Formation of angular power profile via ballistic light transport in multimode optical fibers with corrugated surfaces

We present experimental results on the light transmission and intermode power exchange in multimode fibers with rough surfaces. The experiments were performed with chemically etched $200\text{−}\mu \mathrm{m}\text{−}\text{diameter}$ cores of fused silica fibers that can support up to $700\times 700$ guided modes at the wavelength employed, $\lambda \approx 0.6\mu \mathrm{m}$. After propagation through some rough fiber section the mode power profile acquires a Gaussian shape that propagates along the mean axis of the corrugated fiber, with a slowly reducing radius $\left(\propto 1∕\sqrt{z}\right)$, and practically without attenuation of the central intensity. These features, and other observed characteristics, are reproduced with a theory that assumes a ballistic regime of light propagation through a one-dimensional slab waveguide, relating the characteristic propagation lengths to parameters of the fiber surface roughness.