Transcript G030337-00 - DCC

Thermal noise from optical
coatings
Gregory Harry
Massachusetts Institute of Technology
- on behalf of the LIGO Science Collaboration 25 July 2003
10th Marcel Grossman Meeting
Rio de Janeiro, Brazil
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Yuri Levin's Theorem
Sx(f) = 2 kBT / (p2 f2) Wdiss/F02
•
Levin's theorem more easily handles loss inhomogeneities than
modal expansion
•
Coatings contribution to thermal noise high because of
proximity to laser
•
Other mirror losses (magnets, wire, standoffs) less important
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Theory of Brownian thermal
noise from coatings
1 (1  sub) d Ycoat
Yub
readout  bulk
(
coat || 
coat  )
Ycoat
p (1  2sub) w Ysub
•
Derived from Levin's theorem (Gretarsson et al)
•
Derived independently (Nakagawa et al)
•
Dependance on coat||, coat+, sub, Ycoat, and Ysub
• Noise
decreases as laser spot size increases
Plan to use largest possible (6 cm) spots in Adv LIGO
•
Assumes infinite mirror substrates
FEA modeling by Numata et al shows noise slightly
lower for finite mirrors
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Sapphire Mirrors
160 Mpc BNS Range
Silica Mirrors
140 Mpc BNS Range
10-22
10-22
h (1/Hz1/2)
h (1/Hz1/2)
Advanced LIGO
sensitivity
10-23
10-24
10-23
10-24
10-25
10-25
10
100
f (Hz)
1000
10
1000
100
f (Hz)
Coating used for Initial LIGO (REO tantala/silica)  = 1.5 X 10-4
Advanced LIGO target 200 Mpc BNS Range
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Mechanical loss in
tantala/silica coatings
• Measured Q's of initial LIGO coating on
silica disks
• Measured coatings with varying thickness
and number of layers
Loss depends on amount of materials
Independent of number of layers
• coat||
• silica
• tantala
• coat+
=
=
=
=
2.7 +/- 0.7 10-4 for Q measurements
0.5 +/- 0.3 10-4
4.4 +/- 0.5 10-4
1.5 +/- 0.3 10-4 for thermal noise
• Good agreement between coatings from three
vendors (REO, MLD, SMA/Virgo)
• Loss too high for Advanced LIGO sensitivity
Monolithic suspension and
birefringence readout for thin
silica sample coating
measurments
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Advanced LIGO sensitivity vs
coating loss angle
9 Pa
YBNS
=
200
10
coatRange
 for
vsY = 200 GPa
9
Ycoat
= 70
10
BNS
Range
for
vscoat
Y
= 70Pa
GPa
coat
210
210
Silica 200 million Q
Silica 200 million Q
Binary Neutron Star Inspiral Distance (Mpc)
200
190
180
Silica 130 million Q
Sapphire 200 million
Sapphire 60 million
170
170
160
160
150
150
140
140
130
130
120
0.0E+00
Binary Neutron Star Inspiral Distance (Mpc)
Silica Q=200Silica
10130 million Q200
6 200 million Q
Silica Q=130Sapphire
10
Sapphire 60 million190
Q
Sapphire Q=200 106 180
Sapphire Q=60 106
6
2.0E-05
4.0E-05
6.0E-05
Coating

8.0E-05
1.0E-04
120
1.2E-04
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
Coating

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1.2E-0
Alternate materials in optical
coatings I
Materials other than silica and tantala have been examined
•
Low index material : Alumina (Al2O3 with Ta2O5)
Mechanical loss
From General Optics al2o3 consistent with 0
From MLD
al2o3 = 2.4 10-4
Optical loss about 2 ppm after annealing (goal <1 ppm)
Yal2o3 > Ysio2
•
High index material: Niobia (Nb2O5 with SiO2)
Mechanical loss nb2o5 = 6.7 10-4
Optical loss about 0.3 ppm after annealing (goal <1 ppm)
Ynb2o5 < Yta2o5
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Alternate materials in optical
coatings II
Tantala/silica with dopant added to tantala
•
Dopant is proprietary (SMA/Virgo)
Young's modulus unchanged from Ta2O5 to 0.2 %
Index of refaction unchanged from Ta2O5 to 1 %
Mechanical loss ta2o5 = 2.1 10-4 (was 4.4 10-4)
•
Doped tantala/silica coating in Advanced LIGO
Mechanical loss coat+ = 9.0 10-5
BNS Range 145 Mpc (was 140 Mpc)
•
Work is continuing on dopants in coatings
Possibly related to stress reduction ?
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Theory of thermoelastic
noise from coatings
•
Recent work shows that thermoelastic damping between
the coating and the substrate can be a significant source
of thermal noise (Fejer, Rowan et al, Braginsky et al)
•
Match thermal expansion between coating and substrate
•
Some rough loss values for coating/substrate matches
Silica coating on sapphire
 ~ 1 10-3
Silica coating on silica
 ~ 1 10-5
Alumina coating on sapphire  ~ 2 10-5
Alumina coating on silica
 ~ 2 10-4
•
Baseline is sapphire substrate with alumina in coating
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Future plans:
Improved coatings
•
Coating vendors are responding to request for proposals
Multiple international vendors have replied
Two vendors for R&D phase
One (possibly two) vendors for production of optics
•
Three directions of research
New materials - hafnia, zirconia, titania, alloys
Dopants - aluminum, titanium, designed to reduce stress?
Annealing - known to improve loss in silica
•
Input solicited from material scientists and others
•
Correlate loss with stress in coatings
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Conclusions
•
Internal mode thermal noise fundamental limit to
gravitational wave interferometer sensitivity
•
Thermal noise from coatings represent significant
part of overall thermal noise
•
Noise depends on many thermal and mechanical
parameters of coatings as well as spot size
•
Tantala/silica coatings have been characterized, but
do not meet Advanced LIGO goals
•
Other materials and techniques are being explored
•
Collaboration and plan in place to find a workable
coating for advanced LIGO
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Future plans II:
Measurements
Coatings need to be characterized for all relevant parameters
•
Mechanical loss -ringdown Q experiments (MIT, Glasgow, Stanford,
and Hobart and William Smith)
•
Optical loss- absorption measurements (Stanford)
•
Young's modulus - acoustic reflection experiment (Stanford)
•
Thermal expansion - optical lensing experiment (Caltech, Stanford)
•
Direct thermal noise measurement - (Caltech, Hongo)
Interferometers to measure thermal noise in short cavities
Two different spot sizes ( ~50mm at Hongo, 160 mm at TNI)
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Advanced LIGO sensitivity vs
coating Young's Modulus
coat = 1 10-5-5
-5

=
5
10
-5
BNS coat
Range vs
 Y=for
5 10
BNS Range vscoat
Y=
for
1 10
coat
210
200
200
190
190
180
180
170
170
160
160
150
150
140
Silica 200 million Q
Silica 130 million Q
130
Sapphire 200 million Q
Sapphire 60 million Q
120
10
Binary Neutron Star Inspiral Distance (Mpc)
Binary Neutron Star Inspiral Distance (Mpc)
210
140
130
Silica Q=200 106
Silica Q=130 106
Silica 200 million Q
6
Sapphire Q=200
10million
Silica 130
Q
Sapphire 200
Sapphire Q=60
106 million
Sapphire 60 million
120
100
Coating Young's modulus (GPa)
1000
10
100
Coating Young's modulus (GPa)
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1000