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US: MIT develops laser to deliver secret audio messages

Researchers at the Massachusetts Institute of Technology (MIT) described a way to use a laser beam to deliver a message that could be heard only by the person to whom the beam was directed.

Their paper describing this, "Photoacoustic communications: delivering audible signals via absorption of light by atmospheric H₂O", was published in January 2019. The abstract is as follows:

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.

The laser is said to be eye- and skin-safe. The system can currently send a sound up to 60 dB across a distance of 2.5 meters (8 feet). In the future they are hoping for a range of 100-500 meters (328-1,640 feet). It is useful for applications as diverse as notifying one person in a crowd, and for headphone-free listening. MIT is patenting the technology.

From R. Sullenberger, S. Kaushik, and C. Wynn, "Photoacoustic communications: delivering audible signals via absorption of light by atmospheric H₂O," Opt. Lett. 44, 622-625 (2019). A PDF of the article is here. News stories about this appeared in numerous publications including Digital Trends, Photonics.com and The Sun.