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Researchers at MIT’s Lincoln Laboratory 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.
In The Optical Society (OSA) journal Optics Letters, researchers reported using two different laser-based methods to transmit various tones, music and recorded speech at a conversational volume. The system can be used from some distance away to beam information directly to someone's ear. According to the researchers, it is the first system that uses lasers that are fully safe for the eyes and skin to localize an audible signal to a particular person in any setting.
The new approaches are based on the photo acoustic effect, which occurs when a material forms sound waves after absorbing light. In this case, the researchers used water vapor 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, according to research team leader Charles M. Wynn. The team found that they don't need a lot of water if a laser wavelength is used that is very strongly absorbed by water. This was key because the stronger absorption leads to more sound.
One of the new sound transmission methods grew from a technique called dynamic photo acoustic spectroscopy (DPAS), which the researchers previously developed for chemical detection. In the earlier work, they discovered that scanning, or sweeping, a laser beam at the speed of sound could improve chemical detection. According to Ryan M. Sullenberger, the first author of the paper, the speed of sound is a very special speed at which to work. In this paper, the research team has shown 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-related approach, the researchers change the length of the laser sweeps to encode different frequencies, or audible pitches, in the light. One unique aspect of this laser sweeping technique 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 of light. It also opens the possibility of targeting a message to multiple individuals.
In the lab, the researchers showed that commercially available equipment could transmit sound to a person more than 2.5 meters 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 photo acoustic method that doesn’t require sweeping the laser and encodes the audio message by modulating the power of the laser beam. There are tradeoffs between the two techniques, according to Sullenberger. The traditional photo acoustics 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. They hope that this will eventually become a commercial technology. There are a lot of exciting possibilities, and they want to develop the communication technology in ways that are useful.
Full Paper: Photo acoustic Communications: Delivering Audible Signals via Absorption of Light by Atmospheric H2O by R. M. Sullenberger, S. Kaushik, C. M. Wynn
