INVESTIGATION OF LASER GENERATION OF LAMB WAVES IN COPY PAPER

Experiments have been conducted to study the generation of ultrasonic Lamb waves in copy paper using a pulsed Nd:YAG laser. The signals were detected with an out-of-plane heterodyne Ar+ laser interferometer. Th e effect of the generation laser spot size, energy density, and total energy on the transfer of energy from the generation laser to the two lowest Lamb wave modes, a, and s(0), was investigated. As with metals excited in the linear thermoelastic regime, the signal energy increase s as the square of the input laser energy. However, unlike in metals, significant statistical variations of the waveforms (peak amplitudes, arrival time) were observed and quantified by means of cross-correlati on techniques. Also unlike in metals, it appears that there is an opti mum spot radius of about 90 +/- 20 mu m (i.e. an energy density of abo ut 22.4 J/cm(2)) for efficient generation of both a(0) and s(0) mode i n the copy paper we studied. The shift in frequency content of the Lam b waves was examined as a function of the generation spot size. Damage at the generation spot was assessed by photographic microscopy.