# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Photoacoustic Communication  
Wireless technology between sea water and the air has been an unresolved challenge for several decades. With the advancements in maritime technologies and the need for the investigation of the undersea environment, connectivity between the air and underwater is rapidly increasing [209]. The ocean is very important for the politics, economy, and security of a nation. Marine informatization, communication, detection, and intelligent decision making are being paid more and more attention nowadays. Basically, acoustic transducers are used to create today's sonar systems that are being used to detect and analyze the submarine environment. However, for effective data transmission via acoustic sound, transducers should be physically submerged at an aquatic depth. The air-water interface imposes additional limitations on the communication mechanisms utilized between submerged transducers and the water surface or in-air systems. RF or FSO devices are often placed on the sea surface to communicate with flying (in-air) platforms. Establishing communication between an in-air platform and underwater or submarine platform by exploiting the sound from the light at the air water interface is known as air-optical submarine communication. A PAC system generates acoustic message information in a submerged atmosphere using the optical energy arriving form an in-air platform. The approach describes a way for transferring data at fast rate from an in-air location to a submerged location. Furthermore, it is crucial to have an efficient communication system between overwater and underwater locations to facilitate a direct air to underwater interface for target detection, submarine communication, and other marine-related data collection [210]. Thus, PAC for a direct air to submerged network is immediately required. Transforming EM impulses for acoustic wave generation at the air barrier is a better option since EM waves can travel in the air while acoustic waves can travel underwater [211]. Heat expansion, exterior evaporation, and optical disintegration are the most frequent processes for transforming EM waves to acoustic waves [212]. Existing underwater communication has two modems: an acoustic modem within water and an RF modem for ground station and/or airborne platform connectivity [213]. Nevertheless, such a method is prohibitively expensive and impracticable in larger regions. A technique using laser-generated acoustic waves is a good choice to realize air-underwater communication, which uses both optical and acoustic waves in series under the principle of the \"photoacoustic effect\" [214]. Photoacoustic impact, also known as optoacoustic impact, is a more effective technique to transform light energy into ultrasonic radiation at the air-water interface [215]. In practice, there are mainly three methods [216] for achieving the optoacoustic effect for the conversion of light waves into acoustic waves, namely (i) direct optical beam focusing, (ii) optical beam focusing on the active relay, and (iii) optical beam focusing on the passive relay [209]. In the direct beam focusing method, the transmission optics shape either a high-power continuous wave (CW) or a pulsed wave (PW) optical beam and pass it to the desired point on the water surface. The incident light beam generates the acoustic wave in accordance with its wavelength, incident power, beam width, wavefront quality, and water refractive transparency [217]. In the other two methods, a relay (light absorbers--refer to Figure 31) absorbs the light and generates the acoustic wave by thermal expansion, i.e., temperature rise [209; 218]. These phenomena bring the photoacoustic effect, which is being increasingly used in medicinal [219] as well as industrial applications [220]. Furthermore, because of the high optical absorption coefficient of contrasting agents of relay, the strength of a photoacoustic pulse is directly related to its absorbing factor and it can enhance the acoustic energy transfer performance [209].