Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
Improving seismic motion isolation of gravitational-wave detectors
AEI researchers demonstrate the feasibility of using a suspension platform interferometer to stabilize the length of a suspended optical resonator
February 20, 2023
Current and future groundbased interferometric gravitational-wave detectors are decoupled from ground motion by a passive mechanical suspension system. Additionally, stable measurement operation of the interferometers requires active control of the suspended optical systems. This control can reintroduce noise into the measurement process, mostly at low frequencies, and is a known problem in the current detector generation and one of the major challenges for the future observatories like the Einstein Telescope and Cosmic Explorer. Using the institute’s 10 meter prototype facility, a team of AEI researchers has now demonstrated how a suspension platform interferometer – an interferometric and optical position sensor – can reduce input motion and suppress noise in the seismic isolation. Overall, they stabilized the length of a suspended optical resonator with a length of more than ten meters and reduced the differential motion by three orders of magnitude. The results are relevant for current and future gravitational-wave detectors.
Paper abstract
We report a reduction in motion for suspended seismic-isolation platforms in a gravitational wave detector prototype facility. We sense the distance between two seismic-isolation platforms with a suspension platform interferometer and the angular motion with two optical levers. Feedback control loops reduce the length changes between two platforms separated by 11.65 m to 10 pm Hz−1/2 at 100 mHz, and the angular motion of each platform is reduced to 1 nrad Hz−1/2 at 100 mHz. As a result, the length fluctuations in a suspended optical resonator on top of the platforms is reduced by three orders of magnitude. This result is of direct relevance to gravitational wave detectors that use similar suspended optics and seismic isolation platforms.
The first three graphics (a-c) show the residual Amplitude Spectral Density (ASD) of displacement in length and angle of the AEI-SAS platforms. All data was measured with the sensors of the SPI. Graphic (a) shows the residual length fluctuations between two platforms, graphic (b) shows the pitch of an individual platform, and graphic (c) shows the yaw of one platform. The AEI-SAS can be used without active control, and its passive performance is shown in blue. It is equipped with inertial and relative displacement sensors to the ground. The residual displacement with feedback control with the internal sensors and actuators applied is represented by the red lines. The feedback control with the SPI sensor input is represented by the yellow lines. An additional SPI sensor is used for an independent measurement and is shown in green and the Root-Mean-Square of it is shown in the dashed green curve. (d) Shows the projection of the suspension point motion for the suspended optical resonator, in the direction of its optical axis, extrapolated from the seismic platform motion. The blue curve shows the expected suspension point motion without a SPI and the purple curve with a SPI. The red, yellow, and green curves show the effects of pitch, yaw, and length of the seismic isolation platforms on the suspension point displacement.
The first three graphics (a-c) show the residual Amplitude Spectral Density (ASD) of displacement in length and angle of the AEI-SAS platforms. All data was measured with the sensors of the SPI. Graphic (a) shows the residual length fluctuations between two platforms, graphic (b) shows the pitch of an individual platform, and graphic (c) shows the yaw of one platform. The AEI-SAS can be used without active control, and its passive performance is shown in blue. It is equipped with inertial and relative displacement sensors to the ground. The residual displacement with feedback control with the internal sensors and actuators applied is represented by the red lines. The feedback control with the SPI sensor input is represented by the yellow lines. An additional SPI sensor is used for an independent measurement and is shown in green and the Root-Mean-Square of it is shown in the dashed green curve. (d) Shows the projection of the suspension point motion for the suspended optical resonator, in the direction of its optical axis, extrapolated from the seismic platform motion. The blue curve shows the expected suspension point motion without a SPI and the purple curve with a SPI. The red, yellow, and green curves show the effects of pitch, yaw, and length of the seismic isolation platforms on the suspension point displacement.
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