Study On Optical Fiber Sensors Based On The Fiber Structures With Mode-Field Abrupt Change

Recently, optical fiber sensors have attracted a lot of interest in the sensing fieldbecause of their advantages of high resolution, high sensitivity, lightweight, fastresponse, good stability, compact, low cost, immunity to electromagnetic interferenceand so on. So far, optical fiber sensors have been widely used in various physical,chemical, and biological sensing applications, such as temperature, refractive index,strain, vibration, acceleration, gas. Fiber optical interferometer is one of the mostimportant classes of optical fiber sensors, mainly including Michelson interferometer,Mach-Zehnder interferometer, Sagnac interferometer and Fabry-Perot interferometer. Inthis thesis, we proposed a new structure of mode field abrupt change (peanut-shape),which can realize modes exciting and re-couping between the fundmental core modeand the symmetrical cladding modes. Based on the the new structure, the reported taperand offset structures, several in-line fiber-optic Mach-Zehnder interferometers and anin-line fiber-optic Michelson interferometer were fabricated. Also the sensingcharacteristics of the interferometers were studied. The main works are as followed:①A peanut-shape fiber structure which is a new coupling method was proposed inthis paper. The production process and various optical properties of the peanut-shapestructure were discussed. Based on beam propragation methods (BPM), wedemonstrated that the peanut-shape structure can couple part energy of the core mode(LP01) into the cladding modes (LP0m) and re-couple the cladding modes (LP0m) into thecore mode.②Three kinds of in-line fiber-optic Mach-Zehnder interferometers based on threefiber tapers, two peanut-shape structures, and peanut-shape and offset structures werefabricated, respectly. The experimental results show that the refractive index sensitivityof the interferometer with interference length L=60mm based on three fiber tapers is28.6nm/RI, which is about four times higher than that of the normal two tapersMach-Zehnder interferometer. When the interference length L=22mm, theinterferometer based on two peanut-shape structures has the temperature sensitivity of46.8pm/℃and the strain sensitivity of1.4pm/με, the maximum strain is3670με.Meanwhile, We also measured the temperature sensitivity of the interferometer withinterference length L=21mm based on peanut-shape and offset structures is61.9pm/℃.③A in-line fiber-optic Michelson interferometer based on a peanut-shape structure was fabricated in this paper. When the interference length L=21mm, the device can beheated up to900℃and has a temperature sensitivity of96.3pm/°C.Conclusionly, this paper proposed a new coupling method to further improve thecoupling theory, which has an important academically significance. Meanwhile, itpresent a novel approach for the design of key components in fiber-optic sensing system.So, it is important for the economic fields and socity.