Thermal modelling of Advanced LIGO test masses

TL;DR

This paper presents a thermal model for Advanced LIGO test masses that accounts for radiative heat transfer, enabling better understanding of thermal effects caused by laser power absorption and improving mirror temperature estimates.

Contribution

The study introduces an empirical radiative heat transfer term into the thermal model of LIGO mirrors, refining absorption estimates and thermal behavior predictions.

Findings

Coating absorption estimated at 1.5 to 2.0 ppm.

Scattered light on the ring heater estimated at 0.3 to 1.3 ppm.

Model accurately predicts thermal transients after power build-up.

Abstract

High-reflectivity fused silica mirrors are at the epicentre of current advanced gravitational wave detectors. In these detectors, the mirrors interact with high power laser beams. As a result of finite absorption in the high reflectivity coatings the mirrors suffer from a variety of thermal effects that impact on the detectors performance. We propose a model of the Advanced LIGO mirrors that introduces an empirical term to account for the radiative heat transfer between the mirror and its surroundings. The mechanical mode frequency is used as a probe for the overall temperature of the mirror. The thermal transient after power build-up in the optical cavities is used to refine and test the model. The model provides a coating absorption estimate of 1.5 to 2.0 ppm and estimates that 0.3 to 1.3 ppm of the circulating light is scattered on to the ring heater.