Evanescent field fiber Bragg grating sensors for index of refraction sensing with applications to structural health monitoring

Abstract

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.