Speaker
Description
The current performance of the Virgo Gravitational Wave Detector is limited by an excess noise source, caused by marginally resonant higher order modes in the recycling cavity. Virgo is proposing a substantial update, to shift to a “stable cavity” geometry. This would suppress higher order modes in the recycling cavity. However, the design introduces new challenges with Astigmatism associated with the angles of incidence (AoI) in the folded recycling cavities represents a potentially critical limitation, which affects the interferometer’s sensing and control signals.
Astigmatism introduces higher-order Hermite–Gauss modes that can contaminate longitudinal and alignment error signals, producing offsets, cross-couplings and changes in the sensing response, with potential consequences for the longitudinal controllability and accuracy of the interferometer. The current design considers astigmatism primarily in terms of power losses within the recycling cavities, which are estimated by design to be of the order of a few tens of ppm; while this requirement is adequate in terms of recycling gain, its impact on longitudinal controllability remains to be studied.
In this talk, we present the simulation results for the longitudinal controllability of Virgo in the stable cavities scenario. In close collaboration with the Virgo ISC group, this work investigates how the AoI and resulting astigmatism of the folded recycling cavities affect the higher-order mode content and the locking signals of the interferometer. We compare the resulting offset with the designed AoI values with the accuracy requirements. These results provide a first assessment of the impact of astigmatism on the longitudinal controllability of the upgraded Virgo interferometer and establish a framework for extending the study to other geometrical defects.