# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 4 Underwater Optical Communication
### Photoacoustic Communication
A data-modulated optical beam was transmitted from a UAV platform to a passive relay deployed on the water surface. An array of acoustic sensors placed at the submarine level was used as the acoustic signals were received. The received electrical signal was amplified, pre-processed, and logged in the computer where the investigational information was analyzed to determine the efficiency of opto-acoustic conversion and quality of data communication. The experiment was also conducted for different wavelengths, link ranges, data lengths, and the decoded data was displayed using a single-digit seven-segment display. In Ref. [190], the authors proposed a technique for submersible remote sensing and imaging using an aerial sonar network. The proposed method contained the principle of SAR (electromagnetic imaging) and sonar imaging. A series of PW optical beams was transmitted from a flying UAV platform, which was converted to an acoustic signal at the air-water interface, propagated inside the water, scattered by the targets, and then the echo signal travelled back to the same flying platform. The experimental results in terms of absorption factors as a function of wavelength, water attenuation as a function of frequency, acoustic frequency as a function of receiver height, and sonic frequency as a function of imaging depth were analyzed. In Ref. [211], the authors presented their results about using the dynamic laser-pulse-repeating rates for voice transmission in an optoacoustic communication network. An indoor experimental model was used in their research, through which they converted speech into encoding characters, and then the characters were used to modulate the pulse laser. The modulated optical beam was focused into a point on the water surface. A hydrophone was used to measure the acoustic signal generated by the incident moderated light rays. The transmitted symbols were recovered after processing the electrical signal generated by the hydrophone. The speech quality was analyzed for different optical wavelengths and laser pulse repetition rate. In Ref. [212], the authors used the principle of photoacoustic in underwater remote sensing, mainly for wireless voice communication and contactless measuring instruments. The main applications covered therein were the measurement of composition/the spatial distribution of substances, ultra-sensitive spectrographic examinations of trace gases, photoacoustic tomography, the remote sensing of underwater objects, submarine sensing, material structure imaging, etc. In Ref. [221], the authors proposed a technique for communication at the air-water surface using multiple light sources. The advance of multiple light source schemes supports increased coverage, strong signal propagation, and the forming of electronic beams. In Ref. [222], Stanford University proposed an underwater investigation and high-resolution imaging of the deep ocean. The potential applications of their proposed drone-based opto-acoustic system are biological marine reviews, airborne searches for shipwrecks on a vast scale, and a map of the lowest point of the ocean. Since PAC are rapidly developing nowadays, the experimental verification of photo-acoustic conversion efficiency with direct, active, and passive relay systems as the function of optical wavelength, power, beam divergence, wavefront aberrations, etc., and the determination of the optimum system parameters: type of optical source (CW/Pulsed/normal/oblique beam), optical power, wavelength, beam pattern, polarization, beam divergence, beam incident angle, link range, LM, acoustic sensor depth, laser pulse repetition rate, switching speed, optical footprint area, optimum conversion mechanism (thermal/plasma), acoustic signal level, acoustic beam pattern, etc., are the emerging research requirements still to be addressed in this field, to establish reliable air-to-underwater long-range PAC.