Transcript G080076-00

Recent Results on the Measurement
of Transmission and Scattering
Structure on Doped and Non-doped
Mirrors
Bilenko I.A. Gromova E.S.
March LSC Meeting
March 21, 2008
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Bilenko I.A. Gromova E.S.
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Coatings investigations:
motivations and goal
• We decided to test coatings for presence of small (starting from the size
compatible to wavelength) rare inhomogeneities (defects).
• Such defects may not significantly contribute to the total scattering and
absorption budgets nor be visible on the spatial microroughness Fourier
spectra.
• Such defects may, in principle, became sources of excess mechanical noise
because stress release and optical breakdown thresholds could be much lower
in that points.
• Simple scanning of transmission and scattering is used as a first step before
more sophisticated investigation or direct noise measurements.
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Transmission and scattering
measurements setup: diagram
Mirror sample
Vacuum chamber
AOM
YAG laser
Transmitted light detector
CCD camera for transmitted beam
control
CCD camera for fine focusing
control
Flexible feedthrough
Photoelectron multipliers for
scattered light
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3D transmission stage
(2 high precision stepper motors,
1 PZT drive)
Bilenko I.A. Gromova E.S.
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Transmission and scattering
measurements setup: photo
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Transmission and scattering
measurements setup: features
• 1 mm spatial resolution (close to diffraction limit)
• 1-2 % transmission measurement accuracy for mirrors with
T=10-4 (10 kHz modulation-demodulation technique is used)
• 0.5-1 % scattering variations measurement accuracy
• Power density up to 300 kW/cm2
• Permanent focusing control and correction
• Approx. 5 minutes for the single 500x10 mm strip scan
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Preliminary measurements:
100 mm plates.
Substrate: Russian SiO2, deep polishing. Coating: Ta2O5/SiO2, T=5x10-3
Caltech order (REO?).
Example of the strip with a “spot” where transmission is 12% below
average value (upper chart). No significant changes of scattering in this
place (lower chart)
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Samples preparation: cleaning
0,80
0,76
0,74
0,72
0,70
2
4
6
Y, mm
8
10
100
200
300
400
X, mm
After re-cleaning
0,80
0,78
0,76
0,74
0,72
0,70
2
4
6
8
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Bilenko I.A. Gromova E.S.
T, arb
0,78
10
100
200
300
X, mm
400
7
Y, mm
T, arb
• Ultrasonic pure acetone and methanol
bath has been used.
• We also tried CO2 “snow” cleaner (good
results, can be easily applied for the
cleaning samples in situ)
• In order to prove that the observed
“spots” aren’t a residual surface
impurities, same areas were tested before
and after re-cleaning. There is a good
evidence that they are not (but still no
100% guarantee).
• Sometimes small (1 micron or less) very
bright scattering centers disappeared after
the re-cleaning: presumably dust
particles.
Before re-cleaning
Different types of observed
defects
1,1
1,0
Transmission peak, no extra
scattering
T, arb
0,9
0,8
2
4
6
Y, mm
8
10
Transmission and scattering
peaks at the same place
100
200
300
400
500
X, mm
3,0
2,8
2,6
2,4
Wide scattering peak, no large
transmission variation at the place
S, arb
2,2
2,0
2
4
6
8
Y, mm
10
100
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200
300
X, mm
400
500
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High power density test
0,90
0,88
Before exposition:
3,80
3,75
0,86
0,84
3,70
T, arb
S, arb
0,82
3,65
0,80
2
3,60
4
2
Y, mm
6
8
4
10
100
200
300
Y mm
6
8
400
10
100
X, mm
200
300
400
X mm
0,90
0,88
0,86
0,84
0,82
0,80
3,80
After exposition:
15 mW, 5 mm2
T, arb
(~300 kW/cm2)
for 30 minutes
3,75
3,70
3,65
3,60
2
2
4
6
8
4
Y, mm
6
8
10
100
200
300
400
100
X, mm
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Y, mm
10
200
300
400
X, mm
Bilenko I.A. Gromova E.S.
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S, arb
High power density test
(Ti-doped Ta2O5/SiO2: С07041121)
0,50
Before exposition:
2,0
0,48
1,9
0,46
1,8
0,44
T, arb
1,7
0,42
0,40
1,6
2
2
4
4
6
100
200
300
400
6
Y, mm
8
10
Y, mm
8
10
S, arb
100
200
500
300
400
500
X, mm
X, mm
0,50
2,0
1,9
1,8
1,7
1,6
After exposition:
15 mW, 5 mm2
2
T, arb (~300 kW/cm )
for 30 minutes
0,48
0,46
0,44
0,42
0,40
2
2
4
4
6
8
10
100
200
300
400
6
8
10
Y, mm
100
500
300
400
500
X, mm
X, mm
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Y, mm
Bilenko I.A. Gromova E.S.
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S, arb
Measurements summary
Ta2O5/SiO2 on
100 mm plate
T=5x10-3
Ta2O5/SiO2 on
superpolished 1” SiO2
substrate. T=1x10-4
(LIGO Lab provided)
Ti-doped Ta2O5/SiO2 on
superpolished 0.5” SiO2
substrates. T=1x10-4 (LIGO
Lab provided: С07041121,
С07041122)
Total scanned area
0.5 mm2 (85 strips)
0.1 mm2 (20 strips)
0.3 mm2 (62 strips)
Number of spots observed*
81
7
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Estimated spots area (% of scanned
area)*
0.008 mm2 (1.5%)
0.0006 mm2 (0.6%)
0.002 mm2 (0.6%)
Maximum deviation of transmittance
from average value
60%
7%
30%
Maximum deviation of scattering from
average value
220%
30%
90%
* Deviation of transmittance and/or scattering from the average value exceeds 2%.
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Distribution of the spot number
on the transmission value
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Number of spots
Histogram for the distribution
of the spot number on the
transmission value for the 100
mm plates (Ta2O5/SiO2,
T=5x10-3 ).
10
8
6
4
2
0
<-50 -50..-20 -20..-5 -5..-1 +1..+5 +5..+20 +20..+50 >+50
Deviation of the transmission
from the average value, %
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Discussion and conclusion
Assuming that in the case when transmittance in a spot drops due
to the additional adsorption, it is possible to estimate upper limit
for the spot heating in AdvLIGO case:
T0 
Wads
2 kfd
0.06 K
(Wads=25 mW, kf=33W/m K, d=10-6 m)
Observed defects seems not to be dangerous for mirrors
performance …so far.
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Future plans
 Make scan area wider (80 microns instead of 10) by installation
stacked PZT drive
 Reduce temperature drifts and mechanical backlash for better
repeatability.
 Implement an absorption measurement (?)
 Collect more data to obtain statistically significant difference between
samples (or to confirm absence of difference)
 Test another coating materials and techniques.
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