Research Of Broadband Acoustic Wave Sensing Based On Fiber-membrane Composite Devices

Fiber optic broadband acoustic sensors have great potential applications in many civil and military fields,such as oil and gas pipeline leakage monitoring,high-precision photoacoustic spectrum detection,seismic wave monitoring and anti-submarine monitoring.Compared with the traditional electronic acoustic sensor,optical fiber acoustic sensor has the advantages of high sensitivity,resistant to strong electromagnetic interference,light weight,small size,resistant to corrosion and other irreplaceable advantages.The research of broadband acoustic sensor based on fiber-membrane composite devices structure has been conducted to achieve the goal of high sensitivity,high precision,fast response,ultra-wideband and small size of the acoustic sensor.Four different broadband acoustic sensors based on fiber-membrane composite devices have been designed.It can be divided into two categories:the one broadband acoustic sensor is based on the transmission-type special optical fiber device combined with membrane,and the other ultra-wideband acoustic sensor is based on the reflective EFPI structure combined with the membrane.The specific research results are as follows:?1?Broadband acoustic sensor based on transmission-type special grating device combined with PET membrane:single-side point-by-point writing of thin core fiber?TCF?with cladding diameter of 80?m is carried out by CO₂ laser,and the refractive index modulation period is 1mm.The fabricated thin-core ultra-long period fiber grating?TC-ULPFG?can achieve the high curvature sensitivity of 97.77dB/m^-1.The dynamic curvature modulation of TC-ULPFG is performed on PET membrane loaded by acoustic wave.TC-ULPFG is seamlessly pasted on the PET membrane.The theoretical resonance frequency of the membrane is 5.36Hz,and the static deformation is 208cm/Pa.The experimental test shows that the broadband acoustic response range of 70Hz³kHz is achieved,and the minimum detectable sound pressure is 1.94mPa/Hz^1/2@200Hz.The response is flat in the range of 70-200Hz and 1-3kHz,respectively.?2?Broadband acoustic sensor based on transmission-type special microstructure optical fiber combined with PET membrane:a new type of special microstructure optical fiber is proposed,which is single hole twin core eccentric fiber?SHTECF?.The transmission spectrum is mainly composed of anti-resonance effect and in-line Mach-Zehnder interference?MZI?effect after fusion splicing both ends of the SHTECF and single mode fiber?SMF?.The curvature sensitivity of-1.31dB/m^-1 and-1.54dB/m^-1 is obtained by intensity demodulation.The length of the sensing area is 2.6 mm.Gaussian fitting and FFT bandpass filtering method is used respectively to get the temperature sensitivity of 42.18pm/?and 2.057nm/?,and can simultaneously realize the high precision and large dynamic range of temperature measurement.The measurement in the range of 60Hz³kHz is realized in the broadband acoustic detection,and the minimum detectable pressure is 171.6?Pa/Hz^1/2@60Hz.The size of fabricated acoustic sensor head is one inch.?3?Broadband acoustic sensor based on reflective graphene diaphragm of closed cavity of external Fabry-Perot interferometer?EFPI?:10nm graphene diaphragm is presented as the acousto-optic transducer,and the calculation of reflectivity is 0.07.The reflectivity of end surface of SMF is 0.0338,the cavity length of acoustic sensor head is62?m,and cavity transmission coefficient is 0.46.The reflectivity matching between the fiber end face and the graphene diaphragm is realized,and the FP interference spectrum with low fine and high contrast is obtained.The contrast of interference spectrum in air and underwater was 16dB and 12dB,respectively.The radius of the selected graphene diaphragm is 1mm,and the resonance frequency of the selected graphene diaphragm is calculated to be 101.4Hz in theory.The broadband acoustic measurement range of5Hz⁰.8MHz is achieved.The minimum detectable pressure of infrasonic frequencies is0.77Pa/Hz^1/2@5Hz,the minimum detectable pressure in the audible frequency band is33.97?Pa/Hz^1/2@10kHz.The acoustic sensor head is a closed cavity with a length of 14mm and a diameter of 2.5mm.?4?Broadband acoustic sensor based on reflective gold film open cavity of EFPI:the300nm gold film is proposed to be used as an acousto-optic transducer.The radius of the gold film is 2.5mm,and the value of theoretical resonance frequency is 50.4Hz.It can detect the infrasound signals in lower frequency band.The gold film with a thickness of300nm is grown by electron beam evaporation,and the film-free sensor head is bonded with the prepared gold film through 353ND adhesive.The contrast of interference spectrum obtained by the sensor head in air and underwater is 12dB and 16dB,respectively.The ultra-wideband acoustic wave of 1Hz⁸0MHz is achieved,which is the widest band measurement range in reported reference.The MDP of infrasound band is54.1mPa/Hz^1/2@5Hz.The designed gold film sensor head is of open cavity type.Under the condition of high sound pressure sensitivity,the measurement range of broadband sound wave is broadened and the noise level of low frequency infrasound is reduced.