Photoacoustic communications: delivering audible signals via absorption of light by atmospheric H

We describe a means of communication in which a user with no external receiver hears an audible audio message directed only at him/her. A laser transmits the message, which is encoded upon a modulated laser beam and sent directly to the receiver's ear via the photoacoustic effect. A 1.9 μm thulium laser matched to an atmospheric water vapor absorption line is chosen to maximize sound pressure while maintaining eye-safe power densities. We examine the photoacoustic transfer function describing this generation of audible sound and the important operational parameters, such as laser spot size, and their impact on a working system.