Transcript J. Franc (LMA

Simulation and research for
the future ET mirrors
Janyce Franc, Nazario Morgado, Raffaele Flaminio
Laboratoire des Matériaux Avancés
CNRS
Villeurbanne, FRANCE
Tuesday 18th May 2010
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Contents
• Mirror Thermal Noises
• Solutions to reduce Mirror Thermal Noise
– Find New materials
– Cool down mirrors
– Use Laguerre-Gauss modes
• Mirrors for ET
• Coating research activities at LMA
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Mirror thermal noises and ET
Adv. Virgo
ET
w=6 cm
Infinite mirror
Unless otherwise
specified
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Coating Brownian noise
Reduce the temperature
Sc 
Reduce losses by new coating materials
2kbT c (1   )
2
s
3
 2Ys wf
Increase the Ysub by
changing the material
substrate
Change the beam shape
and size
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Coating materials
TiTa2O5
TiO2
Nb2O5
ZrO2
SiO2
Al2O3

2 10-4
6.3 10-3
4.6 10-4
5.9 10-4
0.5 10-4
2.4 10-4
 (kg.m-3)
6425
4230
4590
6000
2200
3700
 (W.m-1.K-1) 0.6
0.45
1
1.09
0.5
3.3
C (J. K-1.Kg-1) 269
130
590
26
746
310
 (K-1)
3.6 10-6
0.5 10-6
5.8 10-6
10.3 10-6
0.51 10-6
8.4 10-6
 (dn/dT)
14 10-6
-1.8 10-4
1.43 10-5
10 10-5
8 10-6
1.3 10-6
Y (GPa)
140
290
80
200
60
210

