Measurement of laser-induced acoustic waves with a calibrated optical transducer

Abstract

Irradiation of an absorbing material with a short laser pulse generates a thermoelastic stress wave, from which the distribution of absorbed energy can be derived. This method is ideal to measure the light penetration in biological tissue. Especially for in vivo applications, we developed an optical stress transducer that can be positioned directly in front of the irradiated surface, inside the laser beam, in order to avoid distortion of the stress wave due to acoustic diffraction. The detector is based on stress wave-induced changes of optical reflectance of a glass-water interface, probed with a continuous laser beam that is incident at an angle close to the critical angle of total internal reflection to achieve maximum sensitivity. In this study, we describe the theory for the calibration of the transducer and compare the measured with the theoretically predicted signals. In the experiments, an aqueous dye solution is irradiated with pulses from either a Q-switched, frequency-doubled Nd:yttrium aluminum garnet (YAG) laser or from an optical parametric oscillator with pulse durations of 8 and 6 ns, respectively. Good agreement between the measured and calculated waveforms as well as the possibility to obtain photoacoustic absorption spectra from the shape of the recorded signals is demonstrated. From our experimental and theoretical findings, it follows that the detector is characterized by a high temporal and spatial resolution and by an adjustable sensitivity, depending on the incident angle of the probe beam at the glass-water interface. Apart from the applications proposed in the present work, it seems to be possible to use this kind of transducer for the two-dimensional recording of stress waves.