Compensation of thermal effects in future detectors

17/05/2010 A. Rocchi - GWADW 2010 - Kyoto 2

Thermal effects: a brief introduction

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


A. Rocchi - GWADW 2010 - Kyoto

Virgo+ scheme

Mirror B

Mirror A

Single AXICON used to convert a Gaussian beam into an annular beam. Size of the annulus hole can be set by moving L3

Half wave plate and fixed polarizer are used for DC power control. This system does not deviate the beam impinging on the AXICON

To monitor the CO2 beam quality, an infrared camera has been installed on each bench.

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TCS noise: already an issue TCS can inject displacement noise into the detector (see LIGO-P060043-00-Z) Coupling mechanisms:

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

A. Rocchi - GWADW 2010 - Kyoto17/05/2010

TE TR F

Present detectors already require intensity stabilization of the CO2 laser

IR detector noise limited

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A. Rocchi - GWADW 2010 - Kyoto

2.4W total TCS power

4W 6W

6.5W 7W

No TCS,12W IFO

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%.

Virgo+ TCS performances

Coating absorptions play an important role: with new ITMs, you get the same result with only 3W of TCS

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8.5W total TCS power

14.5W IFO

17W IFO

TCS in Advanced detectors/1 One more effect to take care of: displacement of the HR face of all

TMs 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


TCS in Advanced detectors/2

17/05/2010 A. Rocchi - GWADW 2010 - Kyoto

The heating profile must be much more precise than in present detectors 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

axiconOHP

Optimized Heating Pattern

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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


What about 3rd generation detectors?

From S. Steinlechner et al, “Absorptions measurements on silicon”, 2nd ET general meeting, Erice

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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


But if ET is a 2-tone Xylophone? (CQG 27, 2010)

Does ET-HF need TCS?Does ET-HF need TCS?

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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)


Thermal effects in ET-HF

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Compensation plates (same diameter as TMs) properly heated by CO2 laser

Ring heaters to correct mirrors’ radii of curvature


AdVirgo-like TCS…

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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


… or radiative cooling

S. Hild et al

Experiment carried at Caltech with a parabolic mirror (Nucl.Instrum.Meth.A607:530-537,2009.)

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Cold Target

Parabolic Collector

ParabolicReflector

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

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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1m


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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A 10% change in the width of the cooling profile causes a worsening of the thermal lensing compensation quality of a factor of 10

Optical path length.Strength of the residual lens: 3.4·10-4 dioptres 10% larger3.5·10-5 dioptres ideal case-4.8·10-4 dioptres 10% smaller

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Compensation of thermal effects in future detectors