Brillouin scattering for refractive index sensing in non-adiabatic tapers

We demonstrate the use of non-adiabatic tapers for refractive index sensing in optical fibers based on Brillouin scattering. By exciting higher order optical modes along the taper, the Brillouin gain spectrum becomes multipeaked, where each peak exhibits a different sensitivity to the refractive index of the surrounding medium. By this method, we demonstrate a sensitivity enhancement of the Brillouin frequency shift to refractive index changes by a factor of ≈ 4, compared to an adiabatic taper with the same waist diameter. Furthermore, the use of the spectral difference between two Brillouin gain peaks provides a temperature-independent measurement of the external refractive index.