| Fiber Bragg gratings are one of several fiber optic sensor technologies currently being used in structural health monitoring systems. When the effective index of a fiber Bragg grating is changed by external environmental variations (such as temperature, acidity, etc), the wavelength at which the 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 grating sensors having a 1542 nm Bragg wavelength and cladding thickness values from 1--3 mum provide a maximum refractive index sensitivity of 7x10-4. Tilted grating sensors having a 1566 nm Bragg wavelength and written at 4° tilt angle provide a maximum refractive index sensitivity of 5x10-5. Experiments on the side-polished and tilted gratings were done by immersing the fiber sensors in solutions with the index range 1.31-1.44. Since tilted gratings have enhanced sensitivity and the advantage of maintaining their full mechanical strength, they show greater promise as reliable sensors for structural health monitoring applications. |
