Loss mechanisms in optical light pipes

A geometrical optics approach is used to develop a theoretical model for analyzing loss mechanisms in optical light pipes. Five mechanisms are identified: intrinsic absorption, bulk scattering, losses that are due to roughness at the core-cladding interface, losses that are due to large-scale defects at the core-cladding interface, and losses that are due to absorption in the cladding material; and the effects of each of these on light-pipe transmission are considered. An approximate model appropriate for slightly rough surfaces is used to estimate the loss that is due to interface roughness. Optical experiments on commercially available light pipes are done to quantify the various loss processes. These experiments indicate that the interface effects play an important role in limiting the transmission in high-quality light pipes. From the optical measurements a rms interface roughness height in the 30-70-A range is deduced, and these values are confirmed by direct surface profilometry with an atomic force microscope.