Measurements of ultrasonic field and temperature by a fiber optic microprobe

摘要

There have been several approaches to replace piezoelectric needle hydrophone by fiber optic technologies to obtain higher spatial resolution, wider frequency response over mega-hertz region, a toughness against intense sound pressure and an immunity to electro-magnetic noises. The change in optical refraction index of media caused by sound field was measured as an intensity modulation of light reflected from the end of bare optical fiber. The modulation of bending loss at a sharply bent of fiber was utilized for the same purpose. Though these probes are useful for shock pulses or focused ultrasonic fields, their sensitivities are very low for ordinary sound pressure level. In order to gain moderate pressure sensitivity, the use of a small Fabry-Perot resonator as a sensing part was discussed and an optical fiber sensor with a thin film has been made. We have also proposed a fabrication technique of a small Fabry-Perot resonator at the end of an optical fiber and studied its pressure sensitivity. It has been reported that a fiber Bragg grating can work as an ultrasonic sensor.