Fig. 4

Frequency response calibration: F–P measured time-domain signal. The fiber is placed vertically and inserted into the tank, as shown in Fig. 3. The bottom of the tank is a layer of transparent film, and the bottom of the cling film is the ultrasonic transducer, which is vertically upward relative to the optical fiber. The distance between the end face of the optical fiber and the transducer is approximately 1 mm. The signal is measured by adjusting the distance and coupling angle between the transducer and the optical fiber