Mm Scale 3d Silica Waveguide Fabrication Technique for Solar Energy Concentration Systems

Abstract

The overreaching goal of this dissertation research is to achieve fabrication of mm scale waveguide structure for solar energy concentration systems. In the proposed design, a high concentrator photovoltaics (HCPV) with 1000x concentration and >90 % optical efficiency is targeted. The concept consists of three components: lens array, coupler and waveguiding section. Fused silica is assigned as the waveguide material, since it has a high optical transmission and low absorption and it provides the scalability and low manufacturing cost sought in the fabrication technique. To acquire the desired shape in waveguide, femtosecond laser irradiation followed by chemical etching (FLICE) process is used for fused silica light pipes fabrication. Among two widely used etchants potassium hydroxide (KOH) is preferred over hydrogen fluoride (HF) regarding its higher selectivity. FLICE process parameters have been optimized to achieve higher selectivity, higher manufacturing speed and better surface quality. The minimum number of overlapped pulses is reduced to 3.2 which corresponds to 1.25 m/s writing speed at given 2 MHz laser pulse repetition rate at given 2 μm spot size and an acceptable filtered surface roughness of 400 nm for 1 mm^2 area is achieved. The achieved minimum filtered surface roughness is scaled down to 21.8 nm for given 1 mm^2 area. Up to 1X5 staggered and tapered light pipes with up to 12.75x geometric concentration factor with a 45° angled input facet is successfully fabricated. The achieved accuracy of the angled surfaces is smaller than ±0.5° and ±0.01° for 45° and side wall tapered surfaces, respectively. Having evaluated the polishing techniques, CO2 laser polishing is decided to be employed in this study to obtain a smooth surface finish. Surface profiles are measured by atomic force microscopy (AFM) for generally high spatial frequency analysis and white light interferometry (WLI) for low spatial frequency analysis. Measurements demonstrate that the surface root mean square (RMS) roughness is decreased almost two order of magnitude. 95% transmission efficiency is measured for waveguide samples up to 50 mm in length and 1 mm^2 cross sectional area when the Fresnel losses are ignored and incident angles (in air) are averaged according to F/1.5 lens. Complex shapes in waveguides such as angled facets, tapering of the cross-section along the length, and combiners are proven to be possible to fabricate with high precision.