Scientists have devised a way to communicate secretly by sending laser-transmitted messages directly into the area around a person’s ear.
Humans enjoy talking with one another, and often do so in ways that prevent eavesdroppers from listening in. This new research could have potential military applications — but who knows where else it might find use?
“The ability to communicate with a specific subject at a prescribed location who lacks any communication equipment opens up many intriguing possibilities,” the authors write in the paper published recently in the journal Optics Letters.
How would such a device work? It might help to understand what sound really is: Pulses or vibrations that travel via air molecules into our eardrums, which then vibrate, becoming the analogue input that our brains interpret as specific sounds.
The device generates sounds in the air using a process called the photoacoustic effect. The researchers tune the laser light to a specific wavelength that they know will cause water molecules in the air to vibrate but that won’t cause damage to our eyes.
But the laser only makes the molecules vibrate — the researchers needed to introduce another element in order to bestow the air’s ambient water molecules with more sound than just a single drone or pulse. Past experiments, as well as this one, used an instrument called an “acousto-optic modulator” to change the strength of the laser pulse, and in turn the behaviour of the water molecules.
But in this paper, the researchers also developed a new method, where the laser first passes into a quickly spinning mirror, which essentially spreads the laser beam out. Tweaking the size of the spread produces different frequencies.
The new method produced louder sounds than an older method, according to the paper; the sounds were amplified to 60 decibels, about the volume of background music. I asked the study authors to describe what it is like to hear these specially crafted sounds, and what exact messages they sent.
The research team’s leader Charles Wynn from MIT Lincoln Laboratory told Gizmodo, “Perhaps the most fun was listening to the dynamic photoacoustic system. We had to position our ears in just the right spot (a few inches in extent) to hear the audio, otherwise you didn’t hear anything. That was a bit of an odd but satisfying effect.”
As for the messages they transmitted: “At first, we transmitted single tones and frequency sweeps using the traditional and dynamic photoacoustic communication techniques,” said Wynn.
“Later, we transmitted more advanced waveforms such as spoken words and music (including Miley Cyrus’ “Malibu”), using the traditional photoacoustics configuration. With a bit more engineering, we fully expect the ability to design a dynamic photoacoustic communications system with the bandwidth necessary to encode and transmit the more detailed audio messages.”
This work was just a proof-of-concept; the sound could only be transmitted to receivers a short distance away — 2.5m according to the paper. But the researchers are working on expanding their method to work over longer distances.
There are a lot of strange scenarios in which you can imagine needing to send a secret message via laser pulses. In my dreams, they will replace numbers stations, which give me nightmares.