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

Time-resolved piezoelectric detection of wide-band ultrasonic transients in duced by laser pulses in absorbing medium was studied. An optoacoustic tran sducer was developed for measuring the profiles of ultrasonic transients pr opagating in backward direction out of the laser-irradiated medium. For thi s purpose, an optical fiber for delivery of laser pulses to the surface of absorbing medium and a wide-band lithium niobate acoustic transducer were i ncorporated in one compact system, optoacoustic front surface transducer (O AFST). The transducer possesses temporal resolution (rise time) of 3.5 ns, low effective thermal noise pressure (10 Pa), and high sensitivity of piezo electric 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 distrib ution of acoustic sources in water solutions of potassium chromate with var ious concentrations. A temporal course of ultrasonic transients launched in to an optically and acoustically transparent medium, coupled to the absorbi ng medium, was studied. Ultrasonic profiles experimentally measured at the site of laser irradiation were compared with profiles calculated using theo retical model. Experimental curves were in a good agreement with theoretica l profiles. The backward detection mode permitted accurate reconstruction o f the axial distribution of heat deposition in the laser-irradiated homogen eous or layered medium from the measured profiles. OAFST may become a usefu l modality for optoacoustic imaging in biological tissues and nondestructiv e evaluation of industrial materials. (C) 2000 American Institute of Physic s. [S0021-8979(00)02804-8].