Avoid voltage feedback from laser psu to Arduino via pwm pin
You haven't provided many electrical details, but this sounds like a good application for an optoisolator. Connect the Arduino's PWM output to the optoisolator's LED anode through an appropriate resistor and connect the LED's cathode to the Arduino ground. Choose an optoisolator with an NPN transistor output and connect the transistor's collector to the power supply PWM input, probably adding a pullup resistor to the PS logic '1' level. The transistor's emitter connects to the power supply's ground.

Note that there is no connection between the ground of the Arduino and the ground of the power supply. Also, the optoisolator will invert the PWM signal so the programmed duty cycle will need to be changed accordingly.