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Active Vibration Isolation using Active Vibration Isolation using a Suspension Point Interferometera Suspension Point Interferometer
Yoichi Aso, University of TokyoYoichi Aso, University of Tokyo
IntroductionIntroductionSuspension Point Interferometer (SPI) is an active vibration isolation scheme for interferometric Suspension Point Interferometer (SPI) is an active vibration isolation scheme for interferometric gravitational wave detectors using auxiliary interferometers as sensors. The auxiliary gravitational wave detectors using auxiliary interferometers as sensors. The auxiliary interferometers (SPI) are formed at the suspension points of the mirrors of the main interferometers (SPI) are formed at the suspension points of the mirrors of the main interferometer as shown bellow. The seismic vibration can be blocked at the SPI stage by interferometer as shown bellow. The seismic vibration can be blocked at the SPI stage by monitoring the seismic motion with the SPI and feeding the error signal back to the mirrors of monitoring the seismic motion with the SPI and feeding the error signal back to the mirrors of the SPI.the SPI.This scheme is expected to be useful for the next generation detectors such as LCGT, which This scheme is expected to be useful for the next generation detectors such as LCGT, which requires better low frequency vibration isolation and reduction of the vibration from heat link requires better low frequency vibration isolation and reduction of the vibration from heat link wires to cool down the mirrors. wires to cool down the mirrors.
Working principleWorking principle
The figure below shows one arm of a FabryPerotMichelson interferometer equipped with a The figure below shows one arm of a FabryPerotMichelson interferometer equipped with a SPI. When the upper interferometer (SPI) is locked to the laser frequency, the two mirrors of SPI. When the upper interferometer (SPI) is locked to the laser frequency, the two mirrors of the SPI act like a virtual rigid bar and resist to differential forces caused by seismic vibration to the SPI act like a virtual rigid bar and resist to differential forces caused by seismic vibration to change the length between the mirrors. In this way, the differential motion of the SPI mirrors, change the length between the mirrors. In this way, the differential motion of the SPI mirrors, which causes the length change of the main interferometer, is suppressed. While the common which causes the length change of the main interferometer, is suppressed. While the common motion is not suppressed by the SPI, it does not cause the length change in the main motion is not suppressed by the SPI, it does not cause the length change in the main interferometer in principle. However, in the real case, a fraction of the common motion is interferometer in principle. However, in the real case, a fraction of the common motion is converted into the length variation of the main interferometer due to the asymmetry in the converted into the length variation of the main interferometer due to the asymmetry in the suspension system. This asymmetry sets a limit to the performance of the active vibration suspension system. This asymmetry sets a limit to the performance of the active vibration system using a SPI. system using a SPI.
Rigid barRigid bar
LaserLaser
Advantages and ApplicationsAdvantages and Applications
ResultsResultsThe figure below shows the displacement equivalent noise spectra of the main interferometer and The figure below shows the displacement equivalent noise spectra of the main interferometer and an electronics noise spectrum. The red spectrum was taken when the SPI is not locked, while the an electronics noise spectrum. The red spectrum was taken when the SPI is not locked, while the blue curve shows the spectrum with the SPI locked. The green curve is the spectrum of the blue curve shows the spectrum with the SPI locked. The green curve is the spectrum of the electronics noise.electronics noise.Below 2Hz, the noise of the main interferometer was suppressed typically by 20dB using the SPI. Below 2Hz, the noise of the main interferometer was suppressed typically by 20dB using the SPI. The tall peak in the blue curve at 0.8Hz is the pendulum mode of the main interferometer's The tall peak in the blue curve at 0.8Hz is the pendulum mode of the main interferometer's mirrors ,which can be considered to be suspended from a virtual rigid bar. The fact that this peak mirrors ,which can be considered to be suspended from a virtual rigid bar. The fact that this peak has a high quality factor supports the virtual rigid bar picture of the SPI's working principle.has a high quality factor supports the virtual rigid bar picture of the SPI's working principle.
