Jn. Caron et al., GAS-COUPLED LASER ACOUSTIC DETECTION AT ULTRASONIC AND AUDIO FREQUENCIES, Review of scientific instruments, 69(8), 1998, pp. 2912-2917
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.