@sarvan
Usually the barrel jack connector is the power In plug for the laser and fans etc...
The 12V pin on the PWM connector can be used to feed external devices from the same PSU or just use a potmeter to control laser power.
For you it means you could feed the Duet with a 12V supply and use a heater output to feed the laser thru the barrel jack. An emergency button and keylock switch between Duet and Laser can save your eyesight or your childrens health.
The PWM port can be connected as I described in another thread. ( same way as your picture shows)