Research On Development And Applications Of Optical Fiber Cladding Mode Sensors

Recent years, fiber-optic sensing technology, developed from optical fiber communication technology, has gained extensive attention as a result of its unique advantages. With high sensitivity and sensing capacity for a lot of parameters, numerous high-order cladding mode based fiber-optic sensors have been proposed.Firstly, we briefly introduced the principles of fiber-optic sensors, and development of optical fiber refractive index sensors and humidity sensors with their advantages and limitations. After the theoretical analyses of fiber Bragg grating and cladding modes, we proposed the novel cladding modes sensing structure based on enlarged-taper integrated fiber Bragg grating.Secondly we present the fiber-optic refractometer based on this cascaded sensing structure, which excites cladding modes by the taper and demodulates refractive index by measuring the intensity of the cladding modes that were reflected by fiber Bragg grating and influenced by the surrounding refractive index. The sensor is ease of fabrication, has high sensitivity and low temperature cross sensitivity, and can be used to measure the thermo-optical coefficient of liquids.Finally a fiber optical humidity sensor based on the cascaded structure with polyvinyl alcohol coating was proposed. The polyvinyl alcohol film works as a humidity-to-refractive index transducer, whose thickness can be controlled by the simple coating process. Since it has a high humidity sensitivity, fast response, good repeatability and long term stability, and ability for simultaneous measurement of humidity and temperature, the novel humidity sensor shows a promising future in real-time practical humidity monitoring applications.