An ultrasonic method to measure effective temperature in the vicinity of laser-induced optical breakdown

Laser-induced optical breakdown (LIOB) can be used to target microbubbles in tissue for biomedical applications. We have previously developed a system in which high-frequency (50 MHz) ultrasound is used to characterize LIOB in real time. By exploiting the relationship between sound speed and temperature in a medium, this system readily lends itself to temperature measurement in the vicinity of LIOB. Experiments were performed in which an LIOB bubble was created at the bottom of a water tank using a regeneratively amplified Ti:Sapphire laser. During and after laser exposure, a series of ultrasound pulse-echo signals was recorded to monitor arrival time shifts of reflections from the tank bottom in the region surrounding the bubble. A finite-element model was developed using commercial software (FEMLAB; Comsol Inc.). Predicted shifts in pulse-echo arrival time were calculated from the model data and compared with the experimentally measured shifts. The effective heat input in the model was varied and the mean squared error between model and experiment was minimized to determine the experimental heat input. Results suggest that, by adjusting laser parameters, LIOB can be controlled to be a thermally noninvasive process, producing only a small temperature change in the vicinity of the generated bubble.