An express method for measuring the refractive index, which is one of the main optical parameters of transparent fibres, is suggested. The method uses focusing properties of a cylindrical lens, which such a fibre is. The possibility to accurately measure such characteristics of optical fibres as the shell and core diameters, numerical aperture, refractive index profile, loss, and dispersion is equally important for fibre manufacturers and designers of optical communication systems who should choose the fibre that meets their requirements best. Almost all measurement methods use the refraction of light rays at the interface between the media. To do this, one should make samples of given shape and size, which are individual for each measuring instrument. The suggested method takes into account the fact that when light strikes upon a refractive cylinder (glass rod, fibreglass), the focusing occurs perpendicular to its axis with a focal region where light rays converge. Behind this region, the rays diverge again. The position of the focal region is determined by the refractive index of the cylinder. It can be inside the cylinder, outside it, or on the surface of the cylinder. During the observation of the fibre using a microscope, one can see that the light, which has passed through the fibre, forms a bright band on its backside against a dark background. The bandwidth depends on the refractive index of the fibre. The calculations using the methods of geometric optics were carried out. These methods may be applied over a wide range of fibre diameters. Using strict formulas of diffraction theory, the distribution of radiation energy in the fibre and its vicinity was calculated. A digital analysis of the resulting pattern was carried out. The results of the analysis coincided with the results obtained using the methods of geometric optics. An algorithm for determining the refractive index was worked out. The measurements of the refractive indices of artificial and natural fibres like fibreglass, webs and human hair (blonde-haired person, brown-haired person, grey hair) were provided.
