Laser pulse impact characterization using ultrasonic transducers

Abstract

Experiments were conducted with an excimer laser (wavelength 248 nm) to cause damage to gelatinous materials, which have optical properties similar to skin tissue. The laser pulses, when they impact the specimen, generate an elastic wave that propagates through the material. A 150-MHz broadband ultrasonic transducer was placed underneath the specimen to observe the material response and the pressure generated by the laser pulse. Different incident laser energies and laser beam spot sizes were utilized to obtain a range of input parameters, and the corresponding transducer responses were recorded. Fourier analysis of the signals was performed to identify the frequency response from the laser pulse. Additional tests were carried out to observe the effects due to a train of laser pulses. Earlier tests were performed with 200-kHz hydrophone-calibrated transducer but due to the shortness of the laser pulses (∼20 ns), a higher frequency transducer was used to accurately characterize the laser impact. The initial results indicate that there is a dominant frequency response around the 25–30-MHz regime.