Direct measurement of coating thermal noise at low temperatures with a folded optical cavity

2020 Fiscal Year Final Research Report

• Project/Area Number: 18K03681
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 15020:Experimental studies related to particle-, nuclear-, cosmic ray and astro-physics
• Research Institution: National Astronomical Observatory of Japan
• Principal Investigator: Aso Yoichi 国立天文台, 重力波プロジェクト, 准教授 (10568174)
• Project Period (FY): 2018-04-01 – 2021-03-31
• Keywords: 重力波 / 低温 / 薄膜

Outline of Final Research Achievements

In order to reduce the thermal noise, which is one of the main noises of gravitational wave detectors, mirrors are cooled down to a cryogenic temperature in the next generation detectors. However, an adsorbed molecular layer is formed on the surface of such a mirror with time, which increases the optical loss. We have developed a method to simultaneously measure the optical loss and thickness of the cryogenic molecular layer (CML) using a folded optical cavity. The absorption coefficient of the CML was determined by this method, and it was found to be almost the same as the literature value for amorphous ice.
The results of this study indicate that the next generation gravitational wave detectors using silicon mirrors require measures such as the suppression of CML formation by adding low-temperature baffles and/or the heating and desorption of CMLs using a CO2 laser.

Free Research Field

重力波天文学

Academic Significance and Societal Importance of the Research Achievements

本研究では、真空中で鏡表面に形成される薄膜の光学損失と、膜厚を同時に測定する手法を開発することに成功した。またこれを用いて低温鏡の表面に形成される吸着分子層の吸収係数を求めることにも成功した。これは次世代低温重力波検出器の設計に重要な示唆を与える結果である。本研究の成果を活用することで、低温バッフルや、分子層除去システムの設計を最適化することができる。