Intermode power transfer in a multimode optical fiber with a rough surface

We present an experimental and theoretical study of light scattering and propagation in multimode optical fibers with rough surfaces. In the experiments, performed at lambda = 655 nm, we used multimode 200 mum diameter silica glass fibers with an etched rough section. As the guided light reaches the rough part of the fiber it is scattered, feeding, other modes and leaking out into the surrounding space. After some distance, however, the leakage decreases and the light within the fiber is carried primarily by the modes with low transverse wavenumbers. The light then propagates with a relatively narrow (half-width of about 0.15 rad) and slowly reducing angular spread. To understand this behaviour, we develop a diffusion model for the intermode power transfer. The model predicts the formation of a narrow central maximum with a stable propagating angular profile, and relates parameters of the fiber and the surface roughness to the characteristic decay lengths.