Theoretical Simulation And Sensing Characteristics Of The Local Micro-structure Of Long Period Fiber Grating

Fiber grating, as a new type of fiber-optic passive components, has gained widelyattention since it has appeared. It has a lot of advantages. With the development of scienceand technology, a variety of special gratings have emerged, including the localmicro-structured fiber grating, which has special filter and sensor characteristics. The localmicro-structured fiber grating is made by the fiber Bragg grating (FBG) or the long periodfiber grating (LPFG). The clad are partly etched by the corrosive liquid or the femtosecondlaser. So some defects are produced, which forms some reflection or transmission windows inthe original spectrum. Meanwhile some sensing characteristics are changed. So the localmicro-structured fiber grating has a unique advantage in sensing and communication field.The local micro-structured long period fiber grating (LMSLPFG) is analyzed in thethesis. On the basis of our group’s previous research work, the theoretical model ofLMSLPFG is improved. We study the effects of the defect’s parameters on the transmissionspectra and expand its sensing application field. The details are as follows:1. Brief introductions to the fiber grating and special fiber grating are depicted. Thehistory and research status of the local micro-structured fiber grating is presented，and also theadvantages of the local micro-structured fiber grating in sensing area are introduced.2. By means of the coupled-mode theory, the transfer matrix method and the phase shifttheory，the spectrum characteristics of LMSLPFG is analyzed, and the effect of cladding’sradius on the LPFG is studied. Then we simulate LMSLPFG’s spectrum by MATLAB. On thebasis, we study the effects of the defect’s parameters on the transmission spectra in detail.3. Two methods of fabricating the LMSLPFG are introduced in detail. Two LMSLPFGsare made by using the etching technique. The LMSLPFG’s sensing characteristics areanalyzed. We use the1stLMSLPFG to measure the temperature and the refractive index anduse the2ndLMSLPFG to measure the temperature and curvature. There are two stopbands andone passband in LMSLPFG’s spectra. The temperature-sensitive coefficients of the twogratings’ stopbands and passband are0.05nm/oC, approximately. The peaks of the stopbandsand the passband have good linear relationships with temperature. The surrounding refractiveindex (SRI) sensitive coefficients of the1stLMSLPFG’s stopbands are-35.62nm/RIU,approximately, which is smaller than the passband’s,-61.56nm/RIU. The curvature sensitivecoefficients of the2ndLMSLPFG’s stopbands are both about-4.71nm/m-1, and the passband’scoefficient is about-2.61nm/m-1. Combined with sensitive matrix, simultaneous measurementof double parameters using a single grating is achieved.