大型低温重力波望遠鏡KAGRAのための圧搾真空源の開発、導入及び検証

2020 Fiscal Year Annual Research Report

• Project/Area Number: 20F20803
• Research Institution: National Astronomical Observatory of Japan
• Principal Investigator: LEONARDI MATTEO 国立天文台, 重力波プロジェクト, 助教 (90816448)
• Co-Investigator(Kenkyū-buntansha): EISENMANN MARC 国立天文台, 重力波プロジェクト, 外国人特別研究員
• Project Period (FY): 2020-11-13 – 2023-03-31
• Keywords: KAGRA / filter cavity / freq. dep. squeezing / sapphire / optical absorption

Outline of Annual Research Achievements

From December 2020, Dr. Eisenmann participated to the characterization and improvement of the frequency dependent squeezing experiment in NAOJ. Especially he led the implementation of an automatic alignment control system in order to reduce the angular fluctuations of the filter cavity mirrors with respect to the squeezed vacuum states. He was also involved in the characterization and measurement of frequency dependent squeezing. These tasks are useful to both demonstrate a new control scheme of the filter cavity and prepare the future implementation of frequency dependent squeezing in KAGRA. Based on these measurements, he started to build models to estimate the performances of the KAGRA frequency dependent squeezing injection for various filter cavity lengths. He is also participating to the preparation work of the replacement of the optical cavity required to generated squeezed vacuum states called OPO (Optical Parametric Oscillator).
On another topic, he was also involved in the measurement of the optical absorption of sapphire samples. For this purpose, he participated to the calibration of the experimental setup required to measure the absorption of sapphire samples in NAOJ. Following this activity, he was involved in the absorption measurement of several sapphire samples. Finally, he was also involved in the initial phases of the overall optical simulations of the KAGRA interferometer.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

Dr. Eisenmann successfully integrated into the KAGRA frequency dependent squeezing research group and provided good inputs to it. His activity on filter cavity auto alignment system greatly improved the long term stability of it, allowing for better and easier measurements. He also supported the activity of other members of the group helping with the realization of new OPO cavity and testing of new filter cavity locking schemes. On this last topic, a paper is in preparation and he will be coauthor.
He became responsible of the design of the filter cavity which will be implemented into KAGRA before O5 and, thanks to its past experience in Virgo, he provided good insight on this activity.
He also performed research for KAGRA MIR (mirror) subsystem on the topic of improving the current sapphire substrates. Sapphire technology is in fact crucial for the success of KAGRA and in particular, optical absorption and birefringence are the two most critical parameters. He contributed to the calibration of the absorption setup (based on Photothermal Common-Path Interferometry technology) present at NAOJ and used it to characterize several sapphire sample. He also integrated very well with the KAGRA MIR/MIF simulation team, providing good inputs for the design of a global simulation environment for KAGRA.

Strategy for Future Research Activity

As highlighted by the first measurement of frequency dependent squeezing (FDS) performed in TAMA in 2020, the alignment condition of the filter cavity plays a crucial role into the stability of the squeezing level. The work on improving the alignment control was started in FY 2020 and will continue this FY. New sensor will be implemented for all the local controls, leading to improved mirror control, mechanical resonance suppression and position stability prior to engaging the full automatic alignment control. Also, the work on its improvement will continue and we plan to test a novel automatic alignment technique involving the coherent control sidebands. In addition, the design of the KAGRA FDS system will be performed, in collaboration with KAGRA Filter Cavity team. The design effort will be focused on the study of the impact of the filter cavity parameters (such as length, mirror roughness and reflectivity, suspension quality, ...) on the maximum achievable squeezing degree and consequently, KAGRA detector improvement. The upgrades of several components of the TAMA experiment is also planned to achieve the frequency dependent squeezing level planned for gravitational wave detectors.
In parallel to these activities, the measurement of the absorption of several sapphire samples was performed during FY2020. These measurements as well as birefringence will be performed in this FY on several new samples with different growth.