Eta450 Ultra: Optical Microphone


Optical Microphone – World’s first microphone without any moving parts

The Eta450 Ultra membrane-free optical microphone is XARION’s most sensitive device. It is designed for contact-free ultrasound material testing. The frequency bandwidth expands from 50 kHz to 2 MHz making the sensor the perfect match for broadband, laser-ultrasound excitation. This small-sized and contact-free testing setup is particularly suitable for the characterization of composite materials, weld spots and adhesive joints.



  • Membrane-free optical microphone
  • Frequency range 50 kHz–2 MHz
  • Self-noise: 5 µPa (BW: 1 Hz, measured @ 500 kHz)
  • Max. sound pressure for THD < 3%: 40 Pa
  • Dynamic range: 30-130 dB SPL @ 50 kHz-2 MHz


XARION’s patent-protected sensor technology platform offers substantial advantages:


  • Extreme ultrasound frequency range from 10 Hz up to 2 MHz in air, 20 MHz in liquids
  • Acoustic and ultrasound detection greater by a factor of 10 than present state-of-the art
  • Transducer principle with a perfectly linear frequency response. Although the enclosure needs to be carefully designed to minimize its influence on the sound field, the transducer itself is not frequency dependent
  • Sound detection in air and liquids
  • Qualification for ultra-high sound pressure levels (up to 190 dB SPL)
  • Since no moving inert mass is involved, the Optical Microphone has a true temporal impulse response.
  • Inherent phase match in array configurations
  • No metallic parts and glass fiber-coupled, hence operational in high electromagnetic fields.


Eta450 Ultra: Air-Coupled Ultrasonic Inspection of Fiber-Reinforced Plates Using an Optical Microphone
Eta450 Ultra: Impact damage assessment in biocomposites by micro-CT and innovative air-coupled detection of laser-generated ultrasound
Eta450 Ultra: Material characterization via contact-free detection of surface waves using an optical microphone
Eta450 Ultra: Ultrasound inspection of spot-welded joints
Eta450 Ultra: Laser-Excited Acoustics for Contact-Free Inspection of Aeropace Composites
Eta450 Ultra: Thickness measurement via local ultrasonic resonance spectroscopy



For the detection of sound waves, conventional microphones use membranes or other moving parts as intermediaries between the incoming acoustic and the resulting electrical quantity. For acoustic ultrasound sensors based on piezoelectric crystals, the approach is similar: the acoustic wave mechanically deforms the crystal. In contrast, the patented idea behind XARION’s Optical Microphone is to exploit another, completely different property of sound: the fact that sound changes the speed of light!   In a rigid Fabry-Pérot laser interferometer consisting of two miniaturized mirrors, sound pressure changes the refractive index of the air. This alters the optical wavelength and the light transmission which consequently leads to the respective electrical signal. In contrast to conventional microphones, the Optical Microphone is the world’s first microphone without any moving parts. No mechanically movable or physically deformable parts are involved. By consequence, the sensors exhibit a compelling frequency bandwidth, free from mechanical resonances. The sensor principle is highly sensitive. In fact, refractive index changes below 10-14 can be detected with this technology. This corresponds to pressure changes as small as 1 µPa.


Eta450 Ultra: Optical Microphone Datasheet

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