Thermal Compensation in LIGO
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Rupal S. Amin
Thermal Compensation in LIGO
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Outline
LIGO LayoutThermal AberrationsThermal IssuesCompensation SystemsModelClosing Remarks
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Optical Layout
CarrierResonant Sideband pairNon-resonant sideband pair
RM
ETMy
BS
ITMy
ITMx ETMx
Power Recycled Michelson
Fabry-Perot Cavity
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal Aberrations
Causes HEAT GRADIENT
◦Thermal Lensing◦Thermal Expansion◦Induced Birefringence◦…
Effects
◦Differential Expansion
MainLaser Beam
AberrationsCold Hot
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal Issues: eLIGO
PRM Stability Issues
Not stable for RSBCavity g-factor > 1No containment in
cold state
Contrast Defect
Imbalanced thermal lenses increases junk light output
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal Compensation System
TCS Laser Box
Pointing optics
Initial Test MassTowards 4 km arm
CO2 laser light
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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TCS HardwareLIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Solution for eLIGO
(Dramatic music)
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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2) Matlab
0) Matlab1)Comsol
FEMof
ITMs
FINESSE
Interferograms,t(f) results,
Power build up,Beam
parameters
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3
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Model: Matlab+Comsol+FinesseLIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Step 1: FEM of ITMs
Comsol Finite Element Analysis
Simulate mirror heating patterns with and without TCS
Inputs: Boundary ConditionsSubstrate Materials
Outputs: Deformed Surface Thermally Dependent n
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Step 2: In-house sim. code
Config fileWith layout and simple instructions
Luxor GUI config builder
Finesse: http://www.rzg.mpg.de/~adf/Luxor: http://www.aei.mpg.de/~jah/luxor.html
Optical interaction only No mechanical interactions No radiation pressure
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Step 3: FEM + Simulation
LHO AS patterns Mich only
Jan 15, 2009K. Dooley ilog LHO
Simulated Interferogram
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Closing
Interferometers require thermal compensation to maintain stability at high power
Thermal model construction in progress
Sensible control signals are needed
Cannot assume that new TCS behave exactly like old TCS
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thanks to…
Special Thanks toJ. Giaime LSU Associate Prof. of Physics
Phil Willems TCS Group Leader CIT Aidan Brooks Cheryl Vorvick Carl Adams Viginio Sannibale Mohana Mageswaran
NSF grant number:◦NSF-PHY-0605496
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Problems with cut and paste
Unexpected TCS induced ASC signals◦Due to surface expansion and mirror expansion
lzr
lzr
OSEMs
Magnets
Cold
Heated w/ TCS Optical
lever response
OSEM response
~10 mradHalf-story
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Problems with cut and paste
Common DifferentialiLIGO
eLIGO
Sideband power build-up
Working on it
AS_IIn-phase demodulation
Sideband power build-up?
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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8.9985 8.9986 8.9987 8.9988 8.9989 8.999 8.9991 8.9992 8.9993 8.9994 8.9995
x 108
0
0.5
1
1.5
2
2.5x 10
4
QP
Dx
trans
(cts
)
GPS (x)
8.9986 8.9987 8.9988 8.9989 8.999 8.9991 8.9992 8.9993 8.9994
x 108
9325.31
9325.32
9325.33
9325.34
9325.35
9325.36
9325.37
9325.38
GPS time (s)
Fitte
d Fr
eque
ncy
(Hz)
ITMy frequency trends for July 12, 2008 with linear fits
Need to measure mirror absorption ratios
f (Hz)f0 fhot
Heat and cool interferometer mirrors
Results in frequency shifts
FEM of drumhead modefreq. evolution
Step 1: Model actual heatingLIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal issues: eLIGOLIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Problems with cut and paste
Resonant Sideband Imbalance Still Here
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal Issues: iLIGO
-2003-2004 LHO ilog Indicate poor power build-
up PRIOR to mirror replacement
Due to excessive thermal lensing
Sideband overlap and imbalance NOT controlled
-LLO Phase Cam. (A. Gretarsson LLO ilog 2004)
Pwr in
Pwr PRM
S. Ballmer (MIT thesis, 2006)
Resonant Sideband Build-up
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal issues: iLIGO
Effects on Strain Sensitivity Decrease arm cavity gain Arm length sensing Decrease PRM carrier gain Shot noise Decrease PRM SB gain Locking and alignment
precision Increase total carrier contrast defect Shot noise
R. Lawrence (MIT Thesis, 2003)
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Thermal Compensation System
Objective
Control thermal aberrations with auxiliary lasers
◦Reduce optical noise ◦Improve high frequency sensitivity◦Reduce down time with pre-heating
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009
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Step 1: Result
Result:Absorption ratio for ITMX = 3.5 ppm
ITMY = 5 ppm +/- 10%
LIGO-G0900358. Rupal S. Amin GCGW 17 April 2009