When a gravitational wave passes through the interferometer, the distances along the arms of the interferometer are shortened and lengthened, causing the beams to become slightly less out of phase, so some light arrives at the photodiode, indicating a signal.
The distances along the arms of the interferometer are shortened and lengthened, causing the beams to become slightly less out of phase. A gravitational wave passes through the interferometer. Some light arrives at the photodiode. Some light is indicating a signal.