Optical scatter of quantum noise filter cavity optics

TL;DR

This study measures and analyzes light scattering from super-polished fused silica optics with dielectric coatings, revealing increased scatter post-coating and providing insights for optimizing quantum noise filter cavities in gravitational-wave detectors.

Contribution

It provides the first detailed measurement of scattering from coated optics used in quantum noise filter cavities, linking scattering patterns to coating interface roughness.

Findings

Coating increases wide-angle scatter by over 50%.

BRDF patterns show azimuthal maxima related to interface roughness.

Measured scatter levels are acceptable for LIGO's 16 m filter cavities.

Abstract

Optical cavities to filter squeezed light for quantum noise reduction require optics with very low scattering losses. We report on measured light scattering from two super-polished fused silica optics before and after applying highly-reflective ion-beam sputtered dielectric coatings. We used an imaging scatterometer that illuminates the sample with a linearly polarized 1064 nm wavelength laser at a fixed angle of incidence and records images of back scatter for azimuthal angles in the plane of the laser beam. We extract from these images the bidirectional reflectance distribution function (BRDF) of the optics with and without coating and estimate their integrated scatter. We find that application of these coatings led to a more than 50% increase of the integrated wide-angle scatter, to 5.00+/-0.30 and 3.38+/-0.20 ppm for the two coated samples. In addition, the BRDF function of the coated optics takes on a pattern of maxima versus azimuthal angle. We compare with a scattering model to show that this is qualitatively consistent with roughness scattering from the coating layer interfaces. These results are part of a broader study to understand and minimize optical loss in quantum noise filter cavities for interferometric gravitational-wave detectors. The scattering measured for these samples is acceptable for the 16 m long filter cavities envisioned for the Laser Interferometer Gravitational-wave Observatory (LIGO), though reducing the loss further would improve LIGO's quantum-noise limited performance.