Recently I purchased a 5 .5 watt laser module for my laser engraver. While I was waiting for it to be delivered, I told a friend, who also does laser engraving, about it. He told me that he had heard that the 5 watt laser modules only projected a rectangular beam instead of a small circular beam. Sure enough when I got the module, he was right. The picture with the red circle shows how large the rectangular beam was at its smallest focus.
Step 1: Troubleshooting...
I unscrewed the lens assembly off of the module and noticed that the laser diode was an open type instead of the normal laser diode that has a metal can with a small opening in the top for the laser beam to come out of. I wondered that if this was why the beam was rectangular, could I do something to circularize the emitter area of the diode.
Step 2: The Fix.
I put a nylon washer on top of the diode to prevent a short circuit.and a small metal washer on top of the nylon one.
Step 3: All Better.
EUREKA! That did it. I now have a circular beam from the laser module that is capable of a very fine focus.