1. Commissioning the LIGO detectors
Stability and sensitivity improvements
October 21, 2004
Nergis Mavalvala

2. LIGO : Laser Interferometer Gravitational-wave Observatory3 k m ( 1 s )
4 km
2 km LA
4 km

3. Initial LIGO

5. H1 noise history
(science design) S3 S2 S1

6. Interferometer Layout
laser

7. L1 Estimated Noise Limits for S2
(expected)

8. H1 Noise Model

9. Recent commissioning efforts
Reliability & Stability
Seismic retrofit at LLO: Hydraulic External Pre-Isolator (HEPI)
Sensitivity
Operate at high power: achieve designed optical gain
Laser power increase
Thermal compensation system (TCS)

10. Seismic Noise Motion of the earth is a few mm at low frequencies
Passive and active seismic isolation
Amplify mechanical resonances
Get isolation above a few Hz

11. Daily Variability of Seismic Noise
RMS motion in 1-3 Hz band
day
night
Livingston
Displacement (m)
Hanford
PRE-ISOLATOR REQUIREMENT
Time (GPS seconds)

12. Hydraulic External Pre-Isolation: HEPI at LLO

14. X-arm length disturbance on a noisy afternoon

15. HEPI summary Remaining tasks Summary
Complete commissioning of final six chambers
Sensor and whitening filter optimization
Scripting, safeties, man-machine interface software
Summary
All hardware installed
Crucial one-arm test completed successfully
LIGO should soon have two sites capable of night and day operation with reasonable duty cycle
Pre-Isolator is first Advanced LIGO subsystem shown to work at required specification in an Observatory setting

16. H1 noise history
(science design) S3 S2 S1

17. Acoustic Noise Coupling
Peaks occur in Hz band
10 to 100x the design sensitivity
Source of H1-H2 correlations
Acoustic Excitations
loud
quiet

18. Acoustic Mitigation Acoustic enclosures around output tables
Reduce couplings
Float optics tables
Simplify beam path
New stiffer periscope
Reduce source
Muffle fan noise at electronics crates
Racks on isolation legs
Move racks
Reduce HVAC noise
Insulate mechanical room 18

19. H2 Faraday Isolator Replacement
Larger aperture to reduce beam clipping
Reduce thermally induced drift at REFL

20. H1 noise history
(science design) S3 S2 S1

21. What is shot noise?
Laser light comes in discrete packets of photons
Poisson statistics of light arrival times at the gravitational-wave port photodiodes
Statistical fluctuation in detected photons appears as phase (i.e. length or strain) noise in interferometer
Signal-to-noise ratio increases as the square-root of laser power in the ifo

22. What can we do about shot noise?
Increase laser power
Lasers refurbishing, now running at ~8W
Approximately 4W incident on interferometers
Input optics-train modified and aligned for better throughput
Additional photodiodes added to antisymmetric port
Additional power produces “thermal lensing” in interferometer optics
Need Thermal Compensation System (TCS)
Ensure other high-frequency noise sources are reduced accordingly
E.g. “oscillator phase noise”

23. Thermal Compensation System (TCS)
Mirrors of the interferometer absorb laser light and heat up  thermal lensing
Mirror profile (shape) distorted according to the laser beam and absorption profiles
Use external laser beam shaped to match the input beam to the mode supported in the arm cavities
CO2 Laser
ITM
ZnSe Viewport
Over-heat pattern
Inner radius = 4cm
Outer radius =11cm
Over-heat Correction
Under-heat Correction
Inhomogeneous Correction

24. Two CO2 lasers installed on H1
To input test mass (ITM)
High Reflectivity surface

25. TCS on the power recycled Michelson Beam images at antisymmetric port
No Heating mW mW mW
Best match
120 mW mW mW Input beam

26. Output Mode Cleaner -- The Good
Carrier contrast defect improves by 20x
With OMC: carrier 2% of total power
Makes it possible to reduce modulation depth
Removes offset corresponding to m
Reduced AM noise coupling: factor of 60 at 3 kHz
Reduced oscillator phase noise coupling: factor of 2 at 3 kHz
ASI signal decreases by a factor of 7
“ASI locking” symmetrizes RF sidebands
Would be able to operate with a single PD at AS port!

27. Output Mode Cleaner -- The Bad

28. Output Mode Cleaner -- The Ugly
Higher order modes and beam jitter generate a PDH-like signal
Elliptical beam is a problem
Triangular cavity geometry is a problem
beam jitter 5o

29. High-frequency noise improvements
Recent high-frequency noise improvement by factor of 2 to 4
Thermal compensation required to support this gain in sensitivity (otherwise thermal lensing would limit such gains)

30. Many other key commissioning efforts underway, including
Parallel efforts
Many other key commissioning efforts underway, including
Control of angular degrees-of-freedom (d.o.f.)
All 14 angular d.o.f. controlled on Hanford 4km interferometer
Input pointing into the interferometers controlled
Acoustic mitigation
Reducing control noise from other feedback systems in the interferometers

31. Time Line 1999 2000 2001 2002 2003 2004 Inauguration First Lock3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q
Inauguration
First Lock
Full Lock all IFO
Now
Engineering E1 E2 E3 E4 E5 E6 E7 E8 E9
Runs S1 S2 S3 S4
(expected)
Science
First Science Data

32. Looking ahead L1 interferometer: H1 interferometer: H2 interferometer:
Finish Hydraulic external pre-Isolator (HEPI)
Get good spectrum back
Implement improvements from LHO 4km interferometer
H1 interferometer:
High power operations
“output mode cleaner” test version 2
New frequency and intensity stabilization, “common mode” boards, non-resonant sideband photodiodes on interferometer reflected output port
Optimize dewhitening for new low-noise digital-to-analog converters
H2 interferometer:
Implement improvements from the Hanford 4km interferometer
Expect 4-6 week science run beginning Jan 2005
On track to reach design sensitivity and begin an extended science run summer 2005