1. Current progress of developing Inter-satellite laser interferometry for Space Advanced Gravity Measurements Hsien-Chi Yeh School of Physics Huazhong University of Science & Technology 22 May, 2012

2. Outline 1 Motivation and Strategy 2 Current Progress at HUST 3 Scheme and Error Budget Roadmap and Conclusion 4

3.  Orbit precession in the perihelion of planets  Deflection of light by solar gravity  Redshift of spectral lines  Frame dragging  Gravitational waves Motivation: Gravitational Waves Detection in Space

4. Frequency range (Hz) Arm length Displacement noise (pm/Hz 1/2 ) Acceleration noise (ms -2 /Hz 1/2 ) LISA10 -4 ~ 10 -1 5  10 9 m 20 3  10 -15 @1mHz ALIA10 -3 ~ 1 ~ 5  10 8 m ~ 0.1 ~ 3  10 -16 @10mHz ASTROD10 -6 ~ 10 -3 ~ 3  10 11 m ~ 2000 ~ 8  10 -16 @0.1mHz 10 -18 10 -19 10 -20 10 -21 10 -22 10 -23 10 -24 10 -25 LIGO A-LISA (ALIA) (LISA type, 5  10 5 km) ASTROD (2A.U.) 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 10 3 Sensitivity Requirements of GWD Missions

5. Strategy: Treat SAGM as LISA Pathfinder Satellite-to-satellite tracking (SST): Separation: ~100 km Altitude: 250~300 km (declined orbit) Measurement: Laser interferometer (30~50 nm) GPS (1mm) GRACE-like mission Space Advanced Gravity Measurements (SAGM)

6. Transponder-Type Laser Ranging System 200km Beam Collimation & Pointing Control Proof Mass Inertial Sensor Heterodyne Laser Interferometer Satellite Platform Environment Control Drag Free Control Beam Collimation & Pointing Control Proof Mass Inertial Sensor Transponder With Phase- Locked Loop Satellite Platform Environment Control Drag Free Control

7. Error SourceError component Pre-stabilized laser:  f < 50 Hz/Hz 1/2 L = 200 km 30.0 nm/Hz 1/2 Thermal drift of O.B. (fused quartz): thermal variation: 0.01K unbalanced OPL: 1 cm 2.0 nm/Hz 1/2 Divergence angle of laser beam:  div ~ 3.5  10 -5 rad Pointing control:  dc ~ 10 -5 rad,  jit ~ 10 -5 rad/Hz 1/2 9.0 nm/Hz 1/2 Phasemeter resolution 1.0 nm/Hz 1/2 Residual error of OPLL 3.0 nm/Hz 1/2 Shot noise and Ionosphere effect < 0.1 nm (RSS) Total~ 35 nm/Hz 1/2 Error Budget

8. 10-m Prototype of Laser Ranging System Installed at HUST (2009~2010) 5-nm step Driving by PZT stage

9. FPGA-Based Digital Phasemeter (2010~2011) Phase (peak-to-peak) = 0.01 o  30pm 2  10 -5 rad/Hz 1/2 @0.1Hz

10. Ultra-Stable Optical Bench (2011-2012) Cooperation with AEI, Hannover amplitude: 25 pm

11. Transponder-Type Laser Ranging (2012) Proof Mass Optical Bench Proof Mass Optical Bench Phase Meter Phase Locked Control Master laser Slave laser Displacement output PZT 1-nm sinusoidal motion Weak-light: 10 nW Homodyne OPLL

12. Key factors ： Mechanical stability of cavity Thermal stability of cavity Environment control Mode matching F-P cavity for Laser frequency stabilization Laser Frequency Stabilization

13. Beam Pointing Angle Measurement Phase-difference Measurement Divergence angle ： 3.5  10 -5 rad Received power ： 10 -7 W Phase difference  misalignment angle precision ： 10 -7 rad Jitter ： 10 -6 rad/Hz 1/2 Contrast Measurement Divergence angle ： 10 -4 rad Received power ： 10 -8 W Contrast  misalignment angle precision ： 10 -5 rad

14. 2010 2020 2015 2025 2030 Inter-Satellite Laser Ranging For Earth’s Gravity Recovery Inter-satellite distance: 50-200 km Sensitivity: 30-50 nm/Hz 1/2 Transponder-type heterodyne interferometry Pointing control: 10 -6 rad/Hz 1/2 Inter-Satellite Laser Interferometer For Gravitational Waves Detection Inter-satellite distance: 10 5 ~10 6 km Sensitivity: < 1 pm/Hz 1/2 Transponder-type heterodyne interferometry Special methods to decompress laser frequency noise Pointing control: 10 -9 rad/Hz 1/2 Proposed Timeline

15. GW detection (long-term goal) Earths gravity recovery (short-term goal): SAGM as our LISA Pathfinder Preliminary demonstration: transponder-type laser ranging with weak-light phase locking Focused tasks in the next step: (1) space-qualified frequency-stabilized laser (2) laser beam pointing measurement and control (3) simulation experiment of plasma in ionosphere Conclusions

16. Thank you for your attentions!