Investigation on Frequency characteristic of Fiber Optic Probe Hydrophone

A fiber sensor is widely employed in acoustic measurement recently. A reflection type fiber optic probe hydrophone (FOPH) has a lot of advantages in ultrasonic measurement, for example toughness and wide frequency response. Since Eykman equation of typical FOPH principle is independent on frequency, it is expected that FOPH has wide frequency response. However, FOPH sensitivity is not so high in the comparison to a conventional piezoelectric hydrophone. In this report, sound pressure in ultrasonic cleaning machine is measured around 35.5kHz of low frequency by FOPH. Measurement environment in the ultrasonic cleaning machine has cavitation bubble collapse that give severe damage to conventional hydrophone. The authors already reported that FOPH can measure sound pressure in cavitation bubble collapse environment. In the measurement, the output level FOPH around 35.5kHz is lower than that of conventional hydrophone. The frequency characteristics are discussed in the paper.