Gas-coupled laser acoustic detection at ultrasonic and audio frequencies
Review of Scientific Instruments
Airborne acoustic waves have been detected by a laser-beam deflection technique in both the ultrasonic and audio frequency ranges. For ultrasonic applications, a probe beam is directed parallel to the surface of a sample. Ultrasonic waves in the solid are detected when an acoustic wave is radiated from the surface into the ambient air, where the density variations cause a beam deflection. Gas-coupled laser acoustic detection ͑GCLAD͒ has been used to record well-resolved through-transmission and
... gh-transmission and surface-acoustic wave forms in various materials. GCLAD has also been incorporated into a C-scanning system where it has been used to image subsurface flaws in graphite/ polymer composite panels. Because the laser beam is not reflected from the sample surface, the technique is not dependent upon the surface optical properties of the material under investigation. It is particularly useful for testing graphite/polymer composites and other materials with rough surfaces. The beam-deflection technique has been tested quantitatively in the kHz frequency range by passing a probe beam through a cylindrical resonator. The acoustic spectrum of the resonator was measured from 4 to 13.5 kHz by scanning the frequency of a source and recording the acoustic field with both a microphone and the beam-deflection system. The acoustic fields of the lower-frequency modes are well known and enable both qualitative and quantitative tests of the beam-deflection technique. Measurements on the lowest-frequency plane-wave mode were used for absolute calibration of the microphone. The noise level of the beam-deflection measurements at 4.3 kHz was found to be 0.05 nrad ͑rms͒, corresponding to an acoustic pressure of 0.005 Pa ͑rms͒.