1. Yoichi Aso Columbia University, New York, NY, USA University of Tokyo, Tokyo, Japan July 14th 2007 7th Edoardo Amaldi Conference on Gravitational Waves Sydney Convention and Exhibition Centre, Sydney, Australia LIGO-G070446-00- Z

2. Introduction Suspension Point Interferometer (SPI) Active vibration isolation scheme Auxiliary laser interferometers Sensor: Auxiliary laser interferometers Ultra low-frequency vibration isolation Ultra low-frequency vibration isolation Reduced RMS mirror motion Reduced RMS mirror motion Stabilization of the interferometer Robust lock acquisition Reduction of various technical noises Advantage s 1.5m Fabry-Perot interferometers Prototype Experiment : 1.5m Fabry-Perot interferometers Maximum noise suppression below 10Hz (in spectrum) Maximum 40dB noise suppression below 10Hz (in spectrum) Mirror RMS motion Mirror RMS motion 1/9 Mirror RMS speed Mirror RMS speed 1/7

3. Content s Suspension Point Interferometer Principle Principle Theoretical Performance Theoretical Performance Advantages, Applications Advantages, Applications Prototype Experiment Experimental Setup Experimental Setup Results Results Future Prospects Conclusion

4. Main Interferometer (MIF) For GW Detection Suspension Point Interferometer Originally proposed by Drever (1987) SPI Lases Photo detectors Several possible configurations Conceptual diagram

5. Laser Rigid Bar SPI is lockedSPI MIF Working Principle of SPI: Fabry-Perot Arm Fabry-Perot Interferometer Measures the distance between the two mirrors SPI Virtual rigid bar

6. Rigid Bar Laser Working Principle of SPI: Fabry-Perot Arm Differential Seismic Motion

7. Rigid Bar Common mode motion Identical Pendulums No change in the distance Working Principle of SPI: Fabry-Perot Arm Asymmetry ?

8. Theoretical Performance Rigid bar Simple Pendulums Example: CMRR of Simple Pendulums Other Factors Cross coupling from other degrees of freedom Vertical, Pitch, Yaw etc... Cross coupling from other degrees of freedom Vertical, Pitch, Yaw etc... Control gain of SPI Control gain of SPI Noise of SPI Noise of SPI Common Mode Rejection Ratio (CMRR) Transfer functions: Conversion coefficient Common motion Differential motion Differential motion Average resonant frequency: Symmetry is Important Resonant frequency difference: difference:

9. Advantages of SPI Low Noise Sensor Low Noise Sensor Displacement sensor (global sensor) DC sensitivity Displacement sensor (global sensor) DC sensitivity Ultra low-frequency vibration isolation Characteristic s Benefit s Direct reduction of seismic noise in the observation band Reduction of the RMS motion of the mirrors Stable Operation Robust lock acquisition Technical noises Laser noises Actuator noises Up-conversion noise by non-linearity Duty Cycle Improvement to name a few

10. Prototype Experiment PD PD PD Isolator EOM EOM SPI Mirror Main Mirror Recoil Mass λ/4 λ/4 Laser PBS PBS Mode Cleaner SPI MIF 1.5m long Fabry-Perot interferometers 1.5m long Fabry-Perot interferometers Triple pendulum suspensions Triple pendulum suspensions Triangular rigid cavity mode-cleaner: Frequency Stabilization Triangular rigid cavity mode-cleaner: Frequency Stabilization

11. 1.5m Overview of the experimental setup

12. Nd:YAG Laser (1064nm)

13. PD1 PD2 MC 40MHz EOM Telescope Picomotor Mode Cleaner Chamber

14. Suspension System Recoil mass Main mass SPI mass MGAS filter 2 Damping stage MGAS filter 1 1m Triple Pendulum Two MGAS filters (vertical isolation) Resonance ~ 200mHz Temperature drift compensation servo for MGAS filters Recoil mass to actuate the main mirror

15. MGA S Damping Stage SPI Mirror Main Mirror

16. Result s Spectral measurements Spectral measurements Transfer function measurements Transfer function measurements

17. Displacement Equivalent Noise Spectrum (MIF)

18. 40d B RMS: 1/9

19. Mirror Speed Spectrum Speed RMS: 1/7 Easier Lock Acquisition

20. Transfer Function Pitch resonances MGAS resonance SPI OFF SPI ON Model: 0.25% asymmetry Excit e Measur e Up to 20Hz Up to 20Hz ~ isolation ~ 40dB isolation

21. Future prospects LCG T Cryogenic Mirror: heat link wires Cryogenic Mirror: heat link wires Vibration introduced from heat links SPI can suppress it Vibration introduced from heat links SPI can suppress it Low noise nature of SPI Located close to the MIF Important: Low noise nature of SPI Advanced LIGO Cold Stage Heat Link vibratio n High finesse cavity (RSE) is difficult (when seismic activity is high) High finesse cavity (RSE) lock acquisition is difficult (when seismic activity is high) SPI to reduce the mirror motion (considered as an option) SPI to reduce the RMS mirror motion (considered as an option) Various configurations are considered Various configurations are considered location suspension platform, penultimate mass, MIF itself with different color laser, etc... Interferometer type Fabry-Perot, asymmetric Michelson, etc... Australian group is leading the effort Australian group is leading the effort

22. Conclusio n終 SPI: Low-noise low-frequency active vibration isolation scheme Ultra low-frequency performance: Ultra low-frequency performance: RMS reduction Low noise: heat link vibration suppression for LCGT Prototype Experiment 1.5m Fabry-Perot interferometer, Triple pendulum suspension Noise spectrum: maximum 40dB reduction Noise spectrum: maximum 40dB reduction Displacement RMS: 1/9 Displacement RMS: 1/9 Velocity RMS: 1/7 Velocity RMS: 1/7 Vibration isolation performance improvement: more than 40dB up to 20Hz Vibration isolation performance improvement: more than 40dB up to 20Hz Stable operation, Robust lock acquisition, Technical noise mitigation Spectral measurements Transfer function measurements Future detectors are considering the employment of SPI LCGT, Advanced LIGO

23. Extra Slides

24. Z- Stage MGAS Filter Top Stage

25. PicoMoto r Coil-Magnet Actuator Photo Sensor Wire Clamp Torsion Coil Spring

26. Damping Mass Nd Magnet MGAS Filter Balance Weight

27. Photo Sensor Wire Clamp

28. Damping Plate Horizontal Actuator Vertical Actuator Yaw Actuator Eddy Current Damping

29. Mirro r SPI Mass Horizontal Actuator Vertical Actuator

30. Final Stage Recoil Mass MIF Mirror Wire Clamp

31. Coil-Magnet Actuator Motion Limiter

32. Actuator Magnets Damping Magnets Recoil Mass Main Mass

33. Seismic Noise Estimate SPI OFF Suspension TF × Seismic Motion Estimate SPI ON Estimat e (Accelerometer )

34. In-line motion Seismic - IFO output Correlation Vertica l Good correlation when SPI is OFF Good correlation when SPI is ON Seismic motion: measured by an accelerometer an accelerometer SPI ON Vertical Motion is Dominant at low frequencies

35. Laser Noises

36. Electric Noises

37. Thermal noise, Shot noise

38. Total estimated noise Noise shape: OK Magnitude: Small discrepancy

39. Noise of the SPI