# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Photoacoustic Communication
Thus, the relay brings possibilities for high-efficiency (reduced optical scattering and propagation loss) in air-to-underwater photoacoustic conversion. However, in the second method, active relays are widely used, in which they must be energized to actively transform the received data light signals into underwater acoustic signals. The relay sends out the sound waves beneath the water's layer, which replicate the light data in acoustic form [221]. The majority of these devices are found in permanent offshore cell towers, buoys, or boats. Active relays are costly and require routine maintenance. In an open ocean, it is difficult to obtain complete coverage with operational relays [222]. In the third method, photoacoustic conversion is achieved with a passive relay, which is of low cost, capable of reducing energy dissipation, easily installable, and more suitable for complicated (waving and pollution) water conditions [223]. In photoacoustic effects, both the linear (thermal) and non-linear (evaporation and vaporization) mechanism are being explored for opto-acoustic energy transformation [211; 224]. Only the localized heat of the liquid environment, also referred as the linear opto-acoustic regime, is involved in the linear conversion process, whereby laser energy is transformed to sound at the air-water interface. The acoustic pressure is proportionate to the incoming laser power in this method of translation [225]. The non-linear transformation method, also known as non-linear optoacoustic conversion, in which concentrated laser energy is converted to sound at the air-water interface, involves a phase change in the water medium through the vaporization principle, resulting in the formation of a plasma [211]. The acoustic intensity is non-linearly linked to the beam power in this manner of transformation. In comparison to the linear transformation process, the non-linear transformation process is more effective, but less flexible, compared to the generation of underwater acoustic waves. A standard opto-electronic/opto-acoustic system to conduct experiments to obtain an efficient opto-acoustic conversion leading to the establishment of a reliable in-air to in-water PAC is shown in Figure 31. The opto-acoustic generation system generally consists of an in-air platform, high-powered CW/PW mode-locked laser and wavelength (\(\lambda\)) selection optics. The wavelength tuning loss is compensated for by a variable high-gain optical amplifier. This amplifier is also used to vary the air-water junction incident power (Pt). The wide-band free-space transmission optics decides the laser beam parameters such as beam divergence (\(\theta\)), beam diameter on the water surface (\(D\)), beam source diameter (d), beam footprint area, footprint geometry, incident angle (\(\varphi\)), optical wavefront profile, collimation/focus level, beam polarization, etc., and transmits it to the point of interest on the water surface. Instead of a Gaussian optical beam, an oblique beam can also be used since it gives a considerable increment in the in-air to in-water range [211; 216]. Based on the principle of an acoustic signal traveling inside a water medium, the opto-acoustic energy conversion mechanism is classified into linear and non-linear energy dispersion [217]. The opto-acoustic conversion generates the acoustic wave at the air-water surface and it propagates in the underwater medium. A hydrophone receiver array (\(R_{1}\), \(R_{2}\),..., \(R_{N}\)) is deployed (in volume), as shown in Figure 31, to collect the acoustic signal [221]. The signal conditioning (pre-amplifier and DAC) circuits amplify the electrical signals, digitize them, and then send them into the signal analyzer and computer via a communication peripheral interface port. In [210], the authors introduced a low-cost passive relay system  
Figure 31: A top-level scheme for a PAC system and its performance analysis. for a photoacoustic-conversion-efficiency-increased PAC system.