Raman spectroscopy diagnostics of the local time profile of an ultrasound beam in water

It has been demonstrated for the first time that pulsed laser Raman spectroscopy can be used for diagnostics of a local acoustic pressure profile with a peak pressure drop of 50 MPa and a carrier frequency of 2.0 MHz in the focus of an ultrasound beam propagating in water. A 527-nm 10-ns laser pulse has been focused into the waist of the ultrasound beam at an angle of 90$^\circ$. Backscattered photons have been recorded in a gated spectrum analyzer. It has been found that the Raman spectra at the times corresponding to the maximum and minimum acoustic pressures are significantly different. This feature has been used for point-to-point reconstruction of the acoustic pressure profile; for this purpose, the delay between the ultrasound and laser pulses is consequently increased with a step of 50 ns. It has been shown that, within the measurement error, the resulting changes in the position of the center of the stretching OH vibration band of water molecules in the Raman spectrum reproduce the acoustic pressure profile directly measured using a PVDF hydrophone at the laser sensing point. The results obtained can be used to develop a new method for remote diagnostics of the time profile of acoustic pressure and monitoring the local dynamic of the compression-tension processes in water up to critical pressures corresponding to the cavitation rupture, when the use of the hydrophone can lead to its damage.