Thermal lensing in cryogenic sapphire substrates

TL;DR

This study measures key optical and thermal properties of cryogenic sapphire to evaluate thermal lensing effects, finding it negligible for gravitational wave detectors.

Contribution

First comprehensive measurement of sapphire's optical and thermal parameters at cryogenic temperatures for thermal lensing assessment.

Findings

Thermal lensing in cryogenic sapphire is negligible.

Measured optical absorption coefficient, thermal conductivity, and refractive index temperature coefficient.

Estimated shot noise sensitivity indicates minimal impact from thermal lensing.

Abstract

We report the reduction of the thermal lensing in cryogenic sapphire mirrors, which is planed to be used in the Large scale Cryogenic Gravitational wave Telescope (LCGT) project. We measured three key parameters of sapphire substrate for thermal lensing at cryogenic temperature. They are optical absorption coefficient, thermal conductivity and temperature coefficient of refractive index at cryogenic temperature. On basis of these measurements, we estimated the shot noise sensitivity of the interferometer with thermal lensing by using a wave-front tracing simulation. We found that thermal lensing in cryogenic sapphire mirrors is negligible.