Point absorbers in Advanced LIGO

TL;DR

This paper analyzes how tiny point absorbers on LIGO optics cause surface deformations that reduce interferometer power and sensitivity, proposing mitigation strategies for current and future gravitational wave detectors.

Contribution

It provides a detailed analysis of the impact of point absorbers on Advanced LIGO's performance and suggests correction methods to mitigate their effects.

Findings

Point absorbers cause nano-meter scale deformations reducing power build-up.

Performance degradation can reach up to 50% in stored power.

Mitigation strategies can help restore interferometer sensitivity.

Abstract

Small, highly absorbing points are randomly present on the surfaces of the main interferometer optics in Advanced LIGO. The resulting nano-meter scale thermo-elastic deformations and substrate lenses from these micron-scale absorbers significantly reduces the sensitivity of the interferometer directly though a reduction in the power-recycling gain and indirect interactions with the feedback control system. We review the expected surface deformation from point absorbers and provide a pedagogical description of the impact on power build-up in second generation gravitational wave detectors (dual-recycled Fabry-Perot Michelson interferometers). This analysis predicts that the power-dependent reduction in interferometer performance will significantly degrade maximum stored power by up to 50% and hence, limit GW sensitivity, but suggests system wide corrections that can be implemented in current and future GW detectors. This is particularly pressing given that future GW detectors call for an order of magnitude more stored power than currently used in Advanced LIGO in Observing Run 3. We briefly review strategies to mitigate the effects of point absorbers in current and future GW wave detectors to maximize the success of these enterprises.