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Optical Microphone based on Fabry-Pérot Interferometer
[edit]
The Fabry-Pérot Etalon-based optical microphone is a microphone technology that fundamentally differs from traditional microphone designs. While conventional microphones use mechanical vibrations to convert sound into electrical signals, the optical microphone relies on measuring changes in the speed of light caused by sound pressure.
At the core of the technology is a Fabry-Pérot interferometer, which consists of two miniaturized, rigid (i.e. non-movable and non-deflectable) mirrors. When sound waves hit the interferometer, they alter the refractive index of the air inside the device, leading to a modulation of the optical wavelength. This modulation is then converted into a change in brightness, which is subsequently transformed into an electrical signal.[1][2][3]
A main feature of the optical microphone is that it operates without moving parts. This eliminates mechanical resonances that limit the frequency range of conventional microphones. As a result, the optical microphone offers a wide frequency range extending into the ultrasonic spectrum of up to 4 MHz. Due to its high sensitivity, the optical microphone can measure changes in the refractive index that are smaller than 10−14, corresponding to a pressure change of only 1 µPa. This high sensitivity makes it particularly suitable for applications in ultrasonic metrology, such as in non-destructive testing (NDT) of materials.[4]
The optical microphone can also be used as a reference sensor for the calibration of sound and ultrasound emitters. In addition to its unique design, the optical microphone has distinct characteristics compared to traditional microphone and fiber-optic microphones. It features a wide ultrasound frequency range from 10 Hz up to 4 MHz in air, and 20 MHz in liquids. The transducer principle ensures a perfectly linear frequency response, making it less dependent on frequency than conventional devices.
The optical microphone is particularly well-suited for applications in ultrasonic measurement technology, such as in non-destructive testing (NDT) of materials or process monitoring. A key feature of this technology is the contactless detection of ultrasound waves, which offers advantages, particularly in the automotive and aerospace industries. The ability to perform precise measurements without direct contact with the test surface contributes to improved quality assurance in these sectors.[5][6][7]
References
[edit]- ^ B. Fischer: Nature Photonics 10, 356–358 (2016).
- ^ S. Preisser et al.: Biomedical Optics Express 7(10), 4171-4186 (2016).
- ^ R. Haindl et al.: Optics Letters 42(21), 4319-4322 (2017).
- ^ https://xarion.com/en/applications
- ^ Deutsche Gesellschaft für zerstörungsfreie Prüfung: ACUT Richtlinie: https://www.dgzfp.de/leitfaden-zur-durchfuehrung-von-luftgekoppelter-ultraschallpruefung.
- ^ M. Brauns, F. Lücking et al.: Materials Evaluation 01/2021: Laser-Excited Acoustics for Contact-Free Inspection of Aerospace Composites
- ^ N. Meyendorf, N. Ida et al. (Hrsg.): Handbook of Nondestructive Evaluation 4.0 (englisch), Springer 03/2022: https://link.springer.com/referencework/10.1007/978-3-030-73206-6