Backward mode detection of laser-induced wide-band ultrasonic transients with optoacoustic transducer

Time-resolved piezoelectric detection of wide-band ultrasonic transients induced by laser pulses in absorbing medium was studied. An optoacoustic transducer was developed for measuring the profiles of ultrasonic transients propagating in backward direction out of the laser-irradiated medium. For this purpose, an optical fiber for delivery of laser pulses to the surface of absorbing medium and a wide-band lithium niobate acoustic transducer were incorporated in one compact system, optoacoustic front surface transducer (OAFST). The transducer possesses temporal resolution (rise time) of 3.5 ns, low effective thermal noise pressure (10 Pa), and high sensitivity of piezoelectric detection (10 mu V/Pa) over the ultrasonic frequency range from 1 to 100 MHz. Nd:YAG laser pulses at 355 nm were employed to generate distribution of acoustic sources in water solutions of potassium chromate with various concentrations. A temporal course of ultrasonic transients launched into an optically and acoustically transparent medium, coupled to the absorbing medium, was studied. Ultrasonic profiles experimentally measured at the site of laser irradiation were compared with profiles calculated using theoretical model. Experimental curves were in a good agreement with theoretical profiles. The backward detection mode permitted accurate reconstruction of the axial distribution of heat deposition in the laser-irradiated homogeneous or layered medium from the measured profiles. OAFST may become a useful modality for optoacoustic imaging in biological tissues and nondestructive evaluation of industrial materials. (C) 2000 American Institute of Physics. [S0021-8979(00)02804-8].