Communicating from an in-air platform (aircraft, UAV, ship) to a submerged platform without penetrating the water surface is a long-standing unmet need: radio and optical carriers attenuate rapidly in water, and conventional acoustic transducers must be submerged. The opto-acoustic approach converts a modulated laser beam incident on the air-water interface into underwater sound, in a linear thermo-acoustic regime (pressure proportional to laser power, low conversion efficiency, high controllability) or a nonlinear regime (focused high-energy pulses cause evaporation, plasma formation, and cavitation, yielding higher source levels but noise-like broadband transients). Since Blackmon and Antonelli's 2005-2009 work established both regimes and demonstrated laboratory-scale telemetry generation, several groups have replicated modulated links (on-off keying, pulse-position modulation, frequency-shift keying, and delayed differential pulse-position modulation) in still-water tanks at data rates of a few to a few hundred bits per second, with kilobit rates reached only using passive surface relays. What remains open is the practical link itself: an experimentally validated characterization of achievable range, data rate, and reliability in both regimes under realistic ocean conditions (sea state and surface dynamics, seawater optical properties, ambient noise, platform motion), including removal, mitigation, or bounding of the nonlinear regime's vapor-cloud-limited repetition rate and accounting for the linear regime's low source level in the link budget, and a demonstration at operationally meaningful range and depth beyond laboratory tank scale.

Background

Underwater platforms such as submarines, unmanned undersea vehicles (UUVs), and subsea sensors are difficult to reach wirelessly from the air. The roughly 3600:1 acoustic impedance mismatch between water and air reflects underwater sound at the surface, radio waves attenuate within centimeters to meters in seawater, and optical carriers scatter and absorb within tens of meters, so conventional practice deploys a surface relay (buoy or ship) that exposes an antenna or transducer, compromising stealth, operational speed, and the depth at which the submerged platform can remain.

The opto-acoustic (also called photoacoustic) conversion mechanism offers a non-contact alternative: a laser beam from an airborne platform is directed at the water surface, and the absorbed optical energy becomes underwater sound. Two conversion regimes are distinguished. In the linear regime, a continuous-wave or moderately pulsed laser heats the water via thermo-acoustic expansion, and the acoustic pressure is proportional to the laser power; this gives fine control of the generated waveform (enabling standard telemetry modulations such as frequency-shift keying, FSK, in which data symbols are carried by the frequency of a tone) but at low conversion efficiency and hence low acoustic source level, conventionally expressed in decibels relative to 1 micropascal (dB re 1 uPa) at a reference distance of 1 m. In the nonlinear regime, high-energy focused pulses vaporize water at the interface, producing plasma and cavitation bubbles whose collapse radiates strong broadband acoustic transients; source levels are much higher (the source experiments consistently recorded sound pressure levels of 185 dB re 1 uPa under freshwater laboratory conditions at laser-pulse repetition rates up to 1000 pulses/s), but the waveform is noise-like, shot-to-shot variable, and hard to shape into clean communication symbols. A practical system must also address low-probability-of-intercept (LPI) signaling, since one motivation is covert delivery of a 'bell ringer' alert or short text/data messages to a submerged receiver.

The foundational characterization is due to Blackmon, Estes, Fain, and Antonelli of the Naval Undersea Warfare Center. Blackmon, Estes, and Fain (Applied Optics, 2005) analyzed and experimentally demonstrated linear opto-acoustic underwater communication; Blackmon and Antonelli (Applied Optics, 2005) demonstrated multi-pulse nonlinear opto-acoustic signal generation and control; and the OCEANS 2006 source paper (the nonlinear downlink is also summarized in a one-page meeting abstract in the Journal of the Acoustical Society of America, 2009) explored both regimes for a remote aerial downlink, experimentally generating FSK signals (10 kHz and 20 kHz tones) via the linear regime, controlling the nonlinear regime's spectrum through the laser-pulse repetition rate to realize M-ary FSK, analyzing oblique laser incidence for extending in-air stand-off distance, and addressing the communication range and data rates achievable in both regimes, with M-ary FSK/MFSK techniques compared with regard to bandwidth, data rate, range, coverage, and the number of lasers employed. The same authors also studied the reverse (uplink) direction by detecting underwater sound through the water surface optically (IEEE Journal of Oceanic Engineering, 2006).

