Walk through the portals, hit the red button to drop the Companion Cube down and then head back through the first portal and you'll be right back at the start again.
Once again place the second portal over by the Companion Cube and walk through towards it. Pick it up and jump across towards the platform. Set down the Companion Cube and then repower up the laser with a new portal.
Run towards the right-hand end of the platform and then recollect the Cube, walk off, place it on the red button and the exit will open!