This Is the Purest Beam of Light in the World


A team of scientists at the Massachusetts Institute of Technology (MIT) has made the purest laser in the world.

The device, built to be portable enough for use in space, produces a beam of laser light that changes less over time than any other laser ever created. Under normal circumstances, temperature changes and other environmental factors cause laser beams to wiggle between wavelengths. Researchers call that wiggle “linewidth” and measure it in hertz, or cycles per second. Other high-end lasers typically achieve linewidths between 1,000 and 10,000 hertz. This laser has a linewidth of just 20 hertz.

Imagine sitting down to dinner with a group of friends, when a laser tickles the water molecules inside your ear.

“You need to get home right away,” your older child shouts. The younger one has fallen and cut their knee, and might need stitches.

You stand up, excuse yourself, and make for the exit. Your friends have no idea why, but assume you heard a message inaudible to the rest of them in the noisy room, transmitted into your ear by laser light.

That’s the future scientists at MIT imagined when they developed a laser system for sending sound across a room using laser light.

“This can work even in relatively dry conditions because there is almost always a little water in the air, especially around people,” research team leader Charles Wynn said in a statement. “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.”

Other methods now under development, they noted, produce clearer sounds. But those methods (like switching a laser on and off really fast to jiggle the water molecules) don’t make sounds as loud as the wiggling method. (The researchers call it “sweeping” rather than wiggling.)

The point of all this is to send messages to individuals in a crowd without blasting them over loudspeakers.

“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 statement said.

In the paper, the researchers said that some laser-sound techniques are under development by the military.

One remarkable point is that the underlying concept here isn’t very new. The paper notes that Alexander Graham Bell, who invented the first practical telephone, patented a device back in 1880 along with a partner named Charles Sumner Tainter that transmitted sounds via light.

Bell and Tainter’s “photophone-transmitter” was a proposed “instrument for controlling a radiant beam and imparting to it a varying character, whereby in falling on an appropriate receiving-instrument the said beam may be made to produce sound.”

In other words: Wiggle light over some material, and sound should emerge.

The key differences, of course, in the modern MIT system are that the receiver material is just ambient water vapor, and that the light is a precision laser. But the underlying concept is the same.

The next step for the MIT device, the researchers wrote, is to try it outdoors and at longer range.

Their method isn’t the first to transmit sound waves using lasers. But it is the loudest. Their machine, described in a paper published on Jan. 25 in the journal Optics Letters, relies on wiggling a laser back and forth across the water molecules in the air by someone’s ear. That wiggling motion (accomplished with a fast-twitching mirror) jolts the molecules into motion, causing them to bang against the surrounding air molecules and produce sound waves.

To achieve that extreme purity, the researchers used 6.6 feet (2 meters) of optical fibers that were already known to produce laser light with very low linewidth. And then they improved the linewidth even more by having the laser constantly check its current wavelength against its past wavelength and correct any errors that cropped up.

This is a big deal, the researchers said, because high linewidth is one of the sources of error in precision devices that rely on beams of laser light. An atomic clock or a gravitational-wave detector with a high-linewidth laser can’t produce as good a signal as a low-linewidth version, muddling the data the device produces.

In a paper published today (Jan. 31) in the journal Optica, the researchers wrote that their laser device is already “compact” and “portable.” But they’re trying to miniaturize it further, they said in a statement.

One possible use they imagine? Gravitational-wave detectors based in space.

Gravitational-wave detectors sense the impact of massive, faraway events on space-time. When two black holes collide, for example, the resulting shock wave causes space to ripple like a pool of water struck with a stone. The Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected these ripples in 2015 in a Nobel Prize-winning experiment that relied on carefully monitoring laser beams. When those beams changed shape, it was evidence that spacetime itself had been perturbed.

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