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High optical power is necessary to increase the sensitivity of advanced gravitational wave detectors. However, it will also induce instabilities. In this paper we review the effects caused by high optical power and potential methods to control those instabilities. Some recent experimental results are presented.

Frequency-resolved thermal lensing (FRTL) is an approach used to measure thermal diffusivity coefficients of liquid samples. The theoretical model of this approach is based on thermal waves and the Fresnel diffraction approximation. This model predicts that only two experimental parameters are needed to estimate the thermal diffusivity of the sample. Experiments performed on pure solvents validate the theoretical predictions and confirm the feasibility of using this approach to thermally characterize liquid samples. The results show that the method is a simple and sensitive approach to determine thermal diffusivity coefficients in liquid samples.

This paper describes experimental results of thermal diffusivity measurements performed on different concentrations of aqueous Tartrazine solutions. The measurements are performed using the frequency-resolved thermal lensing technique and the results are compared with the thermal diffusivity value of pure water used as a solvent or host liquid. The results show that at low concentrations, the thermal properties of the solution are roughly equal to those of the water. However, when the concentration is increased, the thermal properties of the solutions diverge from that of the host liquid.

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.