Improving the virgo detector sensitivity: Effect of high power input beam and Electrostatic actuators for mirror control

A ground based interferometric detector for gravitational waves (GW) mainly consists of a Michelson interferometer with kilometric arms. The very low GW interaction with matter requires to keep the fundamental noises, limiting the detector sensitivity, at very low levels. In particular the sensitivity of the Virgo antenna is limited, at high frequency, by the shot noise, while could be limited by control noise in the low frequency band. To overcome the first limit, in Advanced Virgo, an increase of the input laser power up to 200 W is foreseen. On this topic we studied the thermal effects induced by the absorption of laser radiation in the bulk of the injection optics. In particular the Faraday isolator is strongly affected by thermal self-action, since the absorption in magneto-optical media (TGG) is relatively high. After the TGG crystal a doughnut-like beam with topological charge was observed. Tomographic analysis showed that this beam acquires orbital angular momentum. Besides we demonstrated that a suitable optical set-up can compensate this effect. About the second topic, the attention was focused on the electrostatic actuators for suspended mirror control, that represent a valid alternative to the current coil-magnets actuators. An accurate characterization of such actuators was performed aimed to a correct system design. Also we showed that the effect of stray patch charge on the mirror can be minimized by modulating the driving signal of the actuator.