Download Fused Silica

Thermal Compensation Review
David Ottaway
LIGO Laboratory
MIT
Overview
1.
Overview of Problem
2.
Road map for design choices (Set by other systems)
3.
Summary of current results from subscale tests and
modeling
4.
Current known unresolved issues
5.
Plans and resources required for next year
Advanced LIGO Technical Review
2
Thermal Distortion



Absorption in coatings and substrates =>
Temperature Gradients
Temperature Gradients => Optical path distortions
3 Types of distortions, relative strengths of which are
shown below:
Sapphire Fused Silica
Thermo-optic
1
26
Thermal Expansion
0.8
1.6
Elasto-optic Effect
0.2
- 0.3
Advanced LIGO Technical Review
3
Thermal Comparison of
Advanced LIGO to LIGO 1
Parameter
LIGO I
LIGO II
Sapphire
LIGO II
Silica
Units
Input Power
6
125
80
W
PRC
Power
0.4
2.1
1.3
kW
Arm Cavity
Power
26
850
530
kW
Substrate
Absorption
5
Coating
Absorption
0.5
10-40 (30) 0.5-1 (0.5) ppm/cm
0.10.5(0.5)
0.1-0.5
(0.5)
Advanced LIGO Technical Review
ppm
4
Effect on Advanced LIGO
Interferometers
Advanced LIGO Technical Review
5
Adaptive Thermal Compensation





Compensate for distortions in the substrates
Essential for Advanced LIGO sensitivity to be realized
Two parts to thermal compensation:
1. Coarse compensation of thermal lensing
using heating ring and shielding
2. Small scale compensation using scanning
CO2 laser
Accurate measurement of sapphire and fused silica thermal
mechanical properties enable accurate models
Good propagation models to set design requirements (Melody
and FFT Code)
Advanced LIGO Technical Review
6
Requirements that flow from other
systems

•
•
Core Optics (Down select)
Sapphire
-Significant possible inhomogeneous absorption
-> Small spatial scale correction (scanning laser)
-Large thermal conductivity
-> Small amount of coarse compensation (ring heater) on compensation
plates
Fused Silica
-Poor thermal conductivity and homogenous absorption (ring heater)
DC or RF read out scheme (Down select)
-Reduces dependence on sidebands, might affect design requirements
Wavefront Sensing (LIGO 1 experience, not fully understood)
-High spatial quality sidebands are probably necessary for accurate
alignment control, may negate the effect of read out scheme
Advanced LIGO Technical Review
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Summary of Subscale
Experiments and Modeling






Accurate measurements of fused silica and sapphire
material properties
Experimental demonstration of shielded heater ring
coarse spatial correction
Experimental demonstration of scanning CO2 laser
fine spatial scale correction
Accurate models of Advanced LIGO Interferometers
style interferometer using Melody and finite element
analysis (Femlab), (Thermal modeling without SRM)
Scaling from subscale to full scale understood
Work done by Ryan Lawrence
Advanced LIGO Technical Review
8
Thermophysical Parameters
Measurement (295-320 K)
Sapphire (C and A axes)
Fused Silica (Corning 7940)
Parameter
Value
Error
Units
dn/dT
7.2
0.5
ppm/K
Parameter
Value
Error
Units
aa
5.1
0.2
ppm/K
dn/dT
8.7
0.3
ppm/K
ac
5.6
0.2
ppm/K
a
0.55
0.02
ppm/K
ka
36.0
0.5
W/m/K
kth
1.44
0.02
W/m/K
kc
39.0
0.5
W/m/K
Advanced LIGO Technical Review
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Heater Ring Thermal
Compensation
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Thermal Compensation of Point
Absorbers in Sapphire
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11
Sub Scale Scanning Laser Test
Advanced LIGO Technical Review
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Scanning Laser Test Result
Uncorrected Optic (6712 ppm scatter from TEM00)
Corrected Optic (789 ppm scattered from TEM00)
Advanced LIGO Technical Review
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Predicted Effected of Thermal
Compensation on Advanced LIGO
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Current Known Unresolved Issues



Gravitational wave sideband distortion and its effect
on sensitivity. Generated within the cavity no
distortion nulling due to prompt reflection. Greater
understanding through incorporation in Melody (Ray
Beausoleil)
Fabry-Perot mode size change due to input test mass
surface deformation => Spot size change (actuate on
arm cavity faces)
Accurate 2D absorption maps of Sapphire to aid in
actuator selection (negative or positive dN/dT
actuator plates)
Advanced LIGO Technical Review
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Plans and Resources for Next
Year
Plans:
 Work with the Gin Gin Facility to determine prototype
 Further modeling
 Design requirements (29th Oct 2002)
 Preliminary design (14th Apr 2003)
Resources:
 Staffing: Mason (1/5 time), Ottaway (1/4 time)
 Ryan Lawrence graduating and leaving LIGO
 Resources: $50 K in MIT LIGO budget to build
prototype
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