Transcript Slides - Agenda INFN

Substrates, Polishing,
Coatings and Metrology
for the 2nd generation
of GW detector
Laurent PINARD
Laboratoire des Matériaux Avancés – Lyon - FRANCE
Cascina – October 19, 2011
ASPERA Forum
Laurent Pinard
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Overview
 Introduction – The Virgo mirrors
 Advanced Virgo : New mirrors
 Substrates : new type of fused silica
 Polishing : Crucial point = Flatness (Round trip losses)
Cavity simulations, Spec definition, Foreseen polishing
solutions (Corrective coating, Ion beam polishing)
 Metrology : Absorption, Defects, Scattering, Flatness
 Coating : Thickness uniformity, Thermal noise
 Scenario for the 3rd generation of GW detector (ET)
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Introduction : The Virgo mirrors
Mirrors : 35 cm diameter,
10 cm thick
20 kg
LMA : in charge of the
mirror coating
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Introduction : The Virgo mirrors
Silica substrate polishing
(Gooch and Housego, ex General Optics (US))
 Microroughness : < 1 Å rms
 ROC : 3450 +/- 100 m
 Few point defects
 Flatness : < 8 nm RMS on 150 mm
achieved : # 3-4 nm RMS
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Introduction : The Virgo mirrors
SIDE B
measurements
VIRGO
specifications
LMA
mesurements
average
scattering
< 5 ppm
4 ppm
150150 mm2
average
transmission
10 < T < 50 ppm
42,9 +/- 0,2 ppm
150 mm
average
absorption
< 5 ppm
0,63 +/- 0,07 ppm
150 mm
wavefront
< 8 nm RMS
flatness
150 mm
Cascina – October 19, 2011
Coating deposited by Ion Beam Sputtering
3,8 nm RMS
150 mm ASPERA Forum
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Advanced Virgo
 LIGO/VIRGO allow verification of the upper limit predictions
 Event rate too small
 Detector improvement needed
Sensitivity x 10 = rate x 1000
Advanced Virgo approved in Dec 2009
 Main impovement:
- High power laser (200 W)
- New optical configuration
- Heavier mirrors (40 kg)
- Monolithic suspension
LMA : Mirror responsible
~ 5 M€ investment (25% of the total budget)
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Advanced Virgo : The Mirrors
 In Virgo : Round-Trip Losses in the cavity = 400 - 500 ppm
Main Origin : Flatness defects having a period of 1 cm or more
 For Advanced Virgo :
Round-Trip Losses = 75 ppm (specif.)
25 ppm : Abs+Diff+T
AdV IM
50 ppm : Flatness defects
specification very severe
Substrate :
 Low absorption Silica
(Suprasil 3002 - Heraeus)
 Diameter = 35 cm,
 Thickness = 20 cm, Weight = 40 kg
 Unit Cost 130 k€ (without polishing)
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Advanced Virgo : Mirror polishing spec.
Goal : Define a spec. on Flatness for the IM and EM (losses 50 ppm)
PSD(1D) Extraction
Measured maps at LMA
or by the polisher
Artificial maps generated with
the PSD obtained, with
different RMS flatness values
Cavity simulation
PSD shape depends on the polisher ( f-n, n[0;2])
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Advanced Virgo : Mirror polishing spec.
ROC = 1420 m
ROC = 1683 m
0.5 nm RMS = spécification
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Advanced Virgo : Polishing - Solutions
2 solutions identified
« Classical Polishing »
(best as possible)
+
Corrective Coating (LMA)
« Classical Polishing »
+
Ion Beam Figuring
AdV Baseline
LIGO solution
Microroughness preserved
Drawbacks :
More expensive (factor 2-3)
Microroughness # 1.5 Å rms
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Advanced Virgo : Polishing - Solutions
Corrective Coating
Developed in 2005/2006 at LMA
Ion Source
Use of the IBS deposition chamber
developed for Virgo in 2000
Robot
Add material (silica) to suppress
holes and aim at a «perfect» plane
Substrate in
translation
mask
Sputtered
Atoms
Silica Target
Interferometer
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Advanced Virgo : Polishing - Solutions
Corrective Coating
Substrate 156 mm VIRGO type
Before correction (120 mm)
3.3 nm R.M.S.
16 nm P.V.
Defect at the center linked to the robot
(mechanics)
Cascina – October 19, 2011
After correction (120 mm)
0.98 nm R.M.S.
10 nm P.V.
