Thermal lensing due to the absorption of the laser beam in core optics of gravitational wave interferometers can represent a strong limitation to their operation and sensitivity. This effect has already been observed in the present detectors and will become more relevant in the future upgraded interferometers, due to the much higher circulating power. A thermal compensation system, based on a CO2 laser projector, has been installed in Virgo, allowing to increase the interferometer input power from 7 to 17 W. The thermal compensation system can introduce displacement noise by means of several mechanisms. This noise depends on the CO2 laser intensity fluctuations and on the power needed to compensate thermal effects. To make the displacement noise compliant with Virgo specifications, a feedback system to reduce the CO2 laser intensity fluctuations has been implemented.
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