Ring-Shaped Field Pattern: The Fundamental Mode of a Multimode Optical Fiber

The observation of a ring-shaped single-mode field pattern at Stokes wavelengths of backward stimulated Raman scattering in a multimode fiber is reported for the first time. This phenomenon and some other novel experimental and numerical results are presented to explain the mechanism. It has been proved that self-focusing plays little role in the formation of the ring-shaped structure. The ring-shaped pattern is attributed to the refractive index profile of the fiber used in the experiment. It corresponds to the fundamental mode of the fiber and mode competition causes the Stokes waves to propagate in the fundamental mode.