Simple method for determining the parameters of singlemode fiber-optic waveguides

An experimental investigation was made of the possibility of using methods of measuring the near- and far-field distributions to determine the core diameter and the difference between the refractive indices of the core and cladding in single-mode fiber-optic waveguides for the near infrared region, by exciting them in a few-mode regime with ultraviolet (λ = 365 nm) radiation. It was found that even for a normalized frequency of V > 4 the output beam divergence corresponded to the numerical aperture of a single-mode optical waveguide, while the half-width of the radiation intensity distribution at the output end corresponded to the core diameter. The accuracy of the proposed method was not inferior to other methods specifically designed for single-mode optical waveguides, but the present method had the advantage of simplicity, especially in the case of a single-mode optical waveguide having a critical wavelength of > 1 μ.