Rectangular pellicle beam splitter design
Fraser, Jacob Lee
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This project investigates a pellicle beam splitter of rectangular form. A pellicle is a thin optical membrane and a beam splitter separates one optical path into two. The beam splitter under investigation is used at Arnold Engineering Development Center (AEDC) for optical component testing and mission simulation in cryogenic vacuum chambers. The conventional beam splitter had undesirable optical performance at cryogenic temperatures. The goal of this project was to use analysis to guide development of pellicle beam splitter prototypes to minimize thermal distortion. Engineers commonly use finite element analysis (FEA) to model structural performance. FEA models were created that represented various prototypes including dimensions, materials, and loads. The models had a rectangular pellicle with a fixed outer boundary. An inner ring of rectangular form was then pressed against the pellicle to create an optical aperture inside the inner ring. Then geometric parameters were varied one at a time to evaluate their effects. The parameters included inner ring corner radii, outer boundary corner radii, inner ring to outer boundary spacing, pellicle thickness, and inner ring displacement. The FEA models indicated that the rectangular form of the pellicle beam splitter contributed to undesirable optical characteristics. Sharp corners of the inner ring create high stress concentration areas causing varying pellicle thickness and undesirable optical quality. Modeling showed that increasing inner ring corner radii and inner ring to outer boundary spacing are affective at increasing optical quality. Other parameters had little to negative affects on optical quality. The rectangular shape of the pellicle beam splitter leads to a non-uniform stress distribution and therefore undesirable optical quality. Stress non-uniformity can be reduced but not eliminated by using large inner ring corner radii and large inner ring to outer boundary spacing. A prototype with large inner ring corner radii was made and tested. The prototype had much better optical performance with little corner affects as compared to previous prototypes.