I have been fighting a problem for some time now - trying to engrave scans (bitmap or vector) caused the darker areas to be output very badly. The same solid black original would be more of a patterned set of stripes (see attached) and photos came out very blotchy. I blamed the controller
, I blamed the power supply ; I even bought (ouch!) 
a new laser tube and installed that without improving results in the slightest. Last night, I was doing more of my endless tests to determine the problem. I had the o'scope hooked up to the PWM output of the controller (laser power OFF) and was watching for inconsistencies (which there were none). Deciding that it might show different results with the laser power on, I threw the switch and tried again - same results on the o'scope. BUT, there was a difference in the output - it was nearly perfect
. Why?Because I was not really trying to engrave a finished product, I had not paid any attention to what was in the laser. What was in the laser was only a spacer I use beneath my customary 1/4" stock. The spacer was a full 1/4" (6mm) further away from the lens than my stock usually is. I did some additional tests and found that the focal length of the currently installed lens is 55mm, not the 50mm I was used to with a previous lens.
In other words, I was working with an unfocused beam. So, after many hours (100+) and many more dollars, it all came down to ensuring the focus is set correctly. I am relieved, the results I ended up with last night are spectacular
and jobs can run much faster now (out of focus = 50 mm/s, in focus = 200 mm/s) . AND I now have a spare laser tube (ouch, ouch, ouch).