Research On The Key Techniques Of The Interferometric Fiber-optic Sensing

Interferometric fiber-optic sensors have a lot of advantages of its extremely high sensitivity,ultra-large dynamic broadband frequency response range and immunity to electromagnetic disturbance.Combining the superiority of the optical fiber sensing technology,fiber-optic sensor has been widely used in petrochemical industry,structure health monitoring and biological and environment monitoring.So the interferometric fiber optic sensor technology as one of the most important sensing technology in the field of optical fiber sensing has attracted much attention.This paper mainly focused on the study of key techniques of interferometric fiber-optic sensor fabrication,response sensitivity enhancement and sensor signal demodulation.Several navel high sensitivity fiber-optic sensors are proposed and demonstrated and the sensor responses to the refractive index,temperature and gas concentration are theoretically analyzed and experimentally studied in this thesis.The main content of the research work in this thesis includes:(1)a kind of optical in-fiber Mach-Zehnder interferometer(MZI)surrounding refractive index(SRI)sensor with single mode fiber-multimode fiber-single mode fiber-single mode fiber misallied fusion spliced point-single mode fiber structure is proposed.The dip power of the transmission spectra responses to the SRI is theoretically analyzed and experimentally studied.The experimental result show that the dip at wavelength 1530 nm linearly increases with SRI in the RI range of 1.3720-1.3922.A high sensitivity of 252.06dB/RIU is achieved.(2)A fiber-optic MZI based on single-mode-multimode-thin-core-single-mode fiber structure is proposed and demonstrated for measuring refractive index and temperature simultaneously.The intermodal interference mainly occurs between the core mode LP01 and the cladding mode LP16 based on a fast Fourier transformation analysis for the transmission spectrum of the sensor.The sensitivities of the sensor response to the change of SRI and temperature are theoretically achieved.The SRI and temperature theoretical sensitivities of the interference dips at 1535 nm and 1545 nm are-55.90 nm per refractive index unit(RIU),0.0501nm/?,and-56.26nm/RIU,0.0505nm/?,respectively.The responses of the sensor are experimentally studied at the SRI range from 1.3449 to 1.3972 and temperature range from 20? to 90?,and the sensitivities-53.03nm/RIU,0.0465nm/ ? and-54.24nm/RIU,0.0542nm/ ? for the two selected interference orders are obtained.(3)The transmission spectrum of the MZI is formed by the superposition of several cladding modes separately interference with the core mode,which correspond to two frequency components in its fast Fourier transform(FFT)spectrum,respectively.The dominantly frequency components is extracted to take the inverse fast Fourier transform(IFFT),thus we can obtain two dual-mode interference(DMI)patterns between the cladding modes and the core mode for RI measuring.Through this way,we can eliminate the influence of multiple interferences and improve the accuracy of the sensor.(4)A new all fiber MZI constituted by single-mode–bitaper–multimode–single-mode–lateral offset joint –single mode fiber structures is experimentally investigated.In this paper,two DMI patterns are separated from the resultant transmission spectrum of the MZI respectively.Accordingly,it can be achieved to simultaneously measure RI and temperature changes.(5)A highly sensitive optical interferometric polar gases sensor based on graphene/microfiber hybrid waveguide has been theoretically analyzed and experimentally demonstrated.The above sensors have advtanges of easy fabrication,compact structure and high sensitivity,combining with the key techniques of response sensitivity enhancement and sensor signal demodulation,which can further improve the measurement accuracy and sensitivity of the sensors,and there is some practical value.