Light pipes for CPV: Simulation and measurement of sidewall loss mechanisms

A geometric optics approximation (GOA) is used to model scattering losses for light pipes due to sidewall roughness that uses root-mean-square (RMS) slope to parameterize profilometer-measured sidewall roughness. We compare simulated loss versus incidence angle to measured losses in the 500nm to 700nm wavelength range. Silica light pipes were fabricated up to 70mm in length using femtosecond laser irradiation followed by selective etching and CO2 laser polishing. Based on surface profilometer measurements, we estimate a root-mean-square slope of 0.015, corresponding to an average local angle deviation of 0.86 degrees. In addition to surface scattering loss, an excess surface loss was introduced. Fitting our simulations to measured data provided an estimate for reflectivity of 99.5% for our fabricated light pipes.