L ocal S uspension P oint I nterferometer for CLIO

1. Local Suspension Point Interferometer for CLIO University of Tokyo, ICRRA, AISTB Takanori Saito, SouichiTeladaB, Takashi UchiyamaA, Shinji MiyokiA, Osamu MiykawaA, MasatakeOhashiA and CLIO Collaborators GWADW at Hearton Hotel Kyoto

2. Outline • Motivation • LSPI • CLIO suspension system • Tandem Interferometer • LSPI Installation in CLIO • CLIO in Kamioka mine • Experimental setup • Experiment and result • Summary GWADW at Hearton Hotel Kyoto

3. 1. Motivation Experiences from CLIO experiment • Locking FP cavity is difficult in cryogenic temperature. • No damping resonant peaks of CLIO suspension effectively. • Problems on present eddy current damping system • Damping force strongly depends on temperature. • Because the damping force depends on resistance of a mass. • The resistance is drastically changed from the room temperature to the cryogenic temperature. Needed a new damping system. • Solution: active damping system • Damping resonant peaks of CLIO suspension. • No large magnets. • Adjustable damping force and frequency range. • Low noise sensor if using interferometer. GWADW at Hearton Hotel Kyoto

4. 2. Current Suspension system CLIO Suspension • 6 stages suspension system. • Upper 3 stages in room temperaturepart • Lower 3 stages in cryogenic temperature part • Eddy current damping is being applied to the cryo-base by some magnets on damping stage. • Existing eddy current damping system will be replaced by active damping system in future. • Sensing and actuation point: cryo-base (not the test mass) Local Suspension Point • Main FP cavity will be used as a sensor to see the damping effect. • We call it • Local Suspension Point Interferometer. Room Temperature part 3-stages vibration isolation system Damping Stage Cryo-Base Upper Mass Test Mass Cryogenic Temperature part GWADW at Hearton Hotel Kyoto Cryostat

5. 2. Polarized Tandem Interferometer • Electrical objects (actuators, PD, Laser source) don’t work in cryogenic temperature. • They are placed in room temp. part. • To align the optics in cryogenic temperature part is difficult. • Not using simple Michelson interferometer. • Aligned from the room temperature part remotely. • Avoid reconstructing the vacuum tank of CLIO. Room Temperature part Mirror1 Room arms S-wave QWP1 Mirror2 HWP1 PBS1 Laser QWP2 P-wave Mirror3 QWP3 PBS3 PD PBS2 HWP2 QWP4 Mirror4 Cryogenic arms PD Cryogenic Temperature part GWADW at Hearton Hotel Kyoto

6. 2. Polarized Tandem Interferometer Room Temperature part Mirror1 • Tandem interferometer • Compensating optical path length difference at cryogenic part by room temperature part. • Compact structure in the cryogenic temperature part. • Polarized interferometer. • High contrast interferometer • Sensing pendulum motion of the cryo-base. Room arms S-wave QWP1 Mirror2 HWP1 PBS1 Laser QWP2 P-wave Mirror3 QWP3 PBS3 PD PBS2 HWP2 QWP4 Mirror4 Cryogenic arms PD Cryogenic Temperature part GWADW at Hearton Hotel Kyoto

7. 3. LSPI Installation in CLIO Per-arm End Suspension CLIO Overview History of LSPI experiment • 2008: Table-top experiment at Kashiwa Lab.. • 2009-2010: First LSPI Installation in CLIO. • Installation of optics in perpendicular arm end suspension. • Control test with existing eddy current damping • Using aLIGO type digital system as servo • Verified damping effect by LSPI. • Future Work • Control test without the eddy current damping. • Cryogenic test. • Installation of second LSPI in other suspensions. • Cooling all the test mass with LSPI systems. GWADW at Hearton Hotel Kyoto

8. 3. Schematic of LSPI for CLIO Overview Room arms Laser • Two interferometersare installed for 2 DOFs • dampingpendulum and yaw motion • no big motion on pitch. • Two laser sources. • Two corner cube mirrors (CCM) are attached on the cryo-base. Cryo- Base PD Upper Mass Cryogenic arms Essentially sensing this length Test Mass GWADW at Hearton Hotel Kyoto

9. 3. Schematic of LSPI for CLIO Overview Room arms Active mirrors Laser Feedback points for length control. • Cryo-base • Damping the suspension system. • Control in low frequency range (< 2Hz) • Active mirrors • Control in high frequency range (2 ~ 20Hz) Cryo- Base PD Upper Mass Cryogenic arms Essentially sensing this length Test Mass Feedback the high frequency component to Active mirrors avoids spoilingcurrent CLIO sensitivity GWADW at Hearton Hotel Kyoto

10. 3. LSPI Setup (Room Temp.) Laser tank M3 PBS3 PBS Wave length: 633nm Power: 0.9mW From cryo. temp. part Into cryo. temp. part Fixed mirror PD Active mirrors GWADW at Hearton Hotel Kyoto

11. 3. LSPI Setup (Cryo. Temp.) From room temp. part Into room temp. part PBS QWP CCM Mirror Mirror Reference mass Coil-magnet actuators GWADW at Hearton Hotel Kyoto

12. 4. Damping test of LSPI Transfer function of CLIO suspension Arbitrary Unit Pendulum First Mode (0.48Hz) • Measurement: • Excite test mass of main 100m FP cavity with/without LSPI lock during FP cavity locked. • Measure transfer function from excitation point to main mirror motion shown at the feedback signal of FP cavity control. Pendulum Second Mode (1.18Hz) Frequency [Hz] GWADW at Hearton Hotel Kyoto

13. CLIO sensitivity with LSPI • Two loops control. • Cryo-base and active mirrors. • Add the 6Hz LPF only to cryo-base loop. • Improving CLIO sensitivity. Displacement [m/rtHz] We can expect that a suitable filter to the cryo-base loop can avoid to spoil the CLIO best sensitivity. Frequency [Hz] GWADW at Hearton Hotel Kyoto

14. 6. Summary • LSPI (Local Suspension Point Interferometer) is • Local control system that has two polarized tandem interferometers to damp the fluctuation of cryo-base. • Installed to 1 of 4 main suspensions. • We succeeded in damping the resonant frequency of CLIO pendulum (0.48Hz, 1.18Hz) using LSPI. • Feeding back low frequency signal to cryo-base and high frequency signal to active mirror successfully avoided spoiling CLIO sensitivity around 20Hz. GWADW at Hearton Hotel Kyoto