Research On Fiber Optical Evanescent Wave Spectrum Sensing Systems And Applications

Traditional direct light absorption sensor has been widely used and played asignificant role in the field of biochemical analysis. But this type of sensors has thedisadvantage of complex structure, large volume, low level of miniaturization anddifficult to realize on-line measurement which resulted in some limitation in itsapplications. In recent years, with the rapid development of optical fiber sensingtechnology, fiber optic evanescent wave sensor as one of the hot research attractedwidespread attention among scholars all over the world. Fiber optic evanescent waveabsorption sensor show a good prospect in the field of biomedical, biochemical processcontrol and environment detection as its advantage of less sample required, insensitivityto scattering and nondestructive testing, etc. It becomes an important developmentdirection of fiber optic evanescent wave sensors.Fiber optic evanescent wave absorption sensor is a new type of sensor which obtainsthe biochemical information of samples by detecting the change of the transmissionpower caused by the evanescent wave absorption in the cladding of the fiber. Currently,study of fiber evanescent wave absorption sensor focus on sensor structure designing,sensing mechanism and signal detection methods, etc. It has been carrying out someexperiments and theoretical explorations. In this paper, the ratio of cladding of the sensorwhich influences the evanescent wave absorption coefficient was discussed based onoptical waveguide theory and various factors which affect the sensitivity were analyzed.Based on the theoretical analyses, two kinds of fiber evanescent wave spectrum sensingsystem were designed and built. The main works of this paper include the followingaspects:(1) Theoretical analysis of fiber optical evanescent wave absorption sensors:Evanescent wave formation principle and its basic characteristics in planar waveguidestructure and fiber optical waveguide structure were studied. The energy transfercharacteristics of fiber optical evanescent wave absorption sensors while working wasanalyzed. The expressions of evanescent wave absorption coefficient of the sensor in thecase of weakly guided and strongly guided mode were derived. The cladding power ratiorelated with the coefficient was analyzed based on electromagnetic theory. According tothe expressions of evanescent wave absorption coefficient in weakly and strongly guidedmode, combined with the energy transfer characteristics, the expressions of sensitivity were obtained.(2) The numerical simulation of the cladding power ratio which has effect onevanescent wave absorption coefficient: In case of weakly guided mode, the relation ofcladding power ratio of LP01mode with fiber diameter, cladding refractive index wassimulated by numerical method. The relation of cladding power ratio of multimode fiberwith fiber diameter, cladding refractive index has also been simulated by numericalmethod. The results show that the cladding power ratio is proportional to the length of thefiber and decreased as the fiber diameter increased.(3) Numerical simulation analysis of sensor sensitivity: The variety of the sensitivityof the sensor with sensing fiber cladding power ratio, radius, length, refractive index ofthe external medium were simulated by numerical methods. The results show that thesensitivity of the sensor is proportional to the sensing fiber length, cladding power ratioand the refractive index of the external medium, while is inversely proportional to thefiber radius.(4) The fabrication of sensor, the design and construction of fiber optic evanescentwave spectrum sensing system: Transmission type fiber evanescent wave absorptionsensor was fabricated by removing a certain region of the cladding using hydrofluoricacid, the relation between fiber diameter and etching time was obtained by experimentalstudy of fiber etching. The corresponding sensing system was designed and built for thesensing structure. Parts of the parameters which have effect on sensitivity wereexperimentally verified. Reflection type fiber optical evanescent wave absorption sensorwas proposed by improving the transmission type sensor and the corresponding sensingsystem was designed and built. The response characteristic of the designed sensor tochanges in KMnO4concentrations was investigated experimentally. The experimentalresults showed that: when the lengths of fiber evanescent wave absorption sensors are thesame, the sensitivity of reflection type is two times that of the transmission type, whichindicate the correctness of the theoretical analysis. On the other hand, when thesensitivity requirements are the same, the required length of reflection type sensor is halfof the length that the transmission type sensor required, and the sensing approach ismore conducive to miniaturization.