Resolution limits of core-cladding intermodal fiber optic refractive index sensors with spectral interferometric interrogation

In this paper, we have considered the refractive index measurement resolution limits of a core-cladding intermodal fiber optic interferometer based on SMF-28 optical fiber, with spectral interrogation in the C-band. The resolution was calculated using the Cramer-Rao bound. To achieve this, we found the sensitivities of the cladding mode propagation constants to the refractive index of the surrounding medium, as well as the cladding mode attenuation coefficients, through full-vector numerical simulation. These calculations were performed for two fiber configurations: with and without a metal coating on the cladding. Our numerical results demonstrate that the refractive index measurement resolution improves when higher-order cladding modes are used. Also we demonstrated that the use of additional thin metal coating of the cladding and plasmon modes does not enhance the interferometric sensor's resolution in this case because their huge losses.