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 &mgr;m 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 &mgr;m 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.
Fiber Bragg grating sensing is a relatively mature fiber optic sensor technology currently being used in structural health monitoring systems. Therefore, there are significant benefits to using this technology as a platform for other sensing modalities. In this work, a side polished fiber Bragg sensor is described for sensing refractive index changes. The effective refractive index of a fiber Bragg grating is a function of the refractive index of the media surrounding it, and its sensitivity may be optimized with appropriate design. As the external refractive index changes, the wavelength at which incident light experiences a maximum reflection from the grating will shift. The sensitivity of a fiber Bragg grating to external refractive index changes increases when the grating is polished on one side. This side-polishing technique enables the Bragg grating to preserve a greater portion of its mechanical strength compared with other techniques such as chemical etching. This work utilizes side-polished fiber Bragg grating sensors centered at a 1542.9 nm wavelength with cladding thickness values of approximately 1-2 μm. The response of these sensors to small refractive index changes was studied. Previous work on fiber Bragg grating sensors has shown that the peak wavelengths can be measured with 3 pm repeatability. With this repeatability, this study demonstrated that a 0.001 refractive index change can be observed. By using materials that change index with moisture or pH, this technique can be used to construct both pH and moisture sensors.