Laser transmission of audible messages to specific people in a crowd demonstrated

Jan. 24, 2019
Sweeping a laser through the air at the speed of sound is an efficient way to create audio signals.

Researchers from Massachusetts Institute of Technology’s Lincoln Laboratory (Lexington, MA) have demonstrated that a laser can transmit an audible message to a person without any type of receiver equipment.1 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.

The researchers tested two different laser-based methods, using a 1.9 μm thulium laser matched to a water-vapor absorption line in the atmosphere to transmit various tones, music, and recorded speech at a conversational volume. 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 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," says research team leader Charles M. 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."

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 the earlier work, they discovered that scanning, or sweeping, a laser beam at the speed of sound could improve chemical detection.

"The speed of sound is a very special speed at which to work," says Ryan M. Sullenberger, one of the researchers. "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."

Distance targeting

For the DPAS-related approach, the researchers change the length of the laser sweeps to encode different 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, meaning 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 m away at 60 dB 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 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," says Sullenberger. "The traditional photoacoustics method provides sound with higher fidelity, whereas the laser sweeping provides sound with louder audio."

The researchers next plan to demonstrate the methods outdoors at longer ranges. "We hope that this will eventually become a commercial technology," says Sullenberger.

Source:

https://www.osa.org/en-us/about_osa/newsroom/news_releases/2019/new_technology_uses_lasers_to_transmit_audible_mes

Reference:

1. R. M. Sullenberger et al., Optics Letters (2019); doi: https://doi.org/10.1364/ol.44.000622.

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

Sponsored Recommendations

Request a free Micro 3D Printed sample part

April 11, 2024
The best way to understand the part quality we can achieve is by seeing it first-hand. Request a free 3D printed high-precision sample part.

How to Tune Servo Systems: The Basics

April 10, 2024
Learn how to tune a servo system using frequency-based tools to meet system specifications by watching our webinar!

Motion Scan and Data Collection Methods for Electro-Optic System Testing

April 10, 2024
Learn how different scanning patterns and approaches can be used in measuring an electro-optic sensor performance, by reading our whitepaper here!

How Precision Motion Systems are Shaping the Future of Semiconductor Manufacturing

March 28, 2024
This article highlights the pivotal role precision motion systems play in supporting the latest semiconductor manufacturing trends.

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!