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I am now designing an experiment that consists of a 400 nm short pulse (~300 fs). In the experiment, we need to magnify the transverse profile of the pulse by 100x. That's the problem.

I was considering using microscope objective lens, but it turned out that it has a lot of lenses and could stretch the pulse a lot (we could tolerate stretching by up to ~7 mm glass, but not more). Another thing that I considered was using spherical mirror for the magnification system, but I had a bad experience using spherical mirror (the astigmatism could spoil the result really much). Now the candidates that I have in mind are using (1) off-axis parabolic (OAP) mirrors and (2) toroidal mirrors. I know both of them suffers from comatic aberration, so my questions are:

  1. Is comatic aberration usually a bad thing or does it usually spoil experimental data?
  2. Are there any difficulties of using OAPs and/or toroidal mirrors?

Any suggestions and comments are welcome.

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One difficulty with OAPs and toroidal mirrors is that they are significantly harder to align than spherical mirrors, and they're also more sensitive to beam pointing. Without knowing more about your experiment I can't really comment on which trade-offs might work best for your specific application.

I don't have much experience working with 400nm light, but maybe there are Fresnel lenses available that would allow you to minimise the thickness of glass whilst achieving acceptable beam quality after the lenses.

Another option would be to add a prism compressor (or chirped mirrors) to your setup to compensate for the glass dispersion (unfortunately I don't think there's lens materials available at 400nm with opposite dispersion properties).

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Without knowing what you are trying to observe (or stimulate) with your expanded beam, it's impossible to comment on how much aberration your experiment can tolerate. As to the dispersive problem (pulse widening), perhaps you could find some optical fiber with reverse dispersion and feed the pulse thru that prior to the freespace optics. And finally, a properly designed all-reflective beam expander (Zemax FTW) should be aberration-free

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