Researchers at UCF’s College of Optics and Photonics (CREOL) have demonstrated a new way to detect underwater sound, and changes in the water itself, using engineered laser light.

Supported by the U.S. Navy’s Office of Naval Research, the work uses light that responds to subtle changes in underwater environments to measure acoustic activity and ocean conditions.

Led by CREOL Professor Eric Johnson and CREOL research scientist Dr. Keith Miller, the team reported its first results in a recent study published in Optics Express, in which the researchers detected sound traveling through a 10-foot water tank by measuring its effect on a laser beam, using just one light detector. Miller is the study’s lead author.

Detecting Underwater Sound with Light

Sound is one of the primary ways information travels underwater. While radio waves weaken rapidly in seawater, sound waves can travel considerable distances, making acoustic sensing important for oceanographic research, marine biology, maritime navigation and naval operations.

Conventional underwater sensors, such as hydrophones, measure changes in water pressure caused by sound waves. The UCF team instead detects those disturbances through their effect on a laser beam passing through the water.

As sound travels through water, it produces small changes in the water’s refractive index, which determines how light bends as it passes through a material. By measuring how these changes affect a laser beam, the researchers can recover the sound itself.

“The exciting opportunity is to structure the light so that its interaction with the environment writes information directly into a signal we can measure,” Johnson says. “Rather than trying to reconstruct everything happening to an optical beam, we can design the illumination and detection together to reveal the physical changes we want to sense.”

Engineering Light to Measure Environmental Changes

The key is engineering the light so it reports on its surroundings. The team’s approach, called frequency-diverse structured illumination, splits a laser into an array of small beams, or beamlets, and gives each one a slightly different frequency, much like assigning each a different musical note. When the beamlets reach a single detector, every pair beats together like notes in a chord, producing a set of tones. Sound and other changes in the water shift those tones, so one detector can follow what is happening across the entire beam.

“In effect, we turn a single light detector into an entire sensor array. The light organizes the information for us, so what comes out is one simple signal that carries a surprisingly rich picture of what’s happening in the water,” Miller says. “It’s an elegant technique that applies far beyond sound, and we’re just scratching the surface of what we can measure with it.”

In the published study, Miller and colleagues combined this technique with heterodyne interferometry, which uses the interference of light waves to detect small changes in an optical signal. In the tank experiments, the system detected underwater sound in the kilohertz range, and ongoing work suggests it can reach into the megahertz range as well, all with one detector. With a single detector doing the work of many, the approach could lead to simpler, more compact sensing systems.