Large-angle scattered light measurements for quantum-noise filter cavity design studies

TL;DR

This study measures large-angle scattered light from high-quality optics to assess optical losses that could impact quantum-noise filter cavities in gravitational-wave detectors, providing data critical for design optimization.

Contribution

It provides the first detailed angle-resolved scatter measurements of superpolished fused silica optics with dielectric coatings for filter cavity applications.

Findings

Total scattered light into large angles can be as low as 4 ppm

Optical scatter levels are consistent with high-quality superpolished optics

Results inform the design and performance limits of quantum-noise filter cavities

Abstract

Optical loss from scattered light could limit the performance of quantum-noise filter cavities being considered for an upgrade to the Advanced LIGO gravitational-wave detectors. This paper describes imaging scatterometer measurements of the large-angle scattered light from two high-quality sample optics, a high reflector and a beam splitter. These optics are each superpolished fused silica substrates with silica:tantala dielectric coatings. They represent the current state-of-the art optical technology for use in filter cavities. We present angle-resolved scatter values and integrate these to estimate the total scatter over the measured angles. We find that the total integrated light scattered into larger angles can be as small as 4 ppm.