Research On Microbending Optical Fiber Sensors And Reflective Intensity-Modulated Optical Fiber Sensors

Modern optical fiber sensors owe their rapid development to the achievements of technologiesin fiber fabrication and communication. They have many advantages over traditional sensors,such as wide application extent, high sensitivity, insensitivity to influence of electromagneticfield, good solution, small volume, low price, etc. Loss theories of bending optical fiber,microbending optical fiber strain sensors, differential microbending optical fiber sensors,linearity compensation of reflective type fiber-optic displacement sensor and a pressure fibersensor are researched in this dissertation.The main contents and creative works of this dissertation are as following:1. Microbend-induced loss in optical fiber has proved itself useful as a transactionmechanism in optical fiber intensity sensors for detecting environmental changes.Microbending optical fiber sensor has been already utilized for detecting force,pressure, displacement, strain, acoustic wave, temperature, water, etc. A fibermicrobend sensor, however, despite its appearance, has a very complicateddepending on the deformation of fiber and fiber parameters. In general, sensingperformance principle of microbend sensors almost are analyzed by means ofmode theories, seldom treated by ray theories. In this dissertation, a novel rayanalyzing method is established. The description of the operating principles ishow to get the ray propagating traces of rays in a microbending optical fiber.Refractive index profile of fiber changed due to fiber bending is different fromthat of straight fiber. A function of refractive index profile is presented firstlyaccording to my knowledge. From the differential equation that applies to raypropagation in an optical inhomogeneous medium, the traces of rays in a bendingoptical fiber were worked out by running Matlab6.5 software in computer. Theresults can triumphantly explain the loss phenomenons in bending optical fibers.2. A differential microbending optical fiber sensor is invented in this dissertation(Patent applying number:200510035613.X). In the configuration of the sensor,two initially straight sensing fibers are sandwiched between two pair of corrugatedplates. The losses in one fiber increase, and decrease in another fiber in all themeasuring time. This sensor not only can be used to detecting the displacement,force, strain, but also can be used to distinguish the directions of the parameters.3. The sensitivities of microbending optical fiber displacement sensors influenced by theparameters of single mode fiber, multimode fibers, and the microbend periods ofdeformers were studied in details.4. Three kinds of microbending fiber sensors for strain monitoring are designed andmeasured in this paper. The key structure of the sensors is made up of an arch elasticdiaphragm and a couple of gears. The sensitivities of the sensors are demonstrated, and thefeatures of the three sensors are compared.5. The author constructed and tested optical time-domain reflectometry-based microbendingoptical fiber strain gauges and microbending optical fiber displacement sensors by usingsingle-mode fibers.6. A random type and a semicircle type reflective optical fiber displacement sensor were usedto compensate the linearity of optical fiber displacement sensor. Output signals of arandom type and a semicircle type reflective optical fiber displacement sensor areprocessed by a circuit in two method, and the range of linear measurement of displacementis simultaneously enlarged. The results are satisfying.7. A novel fiber sensor used for gas or fluid body pressure measurement is designed in thisdissertation. An elastic spherical diaphragm is used as a reflective target. The diaphragmcoils when force is performed upon. Light intensity of receiving fiber changes dependingon the deformation of the elastic spherical diaphragm. The relationship betweensensitivities of the sensor and diameters of the elastic diaphragm, diameters of fibers andthe distance from the terminal of fiber to the initial diaphragm before force performedupon are calculated based on light distributing theory. From the consequence ofcalculations, we give out the optimum condition for this kind fiber sensor design.