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Development of Miniature High-frequency Interferometric Fibre-optic Hydrophones with Folded Air Cavities

Existing interferometric fibre-optic hydrophones have some common challenges in detecting high-frequency underwater acoustic signals. First, to obtain acoustic sensitivity that meets the requirements for detection in practical applications, long arms of the M-Z interferometer are desired, which in turn requires the width of the fibre-winding area to be increased. However, when the lateral size of the hydrophone probe is too large, it no longer fits in the high-frequency signal detection. Second, to meet the practical requirements of sensitivity, the air cavity, which serves as the sensitisation structure of the hydrophone, needs to be slender to increase the fibre-winding area. This may lead to a lower natural resonance frequency and, therefore, result in the poor high-frequency performance of the probe. These problems hinder the expansion of the response frequency of fibre-optic hydrophones. Herein, we propose a small-size interference high-frequency optical-fibre hydrophone based on a folded air cavity. This structure turns the traditional thin air cavity into a folding multi-layer cavity, greatly reducing the length of the two arms of the interferometer and the transverse dimension of the fibre-optic hydrophone without reducing the fibre-winding area. The high-frequency applied performance of the hydrophone is improved as well. The sensor-sensitive and supporting structures of the hydrophone were optimised by finite element analysis. With the folded air cavity structure, a high-frequency fibre-optic hydrophone with dimensions less than Φ20 mm×20 mm was realised. Experimental results show that the high-frequency fibre-optic hydrophone based on the Michelson interferometer has an acoustic sensitivity of about -140dB in the range of 20Hz-20kHz, and the fluctuation of the sound-pressure sensitivity is better than ±1.5 dB, which satisfies the requirements of high-frequency underwater acoustic signal detection.