INVESTIGATIONS ON FIBER GRATING SENSORS FOR STRUCTURAL HEALTH MONITORING

Abstract

In the last few years, fiber grating sensors made a slow but significant entrance in the field of sensors. Conventional sensors based on mechanical or electrical transducer suffer from their large size, weathering effect, corrosion etc. Therefore, fiber grating sensors are commonly used as an alternative to conventional sensors. This thesis will demonstrate the effect of change in fiber parameters (as like refractive index and diameter of core and cladding) due to applied strain on strain sensitivity of FBG based on SMF-28. Typically, strain sensitivity of FBG in single mode fiber is 1.2 pm4iE. But, if these changes in fiber parameters are taken in account, its sensitivity is increased by 18 percent of the typical value and is 1.4 pm/lIE. This thesis will also demonstrate the investigation of fiber grating sensors for strain sensitivity. A strain sensor based on Long Period Grating (LPG) is inscribed in Polymethylmethacrylate (PMMA) photonic crystal fiber (PCF) and silica photonic crystal fiber. The symmetric LPG is designed to realize the coupling between LP01 and LP06 mode of PMMA PCF and asymmetric LPG is designed to realize the coupling between LP01 and LP11 mode of silica PCF. Asymmetric LPGs are constructed only in upper half part of the fiber core. Air holes are arranged in a fixed pattern up to a fixed distance in cladding of PCF. Variation of grating period depends upon the air holes radii and also varies with wavelength. In PMMA, losses are much higher at longer wavelength. But in case of silica, there is no appreciable loss at longer wavelengths. At 1 550nm, strain sensitivity of LPG based on silica PCF with air holes radii 0.543 urn increases 75 percent of the typical value of sensitivity of FBG in single mode fiber. We also demonstrate the special properties of LPGs in PCF by using dispersion factor Y. The dispersion factor gives a better understanding about the real behaviour of LPGs in silica PCF. Then we calculate the strain sensitivity by varying the strain-optic coefficient of cladding (iij) for I =-2.229 and it is observed that sensitivity increases up to 9.12 pm/lIE for ij,j = -0.224 in negative direction of wavelength. We have done the same work for 1 =1 i.e. no dispersion effect and we observed that shift in resonance wavelength is in positive direction of wavelength.