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I'd like to somehow make a laser beam have a flat profile, or at least as flat as possible in the middle.

The source is a diode laser (ThorLabs LD808-SA60 or similar), so it has different beam divergence parallel or perpendicular to the junction. It is possible to collimate it with a spherical lens and cylindrical lens, and the resulting beam is roughly gaussian profile and is a few mm across. I'd like to somehow produce a flat (uniform intensity) spot about 10mm across instead.

What is the best way to modify the beam profile?

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    $\begingroup$ How much intensity are you willing to sacrifice to get a uniform beam? $\endgroup$ – Floris Dec 16 '17 at 1:44
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    $\begingroup$ I'm not an optics expert but my thought is this might be a job for diffractive optics. $\endgroup$ – The Photon Dec 16 '17 at 2:33
  • $\begingroup$ @Floris I'm not sure what is reasonable, but I'd be happy to keep 20% or better 50% of the intensity. If I only get 1% that is too little for this application. $\endgroup$ – Alex I Dec 16 '17 at 8:02
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You're looking for the conversion of a gaussian beam into a flat-top beam (or top-hat beam). You can have a look on https://www.rp-photonics.com/flat_top_beams.html

Different technics exist to do this conversion: DOE, SLM (spatial light modulator), microlens array or beam expander. You can find on the web commercial devices.

The best way to make the conversion will depend on the quality of the intial beam, the flatness you want, the time you are able to spend making it, the material you have in your lab.

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Typical way is a beam expander and then take the central portion of the expanded beam

or a diffractive optical element

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