Transcript A. Rocchi

17/05/2010
A. Rocchi - GWADW 2010 - Kyoto
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Thermal effects: a brief introduction
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In TM, optical power predominantly absorbed by
the HR coating and converted into heat 
temperature gradient inside the substrate
Two different effects are generated:
 Non-uniform optical path length distortions (thermal
lensing) mainly due to the temperature dependency of the
index of refraction  wavefront distortions of the fields in
the SRC and PRC cavities
 Change of the profile of the high reflective face due to
thermal expansion (thermo-elastic deformation), in both
ITMs and ETMs, affecting the FP cavity. This effect is
negligible in current detectors, but becomes relevant in
advanced IFOs
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Virgo+ scheme
Mirror B
Mirror A
Half wave plate and fixed
polarizer are used for DC
power control. This system
does not deviate the beam
impinging on the AXICON
Single AXICON used to convert
a Gaussian beam into an annular
beam. Size of the annulus hole
can be set by moving L3
To monitor the CO2
beam quality, an
infrared camera
has been installed
on each bench.
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TCS noise: already an issue
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TCS can inject displacement noise into the detector (see LIGO-P060043-00-Z)
Coupling mechanisms:
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Thermo-elastic (TE)- fluctuations in locally deposited heat cause fluctuations in local thermal
expansion
Thermo-refractive (TR)- fluctuations in locally deposited heat cause fluctuations in local refractive
index
Flexure (F)- fluctuations in locally deposited heat cause fluctuations in global shape of optic
Radiation pressure
TE
TR
F
Present detectors
already require
intensity stabilization
of the CO2 laser
IR detector noise limited
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Virgo+ TCS performances
With 14.5W of IFO input power, Virgo+ TCS has been tested looking at the phase
camera images to see the effects of compensation on the shape and position of the
sidebands. The optical gain of the ITF increases by about 50%.
2.4W total TCS power
14.5W IFO
17W IFO
No TCS,
12W IFO
4W
6.5W
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8.5W total TCS power
6W
Coating absorptions play an
important role: with new ITMs,
you get the same result with only
3W of TCS
7W
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TCS in Advanced detectors/1
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One more effect to take care of: displacement of the HR face of all
TMs
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Change of the ROC, decrease of the spot size on TMs, increase of thermal noise of
about 15% (see LIGO-T060083-01-D)
One more actuator: ring heaters to control ROCs of all TMs
Present level of intensity stabilization (10-7/√Hz) not enough to heat
with CO2 directly the TM (10-9/√Hz needed)  compensation plates
required
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TCS in Advanced detectors/2
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The heating profile must be much more precise than in present detectors
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Simple system like an axicon is not enough (see VIR-0182A-10)
Too high HOMs content in RF sidebands for MSRC
Necessity to move to active optical elements (MEMs or scanning systems)
to generate CP heating pattern
Optimized Heating Pattern
axicon
OHP
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What about 3rd generation detectors?
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In a cryogenic IFO with silicon TMs, thermal lensing is likely to
be negligible (to be verified with optical simulations)
 Thermal expansion coefficient tends to zero
 Thermal conductivity increases, higher than 10W/(cm K) between
10 and 100K
 dn/dT is small at low temperatures
From S. Steinlechner et
al, “Absorptions
measurements on silicon”,
2nd ET general meeting,
Erice
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But if ET is a 2-tone Xylophone? (CQG 27, 2010)
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S. Hild et al presented a possible 2-tone
configuration for ET at the Erice Meeting
 High frequency detector:
○ High optical power
○ Room temperature
 Low frequency detector:
○ Low optical power
○ Cryogenic
○ Silicon test masses
Does ET-HF need TCS?
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Thermal effects in ET-HF
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ET-HF uses
 Helical LG33 modes
 Fused silica test masses
 Considering 3MW in the FP cavity and coating
absorptions of 0.5ppm, absorbed power is 1.5W (3 times
higher than in AdVirgo)
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AdVirgo-like TCS…
Compensation plates (same diameter as TMs)
properly heated by CO2 laser
 Ring heaters to correct mirrors’ radii of curvature
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… or radiative cooling
If ET-HF and ET-LF are co-located: there would be a lot of
cryogenics
 Directional radiative cooling working principle (LIGO-G080414
00-R)
 The cold source is imaged onto the centre of the test mass
 The central area of the test mass is in radiative contact with the cold
source
 Heat radiated “towards” the cold source is not returned to the test mass
 The energy balance is negative, the test mass is cooled
S. Hild et al
Experiment carried at
Caltech with a
parabolic mirror
(Nucl.Instrum.Meth.A60
7:530-537,2009.)
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System under test (see VIR-0302A-10)
From radiative cooling to Parabolic Radiative Cooling
 The use of parabolic collectors allows to decrease the
dimensions of the cold targets

ParabolicReflector
Cold Target
Parabolic Collector
How PRC would look like in an IFO
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Experimental set-up
Scaled down system designed, simulated (optically and
thermally) and assembled
 First tests performed using LiN2 to cool the cold spots
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First data
Some technical issues during first tests
Experimental data show some discrepancies with simulations
Work in progress to identify and mitigate stray effects
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300
75
A 10% change in the width
of the cooling profile causes
a worsening of the thermal
lensing compensation
quality of a factor of 10
296
286
0
8hrs
Optical path length.
Strength of the residual lens:
3.4·10-4 dioptres 10% larger
3.5·10-5 dioptres ideal case
-4.8·10-4 dioptres 10% smaller
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