"When carrying out ultrasonic measurements for non-destructive material testing, a coupling agent, such as a gel, usually has to be used," explains Dr. Balthasar Fischer, company founder and CEO of Xarion. "A basic problem here is that non-destructive testing of materials should ideally be carried out without contact, for example to avoid contamination of the material, to avoid damaging the test specimen with water or to facilitate robot-assisted testing. Compared to water or gel, however, air is only suitable as a coupling medium to a limited extent. This is because a lot of ultrasonic signal is lost at the material-to-air interface."

Conventional ultrasonic receivers are generally based on the piezoelectric principle, whereby the ultrasonic wave causes the piezoelectric material to vibrate. To ensure that the sound can couple into the detector with as little loss as possible, the acoustic impedance between the test specimen and the detector is equalized using a coupling agent. This prevents a large proportion of the ultrasound from being reflected and lost at the interface between the air and the piezo receiver, making non-destructive testing more difficult. The optical microphone, on the other hand, detects the ultrasound directly in the air; the sound wave is not first coupled into a solid body and then its vibration is detected. Instead, a laser beam contactlessly measures the change in density of the air caused by the propagating ultrasonic wave. This eliminates the need for an air-to-solid interface and the associated undesirable loss of the test signal.

The optical microphone is also used in another area of application: Camera systems are mostly used in industrial process monitoring today. In manufacturing processes where lasers are used, including laser welding and structuring as well as in additive manufacturing, optical systems can reach their limits due to the strong optical process emissions. The camera can literally be blinded by the laser, or smoke can restrict the line of sight. In addition, a camera cannot be used if the manufacturing process is to be monitored for defects that occur below the surface. Examples of this are cracks or pore formation, or a peeling adhesive layer. Acoustic process monitoring is an interesting alternative here. Analogous to the complementary human sensory perceptions, detection is not only carried out optically with a camera (corresponding to the human eye), but also acoustically via the microphone (corresponding to the ears).

The Eta250 Ultra optical microphone. A complete system consisting of the signal processing unit, sensor head and optical fiber. © Xarion

In the case of acoustic monitoring, condenser microphones have been used up to now, but their sensitivity to background noise means that they can only be used to a limited extent for monitoring production and quality control. As the ambient noise in a production hall is often very high, acoustic process monitoring with conventional microphones is out of the question for most processes. The situation is different with optical microphones: The measurable frequency range is so wide that a simple frequency filter can be used to distinguish between background noise (usually below 100 kilohertz) and valuable process signatures (significant portions also above 100 kilohertz).

In order to cover a wider acoustic frequency range, Xarion has opted for an optical rather than electro-acoustic or mechanical approach to sound detection: the company has developed a laser-based measurement method that can be used to carry out material testing and process monitoring in a very wide frequency range from around ten hertz to two megahertz in the air, outperforming conventional sensors by a factor of 20.

Xarion uses an acousto-optical principle with its Eta250 Ultra optical microphone: a laser pulse is first fired at the test specimen to cause the material to vibrate. The signal of this sound-shock pulse, which is then emitted into the air, is then measured. The sound wave vibrations cause a change in air pressure, which in turn influences the wavelength of the laser light in the optical microphone. "The sound wave is sent into a very compact interferometer in the sensor head, which is only two millimetres in size," explains Fischer. "It influences the wavelength and thus changes the brightness of the laser beam in the mini-interferometer, which is then measured." A movable membrane or a deformable piezoelectric material is therefore superfluous. This avoids disturbing natural oscillations, which influence the measurement result in an undesirable way.

The optical microphone itself requires very little space for the measurements: the sensor head is just five millimetres in size. This means that the device can be mounted on robot arms and integrated into existing inspection processes without a great deal of time and effort or complicated conversion and adaptation measures. This not only facilitates non-destructive material testing, but also inline process monitoring (i.e. quality inspection directly during the manufacturing process). The optical microphone is connected to a control unit via a fiber optic cable; this contains the laser, the detection unit and a preamplifier. The metal-free cabling also enables measurements in difficult environments, such as in the vicinity of electromagnetic fields, and also works over very long distances without measurable signal loss or interference. as