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Modal sensitivity enhancement of few-mode fiber Bragg gratings for refractive index measurement

Few-mode fibers (FMFs) offer several distinct features from single mode fibers and multimode fibers, in terms of medium core size which is capable of accommodating several transverse modes (2–6 modes). In this work, fiber Bragg grating (FBG) has been inscribed in FMF which exhibits more than one resonant Bragg wavelengths. Few-mode fiber Bragg grating (FM-FBG) can be used as sensing device because of its modal sensitivity to the changes in an ambient environment, such as temperature and refractive index in terms of concentration of the solution. To increase its modal sensitivity for solution concentration, the cladding of FM-FBG is removed, and the Bragg wavelength difference of the modes increases during the etching process. It has been observed that by decreasing the core radius through chemical etching, the Bragg wavelength difference of respective modes increases whereas the power confinement ratio in the core for the respective modes decreases. Therefore we have optimized the core radius as at approximately 7 µm for the enhancement of refractive index modal sensitivities. The fabricated sensor has been characterized with the different concentrations of sodium chloride (NaCl(aq)) solution.