Fabrication Of Fiber-optic Hydrophone Based On Off-center Diaphragm Reflection

Depending on the rapid development of optical fiber sensing and optoelectronic technology in recent decades,fiber-optic hydrophones have entered a practical stage.Compared with the traditional piezoelectric hydrophones,the fiber-optic hydrophones have advantages of small volume,wide frequency response range,low self-noise,high sensitivity,large dynamic range,strong anti-electromagnetic interference capability,easy formation,suitable for long-distance transmission,etc.As key component of sonar,fiber-optic hydrophones have great potential in the military industry,especially in terms of array and anti-submarine.In this paper,two kinds of fiber-optic hydrophones were prepared by using Au-coated stainless steel diaphragm and Au-coated quartz diaphragm as reflection strain diaphragms.The optical beam collimator located at the off-center position of the strained diaphragm is used to transmit and receive the light beam.Since the fiber collimator can concentrate the beam,it has the advantages of small beam divergence angle,low transmission coupling loss,and insensitive working distance.The prepared fiber-optic hydrophones have the advantages of small volume,high sensitivity and low cost.In addition,by adjusting the diameter and thickness of the strained diaphragm,the sensitivity and response bandwidth of the fiber-optic hydrophone can be flexibly adjusted.In this paper,the static and dynamic test system of fiber-optic hydrophones are built independently,the response of the prepared fiber-optic hydrophones to static pressure and acoustic pressures at different frequencies is detected.The experimental results show that in terms of static pressure detection,the test results of fiber-optic hydrophones based on Au-coated quartz diaphragm are consistent with the theoretical analysis.In terms of acoustic pressures detection（acoustic frequencies are 8KHz,25KHz,200KHz and 1MHz）,the acoustic pressure range is 0.3-500Pa.The test curves of the fabricated fiber-optic hydrophones are linearly good in the detection range.Taking the acoustic frequency of 25KHz as an example,the sensitivity and the minimum detection limit of the Au-coated quartz diaphragm hydrophone with a diameter of 7mm are-186 dB reV/μPa and 0.7 mPa/Hz^1/2,respectively,which is superior to the ceramic piezoelectric transducer with the sensitivity and the minimum detection limit of-191 dB reV/μPa and 1.2 mPa/Hz^1/2,respectively at the center frequency of 25KHz.In the data processing stage,the signal filtering uses the MATLAB narrowband filter ing function based on the fast Fourier transform and matrix multiplication.Unlike conventional band-pass filters,the narrowband filtering function cut off the irrelevant signals beyond the passband frequency range,rather than gradually attenuating.In the actual filtering process,once the center frequency（test frequency）of the passband and the sampling rate of the input signal are determined,the corresponding highest low-pass frequency parameter and the corresponding lowest high-pass frequency parameter of the narrowband filtering function are also fixed.When the test frequency changes,the narrowband filtering function can still obtain ideal filtering effect by adjusting appropriate parameters.The filtering function has good reusability.