Detection of high-frequency ultrasound with a polarization-maintaining fiber

The effect of a high-frequency ultrasound wave incident normally upon a polarization-maintaining optical fiber is investigated theoretically and experimentally. Two different mechanisms accounting for the birefringence change in the fiber resulting from radial strain produced by the ultrasonic wave are identified and studied. A polarization-maintaining fiber can respond to lower frequencies than a single-mode fiber in the polarimetric mode. Moreover, it does so with better sensitivity and requires no real-time control of the polarization state of the input light. The frequency separating the ranges of significance of the two mechanisms is experimentally found to be about 1 MHz (in water). The principle and the realization of a polarimetric ultrasonic sensor using a polarization-maintaining fiber are also described.>