OPTICAL CHARACTERIZATION OF LIGHT PIPES: MEASUREMENT, FABRICATION AND MODELING

Abstract

Light pipes for optical transmission and distribution have been frequently employed in uses such as solar concentrator designs, and one overarching goal is to predict loss mechanisms and minimize these losses. The aim of this work is to detail the fabrication and measurement of fused silica light pipes, and to provide a method of modeling the optical losses based on input characteristics and geometric considerations of waveguide surface features such as roughness. A laser polishing procedure is outlined, and various statistical quantities for rough surfaces, namely root-mean-square height and slope, are described and estimated. The optical transmission of light pipes of different lengths are measured for different wavelengths and incident angles. A geometric optics method utilizing Fresnel’s equations of reflection and transmission is developed to model the optical transmission of light pipes as a function of incident angle and surface roughness statistics. Other possible loss mechanisms are also analyzed. It was shown via simulation that root-mean-square slope plays a significant role in determining loss. Based on surface profile measurements of a test sample, we estimate a benchmark root-mean-square slope of 0.015 or lower. For the measured transmission for various samples, we observe a relatively linear loss trend for different incident angles into the light pipe that Fresnel’s equations alone cannot account for. We estimate a constant sidewall loss of approximately 0.4% to 0.5% being common in our light pipe samples from an imperfect fabrication process and, to a lesser extent, constant reflectivity loss from light pipe support mounts to account for this observation.