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Researchers from the Massachusetts Institute of Technology’s Lincoln Laboratory have demonstrated two laser-based methods to transmit various audio tones and recorded speech at a conversational volume to a person without any type of receiver equipment. The new approaches are based on the photoacoustic effect. To selectively transmit sound, 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,” said research team leader Charles 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 the stronger absorption leads to more sound.”
The researchers use water vapor in the air to absorb light and create sound. By sweeping the laser they can create an audio signal that can only be heard at a certain distance from the transmitter, allowing it to be localized to one person. Courtesy of Massachusetts Institute of Technology’s Lincoln Laboratory.
One of the new sound transmission methods grew from a technique called dynamic photoacoustic spectroscopy (DPAS), which the researchers previously developed for chemical detection. In their earlier work, they discovered that sweeping a laser beam at the speed of sound could improve chemical detection.
For photoacoustics communications using a DPAS-based approach, the researchers changed the length of the laser sweeps to encode different frequencies in the light. They were able to show that sweeping a laser beam at the speed of sound at a wavelength absorbed by water could be an efficient way to create sound. One 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 to everyone who crosses the beam of light. It also opens the possibility of targeting a message to multiple individuals.
In experiments, the researchers showed that sound could be transmitted to a person more than 2.5 meters away at 60 decibels using the laser sweeping technique and commercially available equipment. They believe that the system could be easily scaled up to transmit over longer distances. They also tested a traditional photoacoustic method that doesn’t require sweeping the laser and encodes the audio message by modulating the power of the laser beam, sending a message directly to the receiver’s ear via the photoacoustic effect.
Ryan M. Sullenberger and Charles M. Wynn developed a way to use eye- and skin-safe laser light to transmit a highly targeted audible message to a person without any type of receiver equipment. Courtesy of Massachusetts Institute of Technology’s Lincoln Laboratory.
“There are trade-offs between the two techniques,” said researcher Ryan Sullenberger. “The traditional photoacoustics method provides sound with higher fidelity, whereas the laser sweeping provides sound with louder audio.”
The ability to send highly targeted audio signals over the air could be useful for warning individuals of a dangerous situation, or for simply communicating across noisy rooms. “Our system can be used from some distance away to beam information directly to someone’s ear,” Wynn said. “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.” A 1.9-µm thulium laser matched to an atmospheric water vapor absorption line was used to maximize sound pressure while maintaining eye-safe power densities.
Next, the researchers plan to demonstrate the methods outdoors at longer ranges. “We hope that this will eventually become a commercial technology,” Sullenberger said. “There are a lot of exciting possibilities, and we want to develop the communication technology in ways that are useful.”
The research was published in Optics Letters, a publication of OSA, The Optical Society (https://doi.org/10.1364/OL.44.000622).READ MORE