# Cross-Medium Photoacoustic Communications: Challenges, and State of the Art
## 1 Introduction
For long distance communication from air to underwater optoacoustic (photoacoustic) energy conversion is indeed a viable option. Optoacoustic energy conversion means converting light energy into acoustic energy. Such an energy conversion mechanism was discovered long ago by [PERSON] in 1881 [36]. Basically, an aerial vehicle would transmit a laser beam towards the surface; the beam would then penetrate the water surface and create an acoustic wave inside the water. Such a generated acoustic wave will propagate in the underwater medium and face the classical challenges of spreading, attenuation, and multipath fading. Hence, contemporary channel estimation techniques that take in consideration fading and other underwater characteristics are often applied [37; 38]. This paper focuses on challenges faced at the air-water interface, particularly on issues related to optoacoustic communication. An optoacoustic link can be established by controlling the properties of the laser beam to carefully modulate the corresponding acoustic wave with the desired data. An example application scenario of optoacoustic communication is illustrated in Figure 1, where an airborne vehicle is assisting an underwater network in localizing nodes and forming a global coordinate system. The airborne vehicle instantiates acoustic emitters at multiple points on or close to the water surface through the optoacoustic effect. These points act like underwater or surface nodes. By carefully modulating the laser beams, the generated acoustic waves could be demodulated to infer the GPS coordinates of the position of the emitters (points). The underwater nodes that are within the range of the generated acoustic signal will localize themselves and then attempt to localize other nodes at further depth. Such localization will be critical for performing certain missions such as finding a black box of a crashed plane, where accurate navigation and target position estimation are achieved. As the underwater nodes move as part of the search for the black box or drift due to water current, the aerial unit will repeat the process to sustain robustness. Photoacoustic-based communication is deemed as a game-changer in the realm of sea-based networks; no wonder it is an emerging area of research. A number of fundamental issues are being addressed related to acoustic signal generation and propagation, modulation and encoding, and reliable communication protocols. Moreover, techniques that leverage photoacoustic links in providing UAN services such as localization and topology management of UANs are being developed. This paper opts to provide an overview of such an area of research, summarizes progress made to date, and highlights open issues that are being or are to be worked on. To the best of the authors' knowledge, there is no similar article in the literature. The hope is that scientists, practitioners, and developers could gain sufficient background, understand the unconventional design challenges, and know the state of the art on photoacoustic communication. The contributions of this paper can be summarized as: (1) a detailed summary of photoacoustic signal generation, especially how laser-induced breakdown occurs and consequently breakdown shockwave and bubble expansion-collapse shockwaves are produced, (2) elaborate discussion of different key properties of the linear and nonlinear photoacoustic effect, (3) an analysis of the challenges associated with the photoacoustic based communication, (4) a report on the current state of the photoacoustic communication research, (5) a highlight of the potential of photoacoustic enabled underwater localization, and (6) an outline of open research issues associated with the air-to-water photoacoustic communication. The paper is structured as follows. Section 2 describes how to generate photoacoustic signals. Sections 3 and 4 discuss the properties of linear and nonlinear photoacoustic, respectively.