In-situ characterization of the thermal state of resonant optical interferometers via tracking of their higher-order mode resonances

TL;DR

This paper presents a method to monitor the thermal state of resonant optical interferometers by tracking higher-order mode resonances, enabling improved thermal compensation and sensitivity in gravitational wave detectors.

Contribution

It introduces a technique to measure thermal effects in interferometers through higher-order mode resonance tracking, providing real-time error signals for thermal control.

Findings

Measured changes in LIGO cavity geometry with input power

Inferred mirror surface optical absorption at 1 ppm

Generated a thermal state error signal for cavity control

Abstract

Thermal lensing in resonant optical interferometers such as those used for gravitational wave detection is a concern due to the negative impact on control signals and instrument sensitivity. In this paper we describe a method for monitoring the thermal state of such interferometers by probing the higher-order spatial mode resonances of the cavities within them. We demonstrate the use of this technique to measure changes in the Advanced LIGO input mode cleaner cavity geometry as a function of input power, and subsequently infer the optical absorption at the mirror surfaces at the level of 1 ppm per mirror. We also demonstrate the generation of a useful error signal for thermal state of the Advanced LIGO power recycling cavity by continuously tracking the first order spatial mode resonance frequency. Such an error signal could be used as an input to thermal compensation systems to maintain the interferometer cavity geometries in the presence of transients in circulating light power levels, thereby maintaining optimal sensitivity and maximizing the duty-cycle of the detectors.