Effect of small variations in the refractive index of the ambient medium on the spectrum of a bent fibre-optic Fabry — Perot interferometer

The phase of light propagating through a bent optical fibre is shown to depend on the refractive index of the medium surrounding the fibre cladding when there is resonance coupling between the guided core mode and cladding modes. This shifts the spectral maxima in the bent fibre-optic Fabry — Perot interferometer. The highest phase and spectral sensitivities achieved with this interferometer configuration are 0.71 and 0.077, respectively, and enable changes in the refractive index of the ambient medium down to 5×10^-6 to be detected. This makes the proposed approach potentially attractive for producing highly stable, precision refractive index sensors capable of solving a wide range of liquid refractometry problems.