GAS-COUPLED LASER ACOUSTIC DETECTION AT ULTRASONIC AND AUDIO FREQUENCIES

Airborne acoustic waves have been detected by a laser-beam deflection technique in both the ultrasonic and audio frequency ranges. For ultra sonic applications, a probe beam is directed parallel to the surface o f a sample. Ultrasonic waves in the solid are detected when an acousti c wave is radiated from the surface into the ambient air, where the de nsity variations cause a beam deflection. Gas-coupled laser acoustic d etection (GCLAD) has been used to record well-resolved through-transmi ssion 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. Becau se the laser beam is not reflected from the sample surface, the techni que is not dependent upon the surface optical properties of the materi al 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 frequen cy 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 wi th both a microphone and the beam-deflection system. The acoustic fiel ds of the lower-frequency modes are well known and enable both qualita tive and quantitative tests of the beam-deflection technique. Measurem ents on the lowest-frequency plane-wave mode were used for absolute ca libration of the microphone. The noise level of the beam-deflection me asurements at 4.3 kHz was found to be 0.05 nrad (rms), corresponding t o an acoustic pressure of 0.005 Pa (rms). (C) 1998 American Institute of Physics.