28th January 2019

New technology uses lasers to transmit audible messages to specific people

Researchers have demonstrated that a laser can transmit an audible message to a person without any type of receiver equipment. The ability to send highly targeted audio signals over the air could be used to communicate across noisy rooms, or warn individuals of a dangerous situation such as an active shooter.

Writing in the journal Optics Letters, researchers from the Massachusetts Institute of Technology’s Lincoln Laboratory report using two different laser-based methods to transmit various tones, music and recorded speech at a conversational volume.

“Our system can be used from some distance away to beam information directly to someone's ear,” said research team leader Charles M. Wynn. “It is the first system that uses lasers that are fully safe for the eyes and skin to localise an audible signal to a particular person in any setting.”



Credit: Massachusetts Institute of Technology’s Lincoln Laboratory

The new approaches are based on the photoacoustic effect, which occurs when a material forms sound waves after absorbing light. In this case, the researchers used water vapour in the air to absorb light and create sound.

“This can work even in relatively dry conditions, because there is almost always a little water in the air, especially around people,” said Wynn. “We found that we don't need a lot of water if we use a laser wavelength that is very strongly absorbed by water. This was key, because stronger absorption leads to more sound.”

One of the new sound transmission methods built upon a technique called dynamic photoacoustic spectroscopy (DPAS), which the researchers previously developed for chemical detection. In the earlier work, they found that scanning, or sweeping, a laser at the speed of sound improves chemical detection.

“The speed of sound is a very special speed at which to work,” said Ryan M. Sullenberger, first author of the paper. “In this new paper, we show that sweeping a laser beam at the speed of sound at a wavelength absorbed by water can be used as an efficient way to create sound.”

For the DPAS approach, the length of the laser sweeps are changed to encode different frequencies, or audible pitches. One unique aspect of this method is that the signal can only be heard at a certain distance from the transmitter. This means that a message could be sent to an individual, rather than everyone who crosses the beam. It also opens the possibility of targeting a message to multiple individuals.

(a) Traditional photoacoustic configuration: 1907.2 nm laser light is absorbed by ambient water vapour. The laser beam is

amplitude modulated via an acousto-optic modulator. (b) Dynamic photoacoustic communication amplifies the audible signal.

(c) H2O absorptivity near 1.9 μm, with an overlay of the laser emission from our thulium fibre laser.

In the lab, the researchers found that commercially available equipment could transmit sound to a person more than 2.5 metres away at 60 decibels using the laser sweeping technique. They believe that the system could be easily scaled up to longer distances. They also tested a traditional photoacoustic method that does not require sweeping the laser and encodes the audio message by modulating the power of the laser beam.

“There are trade-offs between the two techniques,” said Sullenberger. “The traditional photoacoustics method provides sound with higher fidelity, whereas the laser sweeping provides sound with louder audio.”

Next, the researchers plan to demonstrate the methods outdoors at longer ranges. “We hope that this will eventually become a commercial technology,” said Sullenberger. “There are a lot of exciting possibilities, and we want to develop the communication technology in ways that are useful.”

One potential application (not necessarily a desirable one) might be personalised advertising of the kind seen in Minority Report, the sci-fi blockbuster directed by Steven Spielberg. That movie, set in the year 2054, includes a scene where Tom Cruise is walking through a crowded mall and encounters a series of holographic advertising displays, which detect his presence and speak directly to him by name.

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