ligo laboratory1 thermal compensation in ligo phil willems- caltech baton rouge lsc meeting, march...
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![Page 1: LIGO Laboratory1 Thermal Compensation in LIGO Phil Willems- Caltech Baton Rouge LSC Meeting, March 2007 LIGO-G070146-00-Z](https://reader036.vdocuments.mx/reader036/viewer/2022062807/5697c0251a28abf838cd5366/html5/thumbnails/1.jpg)
LIGO Laboratory 1
Thermal Compensation in LIGO
Phil Willems- Caltech
Baton Rouge LSC Meeting, March 2007
LIGO-G070146-00-Z
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LIGO Laboratory 2
The Essence of the Problem, and of its Solution
Power recycling cavity Arm cavity
Optical power absorbed by the ITM creates a thermal lens in the (marginally stable) recycling cavity, distorting the RF sideband fields there.
ITM ETMPRM
Add optical power to the ITM to erase the thermal gradient, leaving a uniformly hot, flat-profile substrate.
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LIGO Laboratory 3
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LIGO Laboratory 4
LIGO CO2 Laser Projector Thermal Compensator
CO2 Laser
?
Over-heat Correction
Inhomogeneous Correction
Under-heat Correction
ZnSe Viewport Over-heat pattern
Inner radius = 4cm Outer radius =11cm
•Imaging target onto the TM limits the effect of diffraction spreading
•Modeling suggests a centering tolerance of 10 mm is required
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LIGO Laboratory 5
CO2 Laser Projector Layout
Image planes here, here, and at ITM HR face
over-heat correction
under-heat correction
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LIGO Laboratory 6
Thermal Compensation as Installed
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TCS Servo Control
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LIGO Laboratory 8
Thermal Compensation Controls
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LIGO Laboratory 9
Heating Both ITMs in a Power-Recycled Michelson
No Heating 30 mW 60 mW 90 mW
120 mW 150 mW 180 mW Carrier
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LIGO Laboratory 10
RF Sideband Power Buildup
•Both ITMs heated equally
•Maximum power with 180 mW total heat
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LIGO Laboratory 11
RF Sideband Power Buildup
•Only ITMy heated
•Maximum power with 120 mW total heat
•Same maximum power as when both ITMs heated
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LIGO Laboratory 12
Common-mode Bulls-eye Sensor
Good mode overlap of RF sideband with carrier determines optimal thermal compensation- so we measure the RF mode size to servo TCS.
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LIGO Laboratory 13
Differential TCS- Control of AS_I
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LIGO Laboratory 14
What Is AS_I?
AS_Q: RF sidebands at dark port create swinging LO field- when arm imbalance detunes carrier from dark fringe signal appears at quadrature phase
AS_I: dark fringe means no carrier, RF sideband balance means no LO at this phase- there should be no signal.
Yet, this signal dominates the RF photodetection electronics!--there must be carrier contrast defect--there must be RF sideband imbalance--apparently, slightly imperfect ITM HR surfaces mismatch the arm modes, creating the contrast defect. TCS provides the cure.
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LIGO Laboratory 15
Thermal Time Scales
After locking at high power, the heat distribution in the ITM continues to evolve for hours. To maintain constant thermal focusing power requires varying TCS power.
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TCS Noise Issues
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LIGO Laboratory 17
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LIGO Laboratory 18
TCS Noise Coupling Mechanisms
Thermoelastic (TE)- fluctuations in locally deposited heat cause fluctuations in local thermal expansion
Thermorefractive (TR)- fluctuations in locally deposited heat cause fluctuations in local refractive index
Flexure (F)- fluctuations in locally deposited heat cause fluctuations in global shape of optic
TE TR F
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LIGO Laboratory 19
Flexure Noise- A Simple Model
probe beam
heating
heating
slat mirror
CM lineA skinny LIGO mirror with ‘annular’ heating
The probe beam sees the mirror move at the center due to wiggling far from center
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LIGO Laboratory 20
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LIGO Laboratory 21
TCS Injected Noise Spectrum
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Quality of Compensation
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LIGO Laboratory 23
Projector Heating Patterns
Annulus Mask Central Heat Mask
•Intensity variations across the images due to small laser spot size
•Projection optics work well
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LIGO Laboratory 24
‘Gold Star’ Mask Design
“star”- from hole pattern“gold”- gold coating to reduce power
absorptionHole pattern is clearly not ideal but
diffraction and heat diffusion smooth the phase profile
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LIGO Laboratory 25
Improved Carrier Power with Gold Star Mask
Why this helps the carrier is mysterious, but we’ll take it
optical gain up 5%
carrier recycling gain up 10%
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Enhanced LIGO TCS
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LIGO Laboratory 27
Our Need for Power
Initial LIGO runs at ~7W input power Enhanced LIGO will run at ~30W input power
» 4-5x more absorbed power
» Naively, ~4-5x more TCS power needed
» Practically, more power even than this may be needed since LIGO point design is meant to make TCS unnecessary at 6W
Our current projectors are not adequate
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LIGO Laboratory 28
Test Mass Absorption Measurement Technique-Spot Size
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LIGO Laboratory 29
Test Mass Absorption Measurement Technique-Acoustic Frequencies
test mass acoustic frequencies vary with temperature, so monitor their drift as the IFO power is varied
requires no additional optics measures all ITMs and ETMs
simultaneously
Many thanks to Alessio Rocchi & Viviana Fafone from Virgo for showing us this could work
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LIGO Laboratory 30
Measured Test Mass Absorption
H1 ITMX H1 ITMY H1 ETMX H1 ETMY H2 ITMX H2 ITMY H2 ETMX H2 ETMY L1 ITMX L1 ITMY L1 ETMX L1 ETMY
-5
0
5
10
15
noitprosbampp
H1 ITMX H1 ITMY H1 ETMX H1 ETMY H2 ITMX H2 ITMY H2 ETMX H2 ETMY L1 ITMX L1 ITMY L1 ETMX L1 ETMY
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LIGO Laboratory 31
Enhanced LIGO TCS Projector
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LIGO Laboratory 32
Axicon design proposed by II-VI for Enhanced LIGO
The Axicon