On Micro Optical Elements for Efficient Light Diffusion

Efficient light management is one of the key issues in modern energy conversion systems, be it to collect optical power or to redistribute light generated by high power light emitters. This thesis touches mainly on the subject of efficient light redistribution for high power sources by means of refractive and reflective micro optical elements. Refractive micro optical elements have dimensions that are big enough to neglect diffraction phenomena and small enough to be still manufactured by the methods used in micro fabrication, typically above 50 micron for visible light but below or of the order of one millimeter. The advantage of this limitation that could be called the “refraction limit” is that the design and performance predictions can be based on simple methods such as ray tracing or the edge ray principles for non-imaging optics. In contrary to many studies on engineered diffusers we concentrate here on optical surface where the functional is given by concave shapes! The first part of the thesis treats the development and fabrication of one dimensional small angle diffusers for collimated high power and potentially coherent light sources. The generation of high power laser lines with a uniform intensity distribution is useful for the optimization of laser manufacturing applications such as annealing of amorphous silicon on large surfaces. This is typically needed for the fabrication of TFT’s or thin film solar cells. The one dimensional diffusers discussed in this thesis are based on an array of concave cylindrical microlenses with a typical lens width of 200 μm and a radius of curvature ranging from 300 μm to 1500 μm. In order to avoid diffraction grating effects due to the regular nature of the array a statistical variation of the lens width was introduced. The proposed fabrication process is based on isotropic etching of fused silica in hydrofluoric acid. The fabrication and design parameters were explored and their influence on the final performance determined. Extensive computer simulations based on ray tracing and diffractive beam propagation were compared with the measured performance of fabricated devices. Design rules based on an analytical model were also developed and verified. The performance under real world conditions were tested with good results for the smoothing of laser lines at the Bayrisches Laserzentrum in Erlangen, Germany. The subject of the second part are compact large angle transformers and their possible applications. A short introduction to non-imaging optics and its basic design tools are followed by development of the compound parabolic concentrator (CPC) based on work known for thermal solar concentration. This non-imaging light funnel is concentrating light and has the ability to efficiently transform the angle of an incoming bundle of rays into large angles up to the full half sphere. If inversed, the CPC works as a collimator. The novelty of the approach presented in this thesis lies in the reduced dimensions of the design and the use of the concentrator not as such but rather as an angle transformer with very high efficiency. When the dimensions of the classical solar concentrators are usually of the order of a few 10 cm or more the design developed in this thesis has dimensions of a few mm or less. Different possible applications for a compact CPC array are discussed such as LED collimation at chip level, fiber coupling with large numerical aperture and improved light management for thin film solar cells. The fabrication of a prototype of a compact dielectric filled CPC array as a proof of concept is described and first attempts at its characterization are discussed.