A Consideration on the use of shear waves to improve the sensitivity of an optical ultrasonic sensor for under-sodium viewers

Saruta, Koichi   ; Shirahama, Takuma*; Yamaguchi, Toshihiko ; Ueda, Masashi 

The present work is intended to investigate shear waves induced on stainless steel diaphragms by the incidence of ultrasonic waves, with the main emphasis on the understanding of the generation mechanism of the resultant second wave, in order to explore the possible use of the shear waves for improvement of the sensitivity of an optical ultrasonic sensor used for under-sodium viewers. The response of the diaphragms exposed to ultrasonic waves with different pressure profiles is measured by a heterodyne interferometer to examine the location where the shear waves are excited. The generation of the second wave is interpreted on the basis of interference of these shear waves. The effectiveness of the use of the shear waves is demonstrated by an improved sensitivity of 0.26 nm/kPa for a 3-mm-diameter and 5-m-thick diaphragm. This sensitivity is approximately 7 times as high as the sensitivity achieved with the first wave, which is the displacement of the diaphragm caused by directly incident ultrasonic waves.