Ultrasonic Detection of High-Intensity Focused Ultrasound Field using Quadrature Point Phase Step in a Fiber Optic Interferometric Sensor

A compact low-temperature crosstalk all-fiber ultrasonic sensor based on quadrature point phase step is proposed and experimentally demonstrated for the detection of high-intensity focused ultrasound (HIFU). The end facet of the graded index fiber was microfabricated with a 3π/2 phase step, which forms an ultra-compact two-wave interferometer. The ultrasonic wave front can be simply detected with phase-front modulation. The all-fiber configuration allows the sensor to detect the HIFU field at a high intensity level. More importantly, the proposed sensor is virtually temperature independent due to the ultra-thin phase step. The experimental results show that the ultrasonic sensor has a sensitivity of 324 mV/MPa, and the standard variation of the temperature cross sensitivity is only 1.7%. The proposed fiber sensor is expected to have potential applications in HIFU detection due to its tolerance at high intensity levels and low-temperature cross sensitivity.