Fiber Bragg grating sensors can be used to measure the refractive index of fluids by etching the surface of a D- shaped fiber to expose the evanescent field to the superstrate layer. The cladding is modified and the modal propagation constants are changed thereby shifting the Bragg wavelength of a grating in the etched region. Experiments are reported with etched e-core D-fiber that demonstrate the effect. The Bragg lines of both the fast and slow eigenmodes are blue-shifted when the silica cladding layer is removed and replaced with water or methanol films. Changes in the fiber birefringence are also observed because the perpendicular and parallel modes decay into the cladding at different rates. By using a tunable laser, such as an ECL, with a narrow band Bragg grating filter or three-grating Fabry-Perot interferometer, it may be possible to resolve refractive index variations of 5 by 10-6. Temperature compensation methods are also discussed including the use of an isolated reference grating and the simultaneous combination of birefringence and Bragg line wavelength shift measurements.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.