Laser ultrasound NDT with a novel airborne optical microphone

Fischer, Balthasar; Xarion Laser Acoustics GmbH; Austria

Herbst, T.; Xarion Laser Acoustics GmbH; Austria

ID: ECNDT-0100-2018
Session: Laser UT 2
Room: J1
Date: 2018-06-14
Time: 11:50 - 12:10

A laser ultrasound NDT system consisting of a pulsed laser and a novel fiber-coupled optical microphone is presented.
A short-pulsed laser is focused onto the test object, where it generates ultrasonic waves by heating or ablating the surface. Due to the short excitation pulse, the ultrasonic waves (especially the bulk waves) show a very broadband frequency spectrum from a few kHz to several MHz. In contrast to conventional LUS approaches where typically two-wave mixing interferometers are used for detection, here, a membrane-free optical microphone is employed to measure the ultrasonic waves.
The microphone sensor is based on the fact that sound pressure changes the refractive index of the air. In a miniaturized Fabry-Pérot etalon of 2 mm size consisting of two parallel mirrors, these changes lead to variations in light transmission, which are translated into a respective electrical signal. Since the detector measures ultrasound radiated into the air, rather than the vibration of the sample surface, the probe laser is not leaving the sensor and has no contact with the sample surface. Hence, there are no requirements regarding the sample’s surface structure, its color, or reflectivity. This novel optical receiver has no moving parts, therefore showing a flat frequency response over a 1 MHz frequency bandwidth in air. It hereby exceeds the bandwidth of conventional piezo-based detectors by an order of magnitude. Regarding size and complexity, the all-integrated system undercuts conventional LUS detection systems (as e.g. above mentioned TWM vibrometer), substantially.
We present the system setup together with thorough characterization measurements. Scan results of a representative set of samples are shown. The measurement outcomes provide strong indication of the feasibility for robust, highly sensitive and cost-effective single-sided pulse-echo NDT.