Ring-shaped field pattern: The fundamental mode of a multimode optical fiber

The observation of a ring-shaped single-mode field pattern at Stokes wavele ngths of backward stimulated Raman scattering in a multinode fiber is repor ted for the first time. This phenomenon and some other novel experimental a nd numerical results are presented to explain the mechanism. It has been pr o red that self-focusing pla rs little role in the formation of the ring-sh aped structure. The ring-shaped pattern is attributed to the refractive ind ex profile of the fiber used in the experiment. It corresponds to the funda mental mode of the fiber and mode competition causes the Stokes waves to pr opagate in the fundamental mode.