A high-frequency hydrophone using an optical fiber microknot resonator

We propose a compact fiber-optic hydrophone and use it for acoustic wave measurements. The sensor probe comprises a microresonator formed by simply tying a tapered fiber into a loop structure. When the round-trip phase of light is an integer multiple of 2π, a well-defined resonance spectrum is produced by the resonator, with a quality factor as high as 10⁴. Using spectral filtering technology for power interrogation, the steep linear resonance slope leads to sensitive operation of the sensor for high-frequency acoustic wave detection (of the order of several megahertz). After fabricating the sensor, the knot exhibits low-absorption loss underwater and, thus, the device can possibly be used as a hydrophone. The simple fabrication and small size structure of the sensor make it a good candidate for high-resolution scanning imaging.