[gnea/grbl-Mega Issue#146] changing duty cycle of pwm

I came across a problem while i was designing my own laser module, requirements for the laser is duty cycle should be 0.1% with pulse width of <100ns. am using arduino mega 2560 with ramps 1.6, i have set the frequency to 3khz can anyone help me out how can i change duty cycle it will be very help to me. thanks in advance