Optical method of measuring nanosecond pressure pulse in water

We have developed a method of recording nanosecond water shock pressure pulses by an optical method. A high-pressure shock front can be detected by the interface between water and a BK7-glass prism, where a He–Ne laser beam is focused. The method utilizes the fact that the critical angle of total internal reflection depends on the density of the medium under compression. After the shock wave front arrives at the water–glass interface, part of the incident light beam is transmitted into the water specimen. The ratio of the light intensity of reflected and transmitted beam depends on the values of the refractive index of water and BK7-glass under shock pressure. In this study, a shock wave in water is generated by focusing a Nd:YAG laser beam to an intentionally roughened surface of PMMA block in water. He–Ne laser beam is incident on the BK7-glass prism interface at an angle slightly less than the critical angle, and an avalanche photodiode monitors the reflected light intensity. The pressure history at the interface can be estimated. Estimated pressure value is very sensitive to the angle of incidence. The time resolution of the method is mainly limited by the focused spot size of He–Ne laser on the interface.