Analyzing the optical modes and vortex signals in an optical fiber by means of side illumination

This article discusses a new method for instrumentally (experimentally) finding the mode-distribution parameters of an optical signal propagating along an optical fiber. The proposed method is based on illuminating the optical fiber from the side, followed by investigating the image for the linear and nonlinear (with the Kerr component) cases and then recalculating the mode parameters using the proposed technique. Unlike known methods of analyzing mode distributions that operate only with end irradiation of the optical fiber and are implemented only at definite points of a line with an intermittent signal transmitted along it, the proposed method does not assume breakage of the optical line and can be used at any point along it. The measurement layout is based on the well-known experimental apparatus, intended to obtain the refractive-index profile of an optical fiber. In addition to the known layout, it is proposed to make measurements at various angular positions of the optical fiber of interest and at various longitudinal positions along it. The proposed method can be implemented when the signal that propagates along the optical fiber is characterized by increased intensity, and this makes it possible to record the nonlinear (Kerr) component of the main refractive-index profile.