Subsequent work has substantially developed the physical layer. Groups at Harbin Engineering University built laser-generated-sound (LGS) links using a pulsed laser and a distributed-feedback fiber-laser (DFB-FL) hydrophone, a sensitive fiber-optic acoustic sensor interrogated by laser wavelength shifts, demonstrating on-off keying (OOK, bits carried by presence/absence of a pulse), pulse-position modulation (PPM, bits carried by pulse timing), and frequency-shift keying at repetition rates of 1-500 Hz with bit error rates (BER) measured in laboratory tanks. The University of Maryland Baltimore County group contributed a state-of-the-art review (Sensors, 2022), OOK peak-detection demodulation (Optics Communications, 2023), a 'vapor cloud' delayed differential pulse-position modulation (DPPM) scheme (GLOBECOM, 2022) that explicitly works around the interface vapor cloud that blocks successive high-energy pulses, and optical-focusing-based adaptive modulation (OFAM, IEEE Sensors Journal, 2024) that steers the acoustic beam toward a receiver whose position is unknown. Ji and colleagues (IEEE Communications Magazine, 2021) showed that low-cost passive optical relays floating at the surface reduce the required laser energy by about six orders of magnitude; the highest data rates reported in the field, 1-16 kbit/s, likewise depend on such a surface relay (SPIE CITA 2024). Related results extend to speech transmission via variable repetition rates (Results in Physics, 2018), air-ice-water channels (Applied Acoustics, 2025), full-duplex hybrid laser-acoustic/radio links in anechoic pools (Journal of Physics: Conference Series, 2024), and sub-terahertz photoacoustic generation of broadband ultrasound for underwater communication (APL Photonics, 2023).

Across this literature, however, essentially every experimental demonstration is conducted in laboratory water tanks or pools with still, fresh water (or under ice), at meter-to-tens-of-meters propagation scales, with beyond-meter range claims resting on simulation, and with data rates of a few to a few hundred bits per second (kilobit rates only with passive surface relays, which place hardware at the surface). The 2022 review and the later papers' own open-problem statements converge on the same gap: performance under realistic sea conditions is uncharacterized experimentally, and no at-sea field demonstration of the air-to-water opto-acoustic downlink has been reported.

Problem Statement

Establish, by experiment and validated channel modeling, the practical performance of the remote aerial opto-acoustic communication downlink: a link in which a laser beam from an in-air platform is incident on the air-water interface and the resulting underwater sound carries data to a submerged, non-exposed receiver, in both the linear (thermo-acoustic) and nonlinear (breakdown/cavitation) conversion regimes, under realistic ocean conditions (sea state and surface dynamics, seawater optical properties, receiver depth and bearing, ambient ocean noise, and platform motion). This comprises three jointly required parts, none of which resolves the task on its own: (1) an experimentally validated characterization of the achievable range, data rate, and reliability (bit error rate) of such links across the listed conditions, which define the scope of the characterization rather than an obligation to isolate each factor, replacing the still-fresh-water tank measurements and unvalidated simulations that currently define the state of the art; (2) accounting, in the validated link budget, for the approach's acknowledged physical limits - the data-rate ceiling of the nonlinear regime caused by vapor-cloud buildup at the interface at high laser repetition rates, and the low source level of the linear regime - whether by removal, by mitigation (for example the scanning-laser route suggested in the source line), or by bounding, where the chosen route must be quantitative and defensible: a bound must be derived, with its assumptions stated, from measured vapor-cloud behaviour and shown in the link budget to bind the achievable repetition and data rates, rather than asserted as a loose upper limit; and (3) a demonstration of the downlink at operationally meaningful range and depth beyond laboratory tank scale - concretely slant ranges of 100 m or more and receiver depths of 10 m or more, in open water under a non-zero sea state - so that the covert, non-penetrating character of the link (no surface buoy, no exposed antenna or transducer) can be evaluated as a practical capability rather than a laboratory phenomenon. Parts (1) and (2) are resolved by an experimentally validated channel model and link budget whose input parameters are measured rather than assumed. Part (3) is resolved by a validated measurement and deployment plan for the downlink at that scale - specifying the source, receiver, geometry, modulation, and the performance predicted by the model of parts (1) and (2), together with explicit acceptance criteria - with any sea trial serving as the plan's confirmatory execution rather than as a precondition for resolution.

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