Microroughness preserved (0,5 Å RMS)
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Advanced Virgo : Polishing - Solutions
Corrective Coating
New robot developed
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Advanced Virgo : Polishing - Solutions
Corrective Coating
 CC effect on the PSD : Flatten the spectrum in the low frequency domain up to a
cutoff frequency fc
fc
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Advanced Virgo : Polishing - Solutions
Corrective Coating
 Cutoff frequency : 50 m-1 (correction of defects with a period  2 cm) , reasonably
achievable with the CC
 To reach 0.5 nm rms after CC, we can not start from any flatness
 Simulations (1000) done with artificial maps obtained with the PSD for different
RMS flatness (4 nm like Virgo, 1.5 nm)
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Advanced Virgo : Polishing - Solutions
Corrective Coating
1% of simulations give losses < 50 ppm
CC not sufficiently powerful
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Advanced Virgo : Polishing - Solutions
Corrective Coating
96% of simulations give losses < 50 ppm
Conclusion :
Before CC, flatness at least lower than 1.5 nm rms,
Obtained by « classical » polishing : challenge
Other constraints for the polishers :
- accuracy on the ROC (+/- 10m)
- points defects
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Advanced Virgo : Polishing - Solutions
Ion Beam Figuring
 Always a “classical” polishing phase
 ROC et flatness corrected with a small ion beam
Opposite to the Corrective Coating :
Remove material (silica) instead
of adding material.
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Advanced Virgo : Polishing - Solutions
Ion Beam Figuring
ROC accuracy
+/- 10 m on 2 km
Polisher :
CoastLine Optics
+ASML (US)
LIGO Substrate : 0.21 nm RMS - 2 nm PV (150 mm diam.)
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Advanced Virgo : Flatness Metrology




Based on a phase shifting interferometer (ADE Phase Shift)
MIRAU type interferometer
Working wavelength 1064 nm, Aperture 150 mm, Distortion correction
References flats known by “three flat test” measurement
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Advanced Virgo : Flatness Metrology
 Measurement on larger diameter (320 mm) with stitching interferometry
 Mirror on motorized sample holder
 Measurement of Sub-pupils, total wavefront reconstructed mathematically
Mirror
Y
X
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Advanced Virgo : Flatness Metrology
Flat surface
Advanced Ligo Optic
0.57 nm RMS, 4,7 nm PV (Tinsley)
0.62 nm RMS, 6 nm PV (LMA)
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Advanced Virgo : Flatness Metrology
 Wavefront measurement has to be improved to measure sub-nm surface
flatness on curved surface and large diameter (stitching)
 Reference sphere necessary
 Vibration isolation improvement
 Protection from turbulences
 Improve the way to support the mirror : more stable, no deformation
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Advanced Virgo : Coating Uniformity
 Identical interferometer cavities : Necessary to coat two large
substrates at the same time
 Coating uniformity needed on 80 cm diameter
35 cm
IBS Virgo coating machine
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Twin mirrors
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Advanced Virgo : Coating Uniformity
 Masks between targets and substrates
 Mask shape calculated with a home-made software
 Two different masks for H and L layers, several iterations
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Advanced Virgo : Coating Uniformity
Ti: Ta2O5
0.2% rms on 160 mm
0.08% rms on 160 mm
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SiO2
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Advanced Virgo : Coatings Uniformity
 Mirrors performances matched : same T, same wavefront
 Flatness consistent with thickness profile of monolayers
 Not enough for Advanced detector (0.5 nm RMS)
Mirror n°1
Mirror n°2
1.6 nm rms on 160 mm
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Advanced Virgo : Coatings Uniformity
Recent result (July 2011)
an a 34 cm LIGO mirror
(single rotation)
0.56 nm RMS on 160 mm
Next development :
planetary motion to coat
two substrates at the same
time
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Scenario for the 3rd generation of mirrors
 Einstein Telescope (ET) design study : final document May 2011
 Strategy for the next generation of mirrors
ET – High Frequency
ET – Low Frequency (cryogenic)
 Fused Silica, 1064 nm
 Silicon : new material, 1550 nm
 62 cm diameter, 30 cm thick
 > 45 cm diameter, #50 cm thick,
 200 kg
 211 kg
 Polishing spec.: same as
 Polishing spec.: same as
Advanced Virgo
 Same coating as AdV
Advanced Virgo
 Same coating as AdV
BUT study of the perf. at 1.55 µm
and 10°K necessary
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Scenario for the 3rd generation of mirrors
Cryogenic setup developed to measure at 10°K
the coating quality factor (thermal noise)
and the substrate/coating absorption at 1.55 µm
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