Optical Wireless Communication (UOWC)  
An underwater optical link, with its high communication speed (16.6 Gbps as of today [176]), too small interference, and directionality (compact multi-path properties), offers an enticing alternative to conventional acoustic transmission networks. Underwater optical applications have exploded in recent years, with the applications extending from underwater surveillance/investigation, observation, and recording of seabed features, ship-protected communications, submarine communications, automated underwater vehicle connectivity, torpedo navigation, and underwater device node information routing without interface, among others. In UOWC, there has been significant progress, notably with underwater sensor networks, which are now being implemented more often [177]. Autonomous underwater vehicles (AUVs) are an example of this. AUVs are autonomous submerged vehicles that navigate automatically. Because AUVs are widely utilized for underwater activities with variety of capabilities, they symbolize fast advancements in submerged maritime data transmission system. Earth is a wonderful place with around 71 percent of its total being water, and the oceans retain nearly all that water, making them the heart of Earth. These oceans are used for transportation, food, and communication. Underwater communication systems are appropriate for a variety of reasons, including economy, security, environmental monitoring, search and review in the deep ocean, scientific exploration, and data collection [178]. Due to numerous hazards, most of the oceans remain undiscovered. Classical underwater communication systems encounter several challenges, including power sources, noise, and bandwidth limitations. The performance of these systems can have an impact on all these elements: distribution losses are significant, as well as poor bandwidth, Doppler spread, high latency, and multi-path transmission with temporal variations. These factors limit the performance of acoustic waves for underwater communication [179]. All these factors contribute to acoustic channel temporal and spatial fluctuation, limiting the system's useable bandwidth. Underwater acoustic communication transmits data at 100 s of kbps within a short range and at 10 s of kbps over long distances. When employed for high-data-rate transfer over short distances, as in classical cases, radio frequencies are a promising alternative for underwater wireless communication. The permittivity, permeability, conductivity, and volume charge density of electromagnetic (EM) wave factors vary depending on the underwater conditions and RF employed. RF wave attenuation increases with frequency and is greatly reduced by sea water [178]. RF high-power narrow-beam transmissions, which need a large antenna, endure significant losses in ocean water; hence, optical beam, as the carrier, is the next alternative for submerged maritime high-data-rate transmissions. UOWC can reach several Gbps at a few thousand meters with the high-frequency optical carrier illustrated in Figure 29. Optical wireless communication has gained popularity in recent years for underwater networks due to its capability to provide speedy information with low power and without bulk system requirements [180]. For short-range wireless communications, standard sonic communication methods give lower information rates than UOWC, with less power and posing fewer computing challenges. However, it will not be suitable for the deepest sea to the shore communication, where UOWC has a wide range of potential uses. Even though optical signals in a submerged maritime atmosphere environment confront multiple extreme hurdles due to the consumption of water, dispersion of particulate matter, and high solar disturbance, there are numerous examples of high bandwidth optical networks submerged across reasonable distances. Numerous hypothetical and practical studies on the behavior of optical beams underwater have been conducted [179; 181; 180].