Single-mode fibre with an additional ring fibre for two-channel communication and special applications

Two-channel concentric fibres with a broadband single-mode optical fibre operating at 1.3 μm and an additional multimode ring fibre of thickness 10 μm with an outer fibre diameter of 125 μm are developed for the first time for two-channel communication and special applications. Optical attenuation in the single-mode fibre and in the ring core was less than 1 dB km^-1, while the crosstalk attenuation between the channels was more than 40 dB. It is found that optical attenuation in the ring fibre depends on the radius of the bend and on the tensile force applied during winding of the fibre into coils. Optimal parameters of the refractive index profile are determined, in particular, the value of Δn for the ring for which the losses in the ring fibre on the coils as well as in the straightened state are minimal. A 12-km long fibre line is constructed and the distribution of attenuation over its length is analysed. It is shown that distributed coupling between the channels in such fibres is virtually absent and a high optical isolation between the channels is preserved.