Study of the different responsive behaviour of the sidebands in LIGO I

Gravitational waves may be measured by detecting the length change induced by them in a Fabry-Perot cavity. In order to amplify the effect on the laser beam driving the cavity, the end mirrors are almost totally reflective so that when the resonant conditions are achieved, the electromagnetic field reflected from the cavity only changes in phase when the resonator changes in length. Several techniques may be used for the signal extraction; for example sidebands can be generated by phase modulation of the laser beam, which are minimally sensitive to any length change in the Fabry-Perot cavity when the carrier is resonating in there. Therefore, they can be used as a reference for the measurement of the variation induced by the gravitational wave upon the carrier. In an ideal case they are symmetric around the carrier for an unperturbed configuration. If the symmetry is broken, the combination of electromagnetic fields, which is supposed to be proportional to the gravitational strain, is a non-zero quantity because of a variety of imperfections in the real interferometers. We will discuss the fundamental mechanism beneath this phenomenon that is observed experimentally