Round Spot from Diode Laser

[heruursciences]

If the output is square than it's as good as it gets for maximum power at minimum divergance in minimum BEAM size at aperature. From the sound of it you have a multimode emitter

If you want a round beam and can accept a loss of power but get excellent divergence you will want to run it through a Keplerian collimator with a spacial filter at the focus of the input lens. This will get rid of the fringes from simple aperaturing.

There are other more expensive options such as anamorphic beam compressers, holographic optical elements and the like but you will pay for your round beam with BOTH a loss of power and greatly increased far field divergance.

[Laser Ben]

I do believe the cheapest are the anamorphic compression optics. I believe I have a set of anamorphic prisms here.