Hi Borge,
The laser TTL spec is usually <0.5volts for off and >2.0 for on. Providing it does not draw any appreciable current this input can be connected to the Z axis direction pin on the BOB so that it turns the laser on for -Z moves and off for +Z moves. I disable the Z axis so that the laser focus is not disturbed and use the Z axis DRO scale factor to control the 'laser bite time' (which varies from material to material). If the laser operation is reversed then change the direction active low under ports and pins.
Although I have used this method quite successfully I now favour a simple shutter system to switch the laser beam on and off, again using the Z axis direction signal as I believe that longer laser life will be achieved by using a soft start when it is turned on from cold.
Tweakie.
There is some further information here :-
http://www.cooperman.talktalk.net/index_files/Page4549.htm