The Sensing Systems Based on Etched Chirped Fiber Bragg Gratings

• Main Authors: CHANG, HUNG-YING, 張弘穎
• Other Authors: LIU, WEN-FUNG
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/97x2wb

博士 === 逢甲大學 === 電機與通訊工程博士學位學程 === 107 === This thesis proposes a novel fiber grating liquid-level sensor and two different design tilt sensors based on etched chirped fiber Bragg gratings (CFBGs). The liquid-level detector can be used to measure tens of micro-meter liquid-level variation. Due to the characteristics of the fiber optic sensor itself, this sensor may be used to precisely measure the liquid-level variation in bio-hazardous systems or industrial containers which demand accurate interrogation of the amounts of contents. As for the study of the tilt sensor, the two kinds of fiber grating tilt sensors include the cylindrical and disc type, respectively. The sensing mechanism of the tilt sensor is designed based on the chirped fiber Bragg grating liquid-level sensor proposed by our research, and can be used to simultaneously measure the tilt angle and the tilt direction without being affected by the temperature. The main sensing optical components of sensors are periodic gradient fiber gratings fabricated by using a periodic gradient phase mask with a KrF excimer laser in general communication single mode fibers (SMF-28). The fiber grating is etched with hydrofluoric acid to reduce its diameter for increasing its sensitivity of sensing the refractive index changes of the surrounding environment. The main sensing mechanism is that the etched chirped fiber Bragg grating is immersed in two media with different refractive indices, and the two media will affect the effective refractive index of the fiber grating to form different two reflection wavelengths. The phenomenon of the two reflected lights to be superimposed is observed from the optical spectrum analyzer. The interface position of the two different refractive index liquids on the grating can be obtained by the wavelength corresponding to the energy superposition peak. The fiber grating liquid-level sensor is designed by fixing an etched chirped fiber Bragg grating in parallel with the inner wall of the measuring barrel. The experimental results are demonstrated with a sensitivity of 1.214 nm/ mm for detecting tens of micro-meter liquid-level variation. In the research of a cylindrical tilt sensor, the sensing head is composed of two etched CFBGs which are fixed with 90 degree angular difference in a cylindrical container. The calcium chloride liquid is injected into the container until half-length of the grating to be sunk in the liquid. When the sensor is tilted, the length of the two gratings immersed in the liquid changes accordingly. The phenomenon will result in the superposition peak to be shifted toward the longer wavelength side. The superimposed peak wavelength can be calculated by the formula to obtain the tilt angle and the tilt direction with the sensitivity of the tilt angle of 0.303 nm/degree, corresponding to the resolution of 0.033 degree. The disc type tilt sensor can be made smaller than the cylindrical sensor due to its designed structure. The sensing head is to divide the four etched CFBGs into two pairs, and the two pairs of gratings are orthogonally packaged on the top of a disc container and filled the liquid into the container to produce an air bubble. When the sensor is tilted, due to the movement of the air bubble, the liquid-air boundaries move to result the wavelength variation of the superposition peaks. The inclinometer has a resolution of 0.003 degree in tilt angle measurement and 0.00187 rad in the tilt direction measurement. The three sensors proposed by this research can measure the liquid-level, tilt angle and tilt direction by calculating the observed superposition peak wavelength in the time, for applying in monitor both the slope of the hillside and the bridge to provide an instant change information.