Highly sensitive cladding-etched arc-induced long-period fiber gratings for refractive index sensing

We have investigated the effects of cladding etching in arc-induced long-period fiber gratings. Comparing the position of the resonant wavelengths at each diameter with the corresponding theoretical dispersion curves of effective refractive index, it is found that some of the resonances show a better match with anti-symmetrical cladding modes, while others with symmetrical cladding modes. Etching the cladding diameter to ∼39 μm, results in coupling to a high-order cladding mode that is very sensitive to changes in the ambient refractive index. The resonant peak shifts remarkably (∼230 nm) toward shorter wavelengths as the external refractive index increases. This sensitivity is higher than any previously reported in arc-induced long-period fiber gratings.