 In general relativity, a gravitational wave is a space-time perturbation which propagates at the speed of light.It then curves slightly the space-time, which changes locally the light path.Mathematically speaking, if formula_1 is the amplitude (assumed to be small) of the incoming gravitational wave and formula_2 the length of the optical cavity in which the light is in circulation, the change formula_3 of the optical path due to the gravitational wave is given by the formula