Light hears sound

© Michael Mazohl

Balthasar Fischer’s microphone relies not on a diaphragm, but a laser. This founder’s invention has caught the attention of enthusiastic researchers and entrepreneurs.

The start-up scene is thriving. Armed with ideas and business models, these mini-companies are competing for publicity, investors and sponsors. But not only in Silicon Valley – creative minds and their ingenious ideas are enjoying tremendous success in Europe as well. Optical technologies, for instance, are truly booming, and their start-up scene is especially dynamic. Photonics is regarded as cutting-edge. Whether in Industry 4.0, intelligent lighting solutions of tomorrow or new imaging techniques in medical technology, light has a bright future. Established companies and research institutes are just as sure of this as are aspiring up-and-coming scientists.

Laser instead of diaphragm

Balthasar Fischer is one of the believers – with the invention to prove it. Fischer’s idea? An optical microphone that has no moving parts. This invention made him an entrepreneur. His long list of customers includes research institutes and corporations such as CERN, Fraunhofer, Zeiss and Siemens. TRUMPF Venture is an investor. As for Fischer’s company, Xarion Laser Acoustics GmbH in Vienna, it has graduated from the start-up phase: it now has 15 employees working on the small sensor.

We are revolutionizing the monitoring of processes and machinery in industrial manufacturing

Better than the human ear

Whereas conventional microphones rely on a vibrating diaphragm, Fischer’s invention uses a laser beam. When a sound wave hits the laser light of his optical microphone, the pressure slightly alters the light’s speed. In turn, the laser beam propagates more slowly. The light intensity thus indirectly measures the sound wave, which is converted to an electrical signal.

Laser beam instead of diaphragm: sound waves alter the speed of laser light. The light intensity thus indirectly measures the sound. (photo: Xarion)

Results can then be visualized. Merely a few square millimeters in size, the sensor needs no moving parts and is thus resistant to external influences. The sensor can also measure sounds ranging from low infrasound to ultrasound in the megahertz range. This frequency range is 50 times broader than that of the human ear – and ten times greater than the best airborne-sound sensor available today. “We are revolutionizing the monitoring of processes and machinery in industrial manufacturing,” says Fischer. “If we can listen to a machine that uses a laser for material processing, for instance, then we can ascertain whether a nozzle is clogged or precisely how deep the laser beam penetrates the material. We obtain this information by using the microphone at very high ultrasonic frequencies.” Fischer envisions fields of application in Industry 4.0 and elsewhere.

Hearing the defect

Xarion manufactures optical microphones in a clean room in Vienna. (photo: Xarion)

Another area where the optical microphone could soon be used is nondestructive material testing in fields such as aircraft manufacturing. Non-contact analysis allows the microphone to “look” inside materials, as demonstrated by an experimental setup at Xarion. An ultrasound transmitter was mounted on the top of a carbon-fiber composite plate, with the sensor on the bottom. “The microphone measures how much sound passes through the plate. Because the sensor is very sensitive and measures a very wide range, it can detect even very small defects, and with a high level of accuracy,” stresses Fischer.

Earlier detection of cancer

Small and precise: this microphone with no moving parts measures sounds from low infrasound to ultrasound in the megahertz range. (photo: Xarion)

Thanks to its lack of moving parts, the microphone is resistant to vibration, wind noise and solid-borne sound. And because there is no inertial mass, there can be no reverberations. As a result, the microphone provides reliable measurements at frequencies up to 25 megahertz – even underwater or in a radioactive environment. This makes it appealing for use in medical technology – particularly photoacoustic tomography, which employs laser pulses to make biological tissue vibrate. The microphone then measures the resulting ultrasonic waves. This makes it possible to see whether there have been any changes in the tissue. This facilitates early detection [BW2] of cancer and eliminates the need for invasive techniques.

Ready for the harsh reality of everyday use?

Little sensor, big potential. Fischer was initially by no means certain, however, that his idea would prove good enough to serve as the foundation for a company. “It’s very risky to found any company. You just can’t know if it’ll work out in the long run. You feel like somebody has thrown you in the deep end,” says Fischer. He consequently recruited Leonhardt Bauer, formerly a principal at McKinsey & Company, in 2012 to be co-founder. Bauer had experience with start-ups, which would help put Xarion on solid footing. “Engineers are often so excited about their work that they’re oblivious to key economic aspects that must be considered,” says Fischer.

Engineers are often so excited about their work that they’re oblivious to key economic aspects that must be considered

Balthasar Fischer’s invention paved the way to founding a company. (Foto: Michael Mazohl)

But entrepreneurial risk was not all he faced in the beginning; he also had his hands full with technological challenges. “Nobody had ever measured sound in this way – it was uncharted territory. We regularly had to cope with minor setbacks. But if the laws of physics say no, that’s impossible, then you can’t argue with that – which occasionally cost me some sleep.” Sleepless nights are a thing of the past for Fischer; his product works. The Xarion team is now working to establish their optical microphone in heavy-duty, everyday industrial use. After all, there is no guarantee that a sensor that performs well under laboratory conditions will be similarly reliable in a production facility. “We’re currently integrating our sensor into a lot of exciting projects. Our focus is less on the microphone now and more on our customers’ needs. We keep approaching things from a new angle.”

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