Thermal defocus-free Hartmann Wavefront Sensors for monitoring aberrations in Advanced Virgo

Abstract Earth-based gravitational waves interferometric detectors are shot-noise limited in the high-frequency region of their sensitivity band. While enhancing the laser input power is the natural solution to improve on the shot noise limit, higher power also increases the optical aberration budget due to the laser absorption in the highly reflective coatings of mirrors, resulting in a drop of the sensitivity of the detector and in a limitation of its performance as well. Advanced Virgo exploits Hartmann Wavefront Sensors to locally measure the absorption-induced aberrations by monitoring the optical path length change in the core optics. Despite the very high sensitivity featured by Hartmann sensors, environmental temperature fluctuations can cause a spurious curvature term to appear in the reconstructed wavefront due to the thermal expansion of the Hartmann plate, that could affect the accuracy of the aberration monitoring. We present the implementation and validation of a control loop to stabilize the Advanced Virgo Hartmann Wavefront Sensor temperature at the order of ΔT ≤ 0.01 K, keeping the spurious curvature within the detector’s requirements on wavefront sensing accuracy.