Please check out the M11P1/M10P1 commands for turning a laser ON/OFF and as Gary has mentioned the spindle 0% to 100% PWM signal can be used for controlling laser output power.
M11P1/M10P1 and
PWMMach3 was never really designed to operate a laser so there is no actual provision to synchronize an adjustable digital output to axis velocity therefore d.p.i and p.p.i are not implemented.
However, the Mach3 Impact / Laser engraving plugin does a pretty good job of overcoming this limitation. It’s operation is considerably different to DSP systems and it takes a bit of practice to get its operation just right but, in my opinion, it is well worth the effort.
Engraving.pdf If you skip through this thread you may find some items of interest.
http://www.machsupport.com/forum/index.php/topic,12444.0.htmlHope this helps.
Tweakie.