Eta250 Ultra: Optical Microphone


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

XARION’s Eta250 Ultra membrane-free optical microphone allows coverage of the acoustic frequency bandwidth from 10 Hz to 1MHz with a single sensor head and is ideally suited for process monitoring. It was specially designed for applications with low self-noise requirements.



  • Membrane-free optical microphone
  • Low self-noise level
  • Frequency range: 10 Hz–1 MHz (acoustics and ultrasound)
  • Dynamic range: 50–150 dB (A) SPL


Ultrasound Measurements


Ultrasound Field Characterization

The small size and the linear frequency response make the Optical Microphone the perfect tool for precise measurements of time signals, frequency distributions and acoustic field maps of ultrasound emitters such as air-coupled ultrasonic piezos.

Measurements in High Electromagnetic Field

All-optical components in the sensor head as well as the optical fiber cabling are insensitive to strong electromagnetic fields. Thus, sound can be recorded in applications that are out of reach for classical microphones due to strong EM- or radioactive fields.

Ultra-high Sound Pressure Levels

The Eta100 Ultra was designed to measure extremely high sound pressure levels (up to 180dB SPL). All our microphones are immune to damage by excessive acoustic pressure levels.

Inline process monitoring of hairpin welding

In this recent investigation, XARION´s Optical Microphone was used to capture airborne ultrasound generated during the hairpin welding process.

A multitude of welds were performed under varying quality-affecting parameter conditions, such as positioning misalignments or focal height and laser power deviations. It was found that the real-time ultrasound emission could be utilized as an early indicator for predicting the important quality criterium of final cross-sectional area of connection.

Process Monitoring


Laser Process Monitoring

Laser processes emit light – and ultrasound! The airborne ultrasound emission can be harnessed to monitor the quality of industrial laser material processes, e.g. laser welding, structuring or cutting as well as additive manufacturing such as powder bed fusion and direct energy deposition in real-time.

Crack Detection

The ultrasonic frequencies exceed the audible human hearing range 100-fold. In the ultrasound regime (free from background noise), crack signals of brittle materials, such as ceramics or high-strength alloys, can easily be picked up by the optical microphone to trigger a warning signal to the production line.

AI-based Machine Diagnostics

Due to the Optical Microphone’s immense frequency bandwidth, every recording contains a vast amount of data available for feature extraction. We utilize classification and regression, SVM algorithms, k-means clustering and other methods to achieve unprecedented correlation between acoustic process signal and final product quality.


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.


Acoustic Process Control for Laser Material Processing

Air-Coupled Ultrasonic Inspection of Fiber-Reinforced Plates Using an Optical Microphone

Inline Process Monitoring of Hairpin Welding Using Optical and Acoustic Quality Metrics

Online cracking detection by means of optical techniques in laser‐cladding process

Ultrasound inspection of spot-welded joints

Characterization of polymer-based piezoelectric micromachined ultrasound transducers for short-range gesture recognition applications

Development of an innovative measurement system for audible noise monitoring of overhead lines

Optical microphone hears ultrasound

Acousto-optical Process Control for Laser Ablation and Laser Welding

Air-coupled membrane-free optical microphone and optical coherence tomography keyhole measurements to setup welding laser parameters


Demonstration of Frequency Bandwidth
  • The Optical Microphone is able to detect the entire acoustic frequency bandwidth which is transmitted through air
  • High frequencies often contain significant process-quality information
  • Hence, disturbing background noise (low frequencies) can be separated from process information (high frequencies)


XARION Process Control



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.


Eta250 Ultra: Optical Microphone Datasheet

XARION Ultrasound Field Characterization Information Brochure

XARION Industrial Process Monitoring Information Brochure

Process Monitoring Bundle Brochure

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