this green beam is actually coming for a submarine off the coast that is coming up for the old characters that are locked in the bottom of the light house.
the step you have found is actually part of step four and is ironically the step that i am stuck on.
this is a quick overview of the first three steps.
step 1 - turn on power, go to the bottom of the light house and listen to the doctor. go up one level and get the fuse off the table and put it in the control panel.
step 2 - listen again and then find all the generators with red lights on them (four throughout the level) these will only blow up with grenades (i recommend simtex)
step 3 - (only in multi-player) listen again and then find the bottle of vodka. it will look like a ball of ice with some of the bottle sticking out of the top. it should be frozen to a rail. you will have to have someone under the bottle to catch it so it doesnt break. if you do break it it will respawn somewhere else dont worry.
step 4 - okay this is where i'm stuck. you have to find the golden rod. i think it has something to so with turning on this green beam then shooting somewhere on the green beam with the vr-11 and/or the scavenger. after you shoot the green beam (from what i have heard) the golden rod will appear. you simply pick it up and bring it to the doctor. then you are done. have fun with the new toy you get.
if someone has an exact explanation for step 4 please post.