Hi Fefenin,
Welcome to the forum.
Going off at bit of a tangent…
Obviously it depends on your intended X axis speed but I have found that the Mach3 PWM is not updated fast enough to be really useful for laser power control unless you only require infrequent changes, such as when vector engraving then profile cutting etc.
I would be interested to know what type of laser engraving work you are planning?
Also you may find this thread (and others links within that thread) of interest;
http://www.machsupport.com/forum/index.php/topic,23584.0.htmlTweakie.