0.23
0.28
0.2
0.27
0.17
0.22
n
2.06
2.3
2.32
2.15
1.45
1.63
N.B. Parameters available in J.Franc et al. ET-02109
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Coating materials
Comparison of different coatings
with same transmission (6 ppm)
SiO2-Ti:Ta2O5 coating offers the best result
SiO2-Nb2O5 is also a good candidate (but absorption slightly higher)
N.B. Parameters available in J.Franc et al. ET-02109
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Substrate materials (300K)
TE Noise limits the
Si and Sapphire TN
Silica remains the best
substrate materials at 300K
Also better from an optical
point of view
And available in large
pieces (just need to
increase budget)
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Low temperature: substrate materials
10K
SiO2 is unsuitable
Similar level for Silicon and Sapphire.
Silicon is promising because of its
interest in microelectronics industry.
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Low temperature: Silicon
* Band-to-band absorption reported in
literature (Green et al., 1995)
Silicon absorption
* The total absorption might be higher
due to :
- Impurities
- Residual conductivity
At 1550 nm impurities expected to
be dominant
* The absorption have to be precisely
determined!
* Work in progress at LMA
M. Green and M. Keevers, Optical properties of intrinsic Silicon @
300K, Progress in Photovoltaic research and Applications, Vol. 3, 189192 (1995)
* NB Only 8’’ diameter silicon wafers
with the best optical quality are
available so far.
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Low temperature: Silicon
Large sensitivity variation vs Temperature!
Good results obtained at 4-20K
30K must be avoided
No real interest in working at 18 K because of
Substrate Brownian noise
Coating losses (see next slide)
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Low temperature: Coating losses
18K
10K
I. Martin et al. Class. Quantum Grav. 25 (2008) 055005
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Beam shape (and size)
To reduce Thermal noise the beam shape and size can be changed.
Advantages :
-Best power distribution on the
mirror surface
-Lower Thermal Noise
-Lower Thermal Lensing
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Beam shape: thermal noise reduction
Coating
Brownian
Noise
Substrate
Brownian
Noise
Substrate
TE noise
(frequency
dependent)
TEM00/LG33
Infinite mirror
1.7
1.9
1.06-1.8
TEM00/LG33
Finite mirror
(Ø:45cm, h:30 cm)
1.6
1.4
1.08-1.86
LG55 does not demonstrate better results
ET NOTE with all the results: soon available
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Solutions for ET
Reference : S. Hild et al., CQG, 2010, 27, 015003
Low frequency ITF
High frequency ITF
Temperature
10K
300K
Mirror size
Ø : 45 cm
H : 30 cm
Ø : 62 cm
H : 30 cm
Substrate
Silicon
Silica
Wavelength
1550 nm
1064 nm
Coating
Ti:Ta2O5 & SiO2
Ti:Ta2O5 & SiO2
Phi coat
2.10-4 & 0.5.10-4
2.10-4 & 0.5.10-4
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Solutions for ET
At low frequency : 45 cm diameter + silicon + 10K+ TEM00 is enough
At higher frequency : 62 cm diameter + silica + 300K+ LG33 is possible
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Study of coating losses at LMA
Coating deposit chambers at LMA
* Ta2O5
* Ti:Ta2O5
* SiO2
Mechanical losses measurement at LMA
1. The coating are deposited on
silica cantilevers in the same
coater used for the Virgo mirrors.
2. The Q of the cantilever is
measured before coating
deposition (200 000-300 000)
=> limitation
3. The Q of the cantilever after
deposition is measured and we
can deduce the coating loss
angle.
4. Good system for Tantala. Not
enough sensitivity for silica film.
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Q of silica layers
Cantilever welded to Silica block
Silica block
Welded cantilever
(frequency of
vibration in Hz)
Silica cantilever
1st mode
730 000 (50 Hz)
2nd mode
720 000 (300 Hz)
3rd mode
1 250 000 (800 Hz)
Collaboration INFN Perugia (H. Vocca)
Silica losses (1st mode)
1st cantilever
4.5 (+/- 1) 10-5
2nd cantilever
4.2 (+/- 1) 10-5
In accordance with the reported values of 5.10-5. Need more tests.
The tests with classical cantilever did not permit to measure this value.
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Q of high index materials
Materials
Losses angles
Ta2O5
3 10-4
Ti:Ta2O5
2 10-4
Nb2O5
4.6 10-4
Measurements generally done for a
500 nm thick film.
BUT some variation observed when
different thickness are measured…
Preliminary studies –
Need more measurement
before to conclude!
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Q of multilayer coatings
1. Q of multilayer coating can be deduced from losses of materials and
coating formula.
2. First measurements showed larger losses in multilayer coatings than
foreseen. Thought to be due to defects in the coating coming from poor
cantilever surface quality.
3. Recently, mechanical losses of different multilayer coatings with no
defects have been measured and confirm larger losses than expected.
4. More studies are needed
Some preliminary results :
Coating
Thickness
(nm)
 Coating
measured
 coating
theoretical
 doped-tantala
deduced
0.817H(1.0736L0.8
828H)12 0.0851L
3859
2.5 10-4
1.32 10-4
4.2 10-4
0.87B(HB)20HBB
5222
3.4 10-4
1.34 10-4
5.8 10-4
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Conclusions
- By combining different solutions (larger beams, LG modes, Silicon and low T)
the ET sensitivity seems achievable.
- But a strong R&D program is necessary to:
reduce coating losses
evaluate silicon optical properties
develop LG interferometry
…….
- Study of coating losses at LMA with cantilevers:
Confirm low losses for silica mono layers
Shows some excess of losses in thick coatings to be better investigated
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Solutions for ET
At low frequency : 45 cm diameter + silicon + 10K+ TEM00 is enough
At higher frequency : 62 cm diameter + silica + 300K+ LG33 is possible
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Low temperature: other coating noises?
=6 10-5 (K-1)
At low temperature a lot of the coating
parameters are unknown.
The thermoelastic ( coef.) and
thermorefractive ( coef.) noises can
not be evaluated.
Question: how large should  and  to
become limited by TE and TR noises?
Answer:
-  = 6 10-5 K-1
20 times larger than at 300 K!
-  = 25 10-4 K-1
200 times larger than at 300 K!
Unlikely
Thermal Exp. coefficient limits sensitivity if
value 20 times larger than value @ 300 K
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