There is a laser, and a laser redirection cube, but you can't reach it yet. First, create a portal on the checkered floor near the angled wall with the green target on it. Make a portal on the piece of floor that the light funnel is flowing into. Stand on the checkered floor where the funnel should now be coming out from.
When you reach the ceiling, look down at the portal below you and make a portal of the other color on the angled wall with the target on it nearby. You will be thrown over to a frankencube. Pick up the cube and take it down to where the laser is shining. Put it in the path of the laser to stop the funnel.
Use portals to get back up to the upper area. Put a portal on the angled wall with the target on it, then go over and put a portal of the other color on the angled wall near the Wheatley monitor. Get on the catapult in the upper area and you should be thrown through the portals to the laser redirection cube. Pick it up and take it to the lower area. Put it in the path of the laser to make it shine at a white wall.
Now make a portal down here and another in the upper area, then take the frankencube through. Put it down on the checkered floor below the big red button on the ceiling.
Put a portal where you made the laser shine using the laser redirection cube. Put a portal of the other color on the wall across from the laser receiver near the two platforms near the exit door. Get on the platform that comes down.
Now move the portals to make the laser stop shining into the receiver. Your platform will go up. Now use a portal under the laser redirection cube so it no longer blocks the laser that activates the funnel. Put a portal under the funnel, then another on the floor where you put the frankencube under the big red button on the ceiling.
When the platform lowers, step onto it, then move the portals so the frankencube is no longer pushing the big red button. This raises your platform so you can reach the exit door.