Development of a squeezed light source for the gravitational wave detector advanced virgo

This work of thesis deals with the design and the first steps toward the realization of a squeezed light source for the Gravitational Wave interferometric detector Advanced Virgo, located in Cascina (PI). The final goal of this new generation interferometer is to achieve an improvement in the sensitivity by a factor of 10 with respect to the first generation detectors. One of the most important noise sources in these advanced detectors, dominating at higher frequencies of the detection band (10 Hz- 10 kHz), is the shot noise. This noise is due to the vacuum fluctuations entering the interferometer through its output port; to reduce it, in Advanced Virgo a very high power laser will be employed (up to 125 W), but such a high circulating power into the interferometer can increase thermal effects. The injection of phase-squeezed vacuum states, into the output port of the interferometer makes it possible to reduce shot noise, without the need of higher power and thus the reduction of the related risks. Squeezed vacuum is produced by a below threshold nonlinear process (down-conversion) inside an Optical Parametric Oscillator (OPO). In common squeezing experiments, vacuum squeezed light at MHz frequency band is produced. For a gravitational wave detector the challenge consists in the production of squeezed light in the audio-frequency band. A first sensitivity enhancement of gravitational waves detectors by means of squeezed vacuum was already demonstrated by the British-German GEO interferometer and, in the next few years, it is planned to be finally integrated, for continuous runs, also in the in the American LIGO Scientific Collaboration. This same approach is now being pursued by the Virgo Collaboration, with the goal to inject squeezing states of light also in Advanced Virgo.