Optical probing of acoustic fields-Application to the ultrasonic testing of steam generator tubes

Pulsed acoustic fields transmitted in water by piezoelectric focused transducers were optically probed. The probe beam of an optical heterodyne interferometer is reflected by a thin membrane immersed in water in front of the transducer. Simulations based on modal analysis demonstrate that the membrane follows the motion of the particle of fluid, both in the time and the spatial domain. Absolute measurements of the mechanical displacement were performed on a 15-μm-thick Mylar film coated with an optically reflecting layer of gold. In the frequency range of 0.1-20 MHz, the minimum detectable displacement is 1 Å and the spatial resolution reaches 50 μm. Comparison between the particle velocity in the vicinity of the transducer and the particle displacement at the focus agrees with diffraction theory predictions of impulse response. The ultrasonic field for both longitudinal and transverse waves focused from the inside of a steam generator tube was optically measured on the outside surface. The high signal-to-noise ratio permits plotting the variations of the acoustic energy, both along the axis and along the circumference of the tube, within a 40-dB dynamic range