Optical microphone "sees" broader sound spectrum

June 13, 2016 // By Julien Happich
Founded in 2012 as a spin-out company by the Vienna University of Technology and the Knowles Corporation, Austrian startup Xarion Laser Acoustics GmbH is bringing a membrane-free optical microphone with unprecedented sound resolution.

Sending a telecom-grade 1550nm IR laser beam through an open Fabry-Pérot etalon (in effect two parallel optical mirrors only 2mm apart, the solid-state microphone (no moving parts) records sound pressures as minute changes in the medium's refractive index. These changes alter the optical wavelength and the light transmission which the optical microphone converts into measurable electrical signals.

Schematic illustration of the optical
microphone's working principle.

Xarion's optical microphone can detect faint refractive index changes under 10-14, which translate to pressure changes as small as 1µPa. Due to their rigid construction, they do not suffer from sound-induced mechanical resonances and boast a very flat frequency response, from 5Hz infrasound to ultrasounds in the MHz range (up to 25MHz in liquids). This enables the optical microphone to capture all acoustic data in one recording across a large spectrum otherwise unattainable by conventional membrane-based microphones.

For now, Xarion's prototypes could be considered fairly bulky compared to today's MEMS-based consumer microphones found in smartphones, and EETimes Europe caught up with the company's CEO, Dr. Balthasar Fischer, to better understand which markets the startup is after.

"Of course, why would you want to record sounds that only bats can hear?" jokingly admitted Fischer with regard to audiophile applications. "The frequencies we can record make more sense for industrial applications such as non-destructive testing, ultrasound metrology, acoustic process monitoring or for medical imaging".

"Also, as a startup, we would not be able to address mass-markets, our current devices would not be cost competitive with consumer-grade products so it makes sense for us to start with niche markets" Fischer said.