Options to correct rectangular laser beam profile? We recently switched to a 405 nm laser diode which has a terrible beam profile -- it is a well-defined rectangle with an aspect ratio of about 10:1.  The image below shows the collimated beam which has been expanded slightly for easier viewing.  We would like to convert this to a ~ 5 mm Gaussian or tophat beam profile with minimal power losses.  What options do we have? A few things we have investigated include


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*Using a anamorphic prism pair.  Previously we had a 678 nm laser with a nice elliptical profile which we easily corrected using a 2.5:1 prism pair, but this doesn't work very well for a rectangular beam.  

*Using an engineered diffuser.  We picked up a 1" 20 degree tophat diffuser from Thor, which is constructed from dark optical magics.  We are able to get a nice ~12 mm collimated tophat from this, but further expanding or condensing the beam screws it up because, uh, magic.  

*Using a normal diffuser (similar to half of the configuration in the photo here or with one of these.)  I don't know of any easy way to estimate the losses in this case but may be less than ~60%.  

*Running the beam through a pinhole to get a Gaussian profile, but this would mean ~90% lost power.

*Over-expanding and cropping the beam, but this would mean ~80% lost power.  


Is there any other way to do this that we missed, or is there any sort of standard approach to emission patterns like this?

 A: There are some other options I can add. By using two cylindrical lens you can build a beam expander that only expands in one direction, thus correcting for the elongation of the beam. 
Once you have corrected for this you can then focus the beam on to a pinhole spatial filter. This would only partially work since the laser needs to have a Gaussian profile to start with before going into the spatial filter. To get a better beam quality you would instead focus this into a single mode fibre optic. This solution has the drawback of wasting a lot of light but it should be better than the 80-90% quoted above.
The other method you can use is to make a kinoform or diffractive optical element. To do this use the Gerchberg-Saxton algorithm to convert your measured beam profile into any desired intensity profile. This produces a phase only hologram which needs a spatial light modulator or a custom built diffractive optical element. It works but unless you have a phase only spatial light modulator kicking about it's a rather pricey solution.  
