Light distribution measurements in absorbing materials by optical detection of laser-induced stress waves

A method for optimized generation and detection of thermoelastic stress waves for the measurement of tissue optical properties and structure is investigated. The stress waves are formed by short pulsed irradiation of an absorbing dye solution with a Q-switched Nd:YAG laser at 532 nm. An optical transducer based on pressure-induced reflectivity changes of a continuous laser beam at a glass-water interface detects the stress wave in front of the irradiated sample surface. It is shown theoretically and experimentally that this kind of detector, where the active area is a small spot close to the irradiated surface, minimizes signal distortion due to acoustic diffraction. Comparisons of absorption coefficients measured acoustically and from optical transmission show a good agreement between the two methods. The high sensitivity of the detector (1.5 mV/bar) makes it possible to keep the temperature and pressure rise in the investigated target low, which enables in vivo applications of the optical transducer. (C) 1996 American Institute of Physics.