Side-polished and tilted fiber Bragg grating sensors for structural health monitoring applications

Abstract

Fiber Bragg gratings (FBG) are one of several fiber optic sensor technologies currently being used in structural health monitoring systems. When the effective refractive index of a fiber Bragg grating is changed by external environmental variations (e.g. temperature, pH), the wavelength at which incident light experiences a maximum reflection from the grating will correspondingly shift. To detect small environmental variations that occur during certain chemical processes, one can enhance the sensitivity using either side-polished or tilted fiber Bragg gratings. Enhanced sensitivity in each case is achieved by polishing the fiber on one side or writing the grating at some tilt angle. Side polished FBG sensors having a 1542 nm Bragg wavelength and cladding thickness values from 1-3 mm provide a maximum refractive index sensitivity of 7×10-4. Tilted FBG sensors having a 1566 nm Bragg wavelength and written with a 4° degree tilt angle provide a maximum refractive index sensitivity of 5×10-5. Experiments on the tilted gratings were done using 50, 80, 125 mm diameter fibers immersed in solutions in the index range 1.31-1.44. Since tilted FBGs have enhanced sensitivity and the advantage of maintaining their full mechanical strength, they show greater promise as reliable sensors for structural health monitoring applications.