The laser beams are reflected back and are recombined by the beam splitter. Most of the light passes back towards the laser, only to be reflected onto itself by the input mirror. This process repeats thousands of times until the light power inside the instrument has built up by resonance to be thousands of times brighter than the laser itself.
Gravity waves cause the mirrors to move which modifies a tiny bit of the light.
This light, which carries the gravity wave signal, is directed towards the output mirror, which reflects most of it back into the instrument. This process also repeats itself thousands of times, so the light containing the signal also builds up by resonance. It is detected by a photo detector at the output and recorded and processed by a computer. The computer also records earthquakes and other disturbances as well as very accurate timing information, so that the arrival times of signals can be compared with those from other observatories around the world. Linked together by computer the world gravity wave telescope will be able to listen in to 'sounds' from throughout the universe and pinpoint their direction.