Audio-band Coating Thermal Noise Measurement for Advanced LIGO with a Multi-mode Optical Resonator

TL;DR

This paper introduces a novel multi-mode optical resonator technique to directly measure coating thermal noise in high-precision optical instruments, specifically applied to Advanced LIGO's mirror coatings, achieving high sensitivity in the 30-400 Hz range.

Contribution

A new measurement method using a multi-mode resonator to directly assess coating thermal noise and loss angle in gravitational wave detector coatings.

Findings

Measured coating thermal noise at 10^-17 m/√Hz in 30-400 Hz range

Determined the loss angle of Advanced LIGO coating materials

Demonstrated high sensitivity of the multi-mode resonator technique

Abstract

In modern high precision optical instruments, such as in gravitational wave detectors or frequency references, thermally induced fluctuations in the reflective coatings can be a limiting noise source. This noise, known as coating thermal noise, can be reduced by choosing materials with low mechanical loss. Examination of new materials becomes a necessity in order to further minimize the coating thermal noise and thus improve sensitivity of next generation instruments. We present a novel approach to directly measure coating thermal noise using a high finesse folded cavity in which multiple Hermite-Gaussian modes co-resonate. This method is used to probe surface fluctuations on the order 10^-17 m\rtHz in the frequency range 30-400 Hz. We applied this technique to measure thermal noise and loss angle of the coating used in Advanced LIGO.