There is a th or tl on the laser power supply that serves as the trigger on a high or low signal. There is also a potentiometer "in" terminal that will accept a variable signal from 0v to 5v to control intensity. You can use either of these features to fire the laser tube.
If you have gotten grbl to work, then, no doubtedly, you have the expertise to know the output features of grbl. If grbl allows a pwm output, you can tie this to the potentiometer terminal on the power supply. Otherwise, you will need to use a digital output to control the power supply using th or tl.
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How to wire the Laser Power Supply with an arduino & GRBL ? (I'm not using a blacktooth laser cutter)