Research Of Fiber-optic Interferometer Based Low-frequency Acoustic Sensing Technology

Low-frequency acoustic wave detection and analysis is an important field of research with a vast number of applications,such as warning and fore-casting of natural disasters,leak detection of oil and gas pipeline,geological prospecting and submarine noise-detection.The most common and commercially available low-frequency acoustic sensors are based on piezoelectric crystals,condensers or moving coils.Nevertheless,they do exhibit some limitations,especially when being submitted to harsh environments,having low resistance to chemical agents and high temperatures,and they are not immune to electromagnetic interference and unequal to use in an explosive or flammable atmosphere.Due to many electronic components and high attenuation of electronic signals,they are also less practical for the applications where it is necessary to cover great distances.But fiber-optic acoustic sensors can remove all of these limitations and bring more advantages to acoustic sensing.Because fiber-optic sensing systems have the strong anti-interference and high sensitivity,are not constrained to the limited size and shape,are robust lasting solutions,capable of multiplexing,and can be directly connected to optical transmission links.By combination of the compact size of fiber-optic and high resolution of interferometer,fiber-optic interferometer is becoming one of the research hotspots in fiber acoustic sensors.In this dissertation,the systematic and intensive study on the techniques of low-frequency acoustic sensing based on fiber-optic interferometers is presented,and the major research works and achievements are outlined as follows:Firstly,the deformation behavior of the sensing diaphragm has been both theoretically analyzed and simulated,especially how the material properties and size of the diaphragm affect the resonant frequency and sound pressure sensitivity.The deformation characteristics of center-embossed diaphragm and the acoustic response characteristics of sensing diaphragm which applied a pre-stress are studied extensively at the same time.Through theory analysis and simulation,it can be observed that the resonance frequency of sensing diaphragm will decrease,at the same time,the low frequency response sensitivity will increase by reducing the thickness,augmenting the radius and using material with lower young's modulus.The center-embossed structure can effectively reduce the variation of reflection angle caused by the diaphragm deformation,which has great benefit for increasing the sensor stability.And the applied pre-stress will increase the resonance frequency and reduce the response sensitivity of sensing diaphragm.All these provide fundamental basis for sensor head design.Secondly,a low-frequency acoustic sensor employing polypropylene/poly(ethylene terephthalate)(PP/PET)diaphragm based on optical fiber Michelson interferometer is proposed.The Michelson interferometer is formed by two beams of light that are reflected into optical fiber collimators by both sides of the PP/PET film.The deformation of diaphragm caused by acoustic signal will be magnified twice in the optical path of proposed sensor.For the novel optical structure and good acoustic performance of PP/PET diaphragm,the proposed sensor has a high sensitivity of 1.73 rad/Pa from 90 to 500 Hz and a noise equivalent pressure of 39.7 dB at 500 Hz.At the same time,the proposed sensor exhibits a good linearity with a R-squared value of 0.9997 while applied acoustic pressure is increased from 76.7 to 97.7 dB.Because of its advantages of low cost,simple fabrication process,a relatively small size,the proposed sensor has great potential in low-frequency acoustic sensing and healthy monitoring.Thirdly,in order to reduce the noise such as temperature changes,central wavelength shift of light source and polarization fading induced noise,a novel in-line Mach-Zehnder interferometer based low-frequency acoustic sensor is designed.The acoustic wave can modulate the curvature of in-line interferometer by using a polythylene terephthalate(PET)diaphragm.The in-line interferometer is fabricated by simply splicing a section of single-mode fiber(SMF)with two short sections of Multimode fiber(MMF).The curvature sensitivity and fringe contrast ratio of the in-line interferometer is improved by tapering the middle SMF to excite the higher clad modes and control the propagation loss of clad modes.The maximum extinction ratio of in-line interference pattern can reach 40dB.The proposed in-line interferometer exhibits a very high curvature intensity sensitivity about 153.9 dB/m-1 at the working wavelength of acoustic sensor.And the curvature-temperature cross sensitivity is 0.0032m-1/??The acoustic detection has been performed in the frequency 60-4000Hz,a flat low-frequency response ranging from 60 Hz to 500 Hz is achieved.The sensitivity can reach to 180 mV/Pa,and the noise equivalent pressure is 36.1 dB at 500Hz,which is 3.6 dB lower than that of MI based acoustic sensor proposed in last chapter.Fourthly,a novel UV adhesive diaphragm-based EFPI fiber sensor for infrasound measurement has been proposed and experimentally demonstrated.The UV adhesive diaphragm with a radius of 1.5 mm and a thickness of 2.2?6.4 ?m is formed by the surface tension of UV adhesive solution.Moreover,UV adhesive can be completely solidified under the ultraviolet light and still keep flexibility,reflective and stability after solidifying.The fabrication process of sensing diaphragm is rather simple and low-cost without any other chemical except UV adhesive.Due to the lower thickness-to-diameter ratio and larger elasticity of UV adhesive diaphragm,This sensor can detect acoustic wave in a very wide range from 1 Hz to 20 kHz.Moreover,it presents a flat frequency response ranging from 1 Hz to 2000 Hz with a small fluctuation about ± 1.5 dB and a high sensitivity of 57.3 mV/Pa at 1000 Hz.And the sensor exhibits the noise limited detectable pressure level of 52.4?Pa/Hz1/2 at 10 kHz and 11.2m Pa/Hz1/2 from 1 to 20 Hz.Such EFPI fiber acoustic sensor can be applied as high sensitive and very low frequency acoustic sensor for earthquake sound detection.