At high optical power densities, materials that would normally be classed transparent, break down. The mechanisms by which high purity fused silica optical fibers fail are discussed in this paper. Multimode fibers with a core diameter of 400 micrometers have been tested with a Nd:YAG laser with a view to transmitting the maximum amount of energy. The importance of surface finish has been verified by implementing polishing schedules of varying degrees. The front face of many of the fibers would be improved during laser testing, due to plasma formation which acts to anneal the surface. It has been found that the energy level at which this effect first occurs gives a good indication of the initial surface roughness. Atomic force microscopy has been used to confirm surface roughness measurements as low as 3 nm and excellent agreement between high power transmittance and surface quality has been found.