Active Vibration Isolation using a Suspension Point Interferometer Youichi Aso Dept. Physics, University of Tokyo ASPEN Winter Conference on Gravitational Waves Feb. 20 ASPEN Center for Physics LIGO-G Z

S uspension P oint I nterferometer (SPI) Active vibration isolation method Originally proposed by Prof. R. Drever using a laser interferometer itself Introduction Low Freq., Improve Stability, Cryogenic Interferometer Prototype Experiments SPI VSPI -40dB in noise spectrum below 3Hz limited by vertical vibration -40dB in transfer function measurement Current experiment: Combination of SPI and VSPI Development Status

Contents SPI – Principle – Advantages, Applications – Proto-Type Experiment VSPI – Principle – Proto-Type Experiment SPI+VSPI – Overview – Experimental apparatus – Development Status, Future plans Summary

Main Interferometer SPI Lasers PD's For GW detection Suspension Point Interferometer

Working principle of SPI Rigid bar Laser Lock SPI

Working principle of SPI Laser Vibration is suppressed Rigid bar

Advantages and Applications Interferometer = Ultra Low Noise Sensor Reduction of RMS motion Vibration isolation of heat links High performance active vibration isolation system Stabilization Easy lock acquisition Two Interferometers Coincidence Analysis Dual Band Actuator noise reduction SPI High power laser Low Shot Noise Seismic vibration Cryogenic Interferometer Main Interferometer Low power laser Low Radiation Pressure

Theoretical performance Main factors Control Gain CMRR Coupling from other degrees of freedom

Asymmetry in the suspension

at high frequencies

Prototype experiment

Results from the proto-type experiment Triple pendulum Fabry-Perot Cavity Vertical vibration is dominant above 3Hz 40dB Vertical vibration

Transfer function measurements

WFS test

Upper More sticky to the inertial frame Reference Lower mass= Lower Assumption External disturbances are only introduced from the suspension points Disturbance Stop the lower mass in the inertial frame Goal Fabry-Perot Interferometer Relative motion between the upper and lower mass Cancel the disturbance Vertical Suspension Point Interferometer

VSPI Experiment Coil-MagnetActuatorsMGASLaser EOM Eddy current damping Phase Mod.15MHz Faraday Isolator ModeMatchinglenses Photo Detector λ/4 PBS PDH Method PhotoSensor BladeSprings Mirrors

Results of the VSPI experiment Middle Lower Upper P.S. Excite Working fine up to 10Hz Excitation Voltage Displacement of the lower mass VSPI ON 約 40dB

Next Step Combination of SPI and VSPI Aim: Achieve good vibration isolation performance above the resonant freq. of the pendulum.

Overview of experimental setup Laser EOM EOM MC

Recoil mass Main mass SPI mass MGAS filter 2 Damping mass Eddy current plate MGAS filter 1 MGAS=Monolithic Geometric Anti Spring Low freq. vertical spring

Last stage: 40cm pendulum Other stages: 20cm Wire Q=1000 Mirror Q= mW at input Finesse = 500 V-H Coupling 1% Asymmetry 1% torr Noise Budget Vacuum Horizontal seis. SPI ON Vertical seis. Transverse

Development Status Vacuum Chamber: done Suspension design: done MGAS test: done Waiting for the suspension parts to arrive Optical components: done To Do Test the suspension system Assemble the whole system Electric circuits Noise hunting

Summary SPI, VSPI High performance active vibration isolation scheme Next generation detectors Prototype experiments Good results below the resonant freq. of the pendulums Limited by vertical vibration Next experiment SPI + MGASF + VSPI better vertical vibration isolation Test and assembly of the suspension is going on