I have been trying to use OpticStudio20 to simulate a ~5x beam expander consisting of 2 plano-convex lenses, of 125mm and 25.4mm focal lengths (THORLABS LA1986-B and LA1951-B respectively). I am attempting to find the optimal distance between the lenses for collimation, while also providing the desired magnification of the input beam.
I have a 2.6mm 1 over e^2 beam diameter incident upon the first lens, with a wavelength of 1.03 micron. I have been using a merit function (REAY Operand) to set the target output beam radius to 6.938mm, but the beam is divergent after the second lens instead of being a collimated output. It also does not expand to the correct diameter. My idea was to set the Thickness (distance) between the lenses to be variable. This idea came from the "How to Design Afocal Systems" Knowledge-Base article, albeit here they were changing the lens radii whereas mines are fixed:
I have also added an additional Operand to aim for an M^2 of 1.3 (to best simulate my laser).
Thanks for your time.
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Have a look at Keplerian beam expander at the Newport site.