At frequencies above 2Hz, there is no difference between the blue and red curves. This means the At frequencies above 2Hz, there is no difference between the blue and red curves. This means the sensitivity of the main interferometer is limited by noises other than the horizontal seismic motion. sensitivity of the main interferometer is limited by noises other than the horizontal seismic motion. We suspect that it is coupling from other degrees of freedom such as alignment fluctuation.We suspect that it is coupling from other degrees of freedom such as alignment fluctuation.
ExperimentExperiment
Recoil massRecoil massMain massMain mass
SPI massSPI mass
MGAS filter 2MGAS filter 2Damping massDamping mass
Eddy current plateEddy current plate
MGAS filter 1MGAS filter 1
40cm40cm
1m1m
Main InterferometerMain Interferometer
SPISPI
Future worksFuture works ReferenceReference
Y. Aso et. al., Y. Aso et. al., Physics Letters A, 327 (1) 18, 2004Physics Letters A, 327 (1) 18, 2004
R.W.P. Drever, R.W.P. Drever, LIGO Document T87000100R, 1987LIGO Document T87000100R, 1987
66thth Edoardo Amaldi Conference on Gravitational Waves Edoardo Amaldi Conference on Gravitational Waves at Bankoku Shinryoukan, Okinawaat Bankoku Shinryoukan, Okinawa
MC TankMC Tank
Suspension systemSuspension system
1.4m1.4m
Objective: Objective: Demonstration of the active vibration isolation using an SPI.Demonstration of the active vibration isolation using an SPI.
● 1.4m long FabryPerot interferometer.1.4m long FabryPerot interferometer.● Triple pendulum suspension with two lowfrequency vertical Triple pendulum suspension with two lowfrequency vertical
vibration isolation stages.vibration isolation stages.● 20cm rigid triangular cavity mode cleaner for laser mode cleaning 20cm rigid triangular cavity mode cleaner for laser mode cleaning
and frequency stabilization.and frequency stabilization.● 200mW Nd:YAG Laser.200mW Nd:YAG Laser.● Wave Front Sensors for alignment stabilization.Wave Front Sensors for alignment stabilization.
Low noise active vibration system Low noise active vibration system
Reduction of the RMS motion of the mirrorsReduction of the RMS motion of the mirrors
Easy lock acquisitionEasy lock acquisition
Stable operation Stable operation
Apparatus:Apparatus:
Cryogenic interferometers (LCGT etc.)Cryogenic interferometers (LCGT etc.)
Reduction of actuator noiseReduction of actuator noise
The performance of an active vibration isolation system is The performance of an active vibration isolation system is usually limited by the noise of the sensors used. In our usually limited by the noise of the sensors used. In our scheme, sensor is an interferometer which can essentially scheme, sensor is an interferometer which can essentially be as sensitive as the main interferometer. This lownoise be as sensitive as the main interferometer. This lownoise feature makes the SPI a high performance active vibration feature makes the SPI a high performance active vibration isolation system.isolation system.
Cryogenic interferometers such as LCGT use heat link Cryogenic interferometers such as LCGT use heat link wires to suck out the heat generated in the mirrors. wires to suck out the heat generated in the mirrors. However these wires also introduce extra vibration from However these wires also introduce extra vibration from cold stages.cold stages.SPI can also be used to suppress the extra vibration as SPI can also be used to suppress the extra vibration as well as the seismic vibration. well as the seismic vibration.
Cold stageCold stage
Since SPI is an active system, it works well at very low frequencies. Since SPI is an active system, it works well at very low frequencies. Consequently one can reduce the residual motion of the mirrors.Consequently one can reduce the residual motion of the mirrors.
Heat linkHeat link
Expected advantages from Expected advantages from the RMS reduction includes:the RMS reduction includes:
Identify the limiting noise source at frequencies above 2Hz.Identify the limiting noise source at frequencies above 2Hz.
Improve the sensitivities of the Wave Front Sensors.Improve the sensitivities of the Wave Front Sensors.
Development of effective ways to adjust the alignment Development of effective ways to adjust the alignment of each interferometer independently.of each interferometer independently.
Suspension system modification.Suspension system modification.
Displacement equivalent noise spectra of the main interferometer.Displacement equivalent noise spectra of the main interferometer.