What kind of controller are you using? (I assume GRBL, but which version?)
If it’s 1.1f or higher, you need to set $32=1 (laser mode). If it’s earlier than that, you need to use the GRBL-M3 device in LightBurn, so it explicitly turns off the beam with every G0 move (laser mode normally manages that).
Regarding the sawtooth edges, you have likely got a slightly loose belt on the X axis.