This paper discusses the possibility of measuring the refractive index of liquids from the light losses in bent cylindrical lightguides. An improved mathematical model is proposed for the propagation of light in a torus and is used for calculating how the light losses in a bent lightguide depend on its geometrical parameters and the refractive index of the liquid media in which it is immersed. According to the calculations carried out here, the light losses in lightguides of various configurations can vary within the limits of 3-60 dB as the refractive index of the liquid varies from 1.33 to 1.44.
A. G. Borisov, V. A. Gan'shin, G. V. Volkova, S. A. Krutovertsev, and D. E. Sudakov, "Refractometry of liquids using a lightguide," J. Opt. Technol. 70, 628-632 (2003)