Multi-Parameter Fiber Optic Sensing Technology Based On Optical Frequency Domain Reflectometer

Optical frequency-domain reflectometry(OFDR),as a kind of optical fiber sensing technology,has excellent characteristics such as high spatial resolution and strong multiplexing capability,which has received wide attention from researchers at home and abroad,and has made great progress in the engineering field.However,there are still shortcomings in the multi-parameter measurement requirements,and the Rayleigh scattering in conventional OFDR is sensitive to only some of the parameters and has low sensitivity,which restricts its application greatly.Since the discrete optical fiber sensing unit has the advantages of high sensitivity and fast response,using OFDR combined with discrete optical fiber sensing unit is an effective method to realize multi parameter optical fiber sensing.Therefore,this paper addresses the high spatial resolution and multi-parameter measurements in OFDR,which are mainly as follows.1.We introduced the principle of optical heterodyne detection technology,and described the basic structure and process of OFDR.Then,the signal models of OFDR were built,and the expression of photocurrent corresponding to the beat frequency signal was derived and analyzed.In addition,the influence of instrument parameters on the performance of OFDR was analyzed.2.The nonlinear tuning effect of the light source in OFDR technology was studied,and its compensation method was introduced.The feasibility of the one-dimensional interpolation algorithm and the deskew filter algorithm was verified by simulation.And combined with the experiments,it is confirmed that the one-dimensional interpolation algorithm has a very high spatial resolution on the short-distance fiber link,with 0.13 mm and 0.198 mm spatial resolution obtained on 10 m and 59 m fibers,respectively.3.We proposed a quasi-distributed temperature and strain sensing method based on OFDR technology combined with cascaded fiber Bragg grating(FBG),the former for multiplexing and localization,and the latter for temperature and strain sensing.The difference in the frequency of beat signals generated by FBGs at different locations was used for localization,and the relationship between time domain and wavelength domain,frequency domain and position domain was deduced.By OFDR multiplexing three sets of cascaded FBGs,the link information of the 16 m sensing fiber was acquired experimentally.The spatial resolution was up to 50.15 mm,and the dynamic range was up to 22.68 d B.Temperatures ranged from 20 °C to 90 °C and strains ranged from 0 μεto 7000 με,with sensitivities up to 10.21 pm/°C and 1.163 pm/με for temperature and strain,respectively.And the strain and temperature values could be demodulated by the sensing matrix.It is confirmed that the method can achieve the simultaneous measurement of temperature and strain.4.We proposed a quasi-distributed temperature and relative humidity sensing method based on OFDR technology combined with cascaded Fabry-Perot(FP)cavities,in which OFDR technique is used for multiplexing and localization,and FP cavities are used for temperature and relative humidity sensing.A digital signal processing algorithm based on one-dimensional interpolation was proposed to realize that the FP sensor can be addressed in the spatial domain and its interference spectrum can be demodulated in the wavelength domain.Experimentally,three sets of FP cavities for temperature and humidity sensing were fabricated,and every two FP cascades with different structures were coupled as a group.The three sets of sensors were connected to the sensing system in parallel to ensure that the positions on the fiber optic link do not overlap.By OFDR multiplexing three sets of cascaded FP cavities,13.6 m of fiber link information was obtained with a spatial resolution of up to 41 μm.The temperature and humidity sensitivity of the FP cavity can reach up to 32.77 pm/°C(25 °C ～ 50 °C)and-0.157 nm/%RH(55 %RH ～ 85 %RH),respectively,with linearity higher than 95%.The sensing system inherits the high accuracy positioning capability of OFDR technology and can overcome the high difficulty of single-point FP multiplexing to achieve the simultaneous measurement of temperature and relative humidity.