Optically Detecting Wavefronts and Wave Speeds in Water Using Refracto-Vibrometry

Abstract

Refracto-vibrometry is a technique that uses a laser Doppler vibrometer to measure acoustic pressure fields. The vibrometer laser is directed through a medium towards a stationary retroreflective surface. Acoustic waves (density variations) for which the wavefronts pass through the laser, as the beam travels from the vibrometer to the retroreflector and back, cause variations in the integrated optical path length. This results in a time-varying modulation of the laser signal returning to the vibrometer, enabling optical detection of the acoustic wavefronts. In the current experiment, a Polytec PSV-400 scanning laser Doppler vibrometer, sampled at 100 MHz, monitored the waves emitted by a 1 MHz Panametrics V303 ultrasound transducer immersed in a water tank. The time-varying signal detected by the vibrometer at numerous scan points was used to generate videos of the time evolution of acoustic wavefronts; these videos will be presented. Refracto-vibrometry was also used for optical measurements of the time of flight of ultrasonic waves through different materials, including samples of lead and fabricated bone. This enabled determination of wave propagation speeds. The wave speeds obtained with optical detection using refracto-vibrometry were in agreement with measurements using a conventional ultrasonic transducer to detect the wavefronts.