Compact integrated optical sensors and electromagnetic actuators for vibration isolation systems in the gravitational-wave detector KAGRA

TL;DR

This paper details the design, testing, and performance validation of a compact optical sensor and electromagnetic actuator module, called OSEM, used for vibration isolation in the KAGRA gravitational-wave detector, demonstrating comparable performance to previous designs.

Contribution

The paper introduces a modified, compact OSEM module with improved sensor spacing, validated performance, and integration into KAGRA's vibration isolation system, contributing to gravitational-wave detection technology.

Findings

Sensor noise level of 0.5 nm/rtHz at 1 Hz

Linear sensor range exceeds 0.7 mm

42 modules operational at KAGRA as of 2020

Abstract

This paper reports on the design and characteristics of a compact module integrating an optical displacement sensor and an electromagnetic actuator for use with vibration-isolation systems installed in KAGRA, the 3-km baseline gravitational-wave detector in Japan. In technical concept, the module belongs to a family tree of similar modules called OSEMs, used in other interferometric gravitational-wave detector projects. After the initial test run of KAGRA in 2016, the sensor part, which is a type of slot sensor, was modified by increasing the spacing of the slot from 5 mm to 15 mm to avoid the risk of mechanical interference with the sensor flag. We confirm the sensor performance is comparable to that of the previous design despite the modification. We also confirm the sensor noise is consistent with the theoretical noise budget. The noise level is 0.5 nm/rtHz at 1 Hz and 0.1 nm/rtHz at 10 Hz, and the linear range of the sensor is 0.7 mm or more. We measured the response of the actuator to be 1 N/A, and also measured the resistances and inductances of coils of the actuators to confirm consistency with theory. Coupling coefficients among the different degrees of freedom were also measured and shown to be negligible, varying little between designs. A potential concern about thermal noise contribution due to eddy current loss is discussed. As of 2020, 42 of the modules are in operation at the site.