thermal effects and their compensation in advanced virgo

Thermal effects and their Thermal effects and their compensation in Advanced Virgocompensation in Advanced Virgo

E. Coccia1,2, M. Di Paolo Emilio2, V. Fafone1,2, V. Malvezzi2, Y. Minenkov2, A. Rocchi2, L. Sperandio1,2.

1University of Tor Vergata2INFN Roma Tor Vergata

46th Rencontres de Moriond46th Rencontres de Moriond26.03.201126.03.2011

Thermal effectsBrief introduction

Compensation principleAn example: Virgo+ TCS

Thermal Compensation System for AdVLayoutHeating pattern optimizationRing heater developmentWave-front sensing

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Thermal effects: introduction

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z

TE dzzrTdT

dnrS ),()(

High Reflectivity coating and substrate of the Test Masses absorb some (O(ppm)) power stored in the Fabry-Perot and recycling cavities.

Due to the low thermal conductivity of SiO2, a thermal gradient is established in the substrate.

Thermal lensing: The refraction index is temperature dependent

(dn/dT≠0, O(ppm/K)). The optical path inside the substrate of the TMs

is not uniform. This is equivalent to putting a lens in the

substrate of the ITMs. Thermo-elastic deformation of the HR surface

of all the TMs.

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Thermal effects: consequences Thermal lensing: wave-front distortions of the

fields in the SRC and PRC cavities. Decrease of the optical gain in the RCs; If lensing is too strong, ITF cannot acquire the lock. FOM: coupling losses.


Thermo-elastic deformation: changes the RoC in both ITMs and ETMs, affects the FP cavity. This effect is negligible in current detectors, but becomes relevant in advanced IFOs. Decrease of the spot size on TMs increase of thermal noise of about 15%.

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2( ) ( )r OPL r

( )20 0 0 ( )

00 0 0 0 0

( ) 2i r a

i rE E E e ES E r e rdr

E E E E

*1L S S [ppm]


Thermal lensing is only due to the temperature gradient along the radial direction.

So, we can heat the peripheral of the test mass to flatten the gradient and, thus, the optical path length.

Working principle of TCS

z

TE dzzrTdT

dnrS ),()(

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Current compensation systems

All use CO2 (=10.6m) lasers to heat the peripheral of the input test masses.

CO2 Laser

Annular heating Gold star mask

AXICON

Initial LIGOEnhanced LIGO and Virgo+

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A. Rocchi - Rencontres de Moriond 2011- La Thuile

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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Noise from TCS TCS can inject displacement noise into the detector. 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 (negligible)


TE TR F

Present detectors already require intensity stabilization of the CO2 laser.

Achievable level of intensity stabilization (10-7/√Hz) not enough to heat with CO2 directly the TM in advanced detectors (10-9/√Hz needed) compensation plates required.

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RIN = relative intensity noise

9

Green dots: heating ringsBlue rectangles: CPs

Compensation plates shined with CO2 laser will correct thermal effects in the RCs

Ring heaters will compensate HR surface deformations

This set up allows to control independently the thermal lensing and the ROCs


TCS in Advanced detectorsPower absorbed by TMs is about 0.5W, wrt ~20mW in initial detectors

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Some details• RH at 5 cm from the anti-reflecting face of the TM to maximize the efficiency• CP diameter: 350 mm (same as TMs)• CP thickness = 35 mm• CP must be “far” from the TM. Distance fixed at 200 mm

Sym

me

try

axi

s

Heat escaping from the barrel of the CP

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TM heated by radiation fromthe CP if CP-TM distance = 1 cm

TM Tmap

HR face

Sym

me

try

axi

s

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CP heating pattern


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

Simple system like an axicon (Virgo-like) is not enough Too high content of higher order modes in RF sidebands

Necessity to optimize the CP heating pattern Linear iterative optimization process based on FEM developed, to

take into account radiative coupling btw TM and CP and the presence of the RH. Uses the OPL increase as error signal.

11

with single axicon = 2200 ppm

Optimal heating patternOptimal heating pattern Resulting Optical Path LengthResulting Optical Path Length

Residual coupling losses= 26 ppm

Coupling losses = superposition integral between “unperturbed” beam and single pass through thermal lensing.

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No TCS = 5·105 ppm

Solution using known technology:modulate rings dimensions by changing distances between lenses and axicons and modulate power in each ring (Double Axicon System)

-With DAS, residual coupling lossesaround 102 ppm.Residual focal length O(103 km)-With single axicon, residual coupling losses around 2000 ppmResidual focal length O(102 km)

Heating pattern generation: DAS

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The beam is split by polarization and recombined with round polarizer at 45 deg AOI. The optical layout is very flexible! Allows to change independently the two rings:

Inner and outer diameters of each ring Power density of each ring

Heating profile simulated with Zemax gives 200 ppm coupling losses Experimental DAS gives 210 ppm


Experiment vs. simulation

Heating pattern generated by the DAS

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Main concerns: Stray magnetic field: AdV will use electro-magnetic actuators to control the TMs; Vacuum compatibility.

Down-scaled prototypes with different geometries and materials realized: Annular winding with counter-flux coils; Toroidal winding:

○ Single coil;○ Double coil (counter-flux).

Magnetic field measurements: Detailed maps as a function of the position; Frequency domain transfer function; 3D magnetic field simulations.

Annular winding Toroidal winding

Ring heater development


x

y

z

Measured Simulated

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Noise projection• Simulated a RH of 400 mm diameter and considered the magnetic field and gradient in the position of the magnets• Assumed a flat current noise of 1nA/Hz


z

B

Lm

gh zz 2

LI

BDgh yx

yx 2

,,

g = magnets symmetry = magnetic momentm = mirror massI = moment of inertiaL = arm cavity lengthD = beam displacement

B

D

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Materials optimization


To avoid contamination from the insulation of the wire we have chosen a design with bare wire on insulated support.

Under investigation:glass corealuminum core with insulating coating

Aluminum samples machined and coated with:inorganic paint plasma spray with aluminadiamond like coating

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The full TC System to be implemented on Advanced Virgo will comprehend sensors which sample the thermal wave-front distortions within the interferometer, either from samples of the IFO beam picked off at appropriate points, or directly from the optics themselves using dedicated probe beams and servo electronics that will derive control signals from the sensors and feed them back to the heater elements.

Advanced Virgo will use a Hartmann sensor developed by the University of Adelaide, that has demonstrated to have a shot-to-shot reproducibility of λ/1450 at 820 nm, which can be improved to λ/15500 with averaging (Opt. Express 15 (16), 10370-10375, 2007).

TCS sensing scheme


Measurement taken at the Gingin HOPTF

L

x

x

W

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Thermal compensation systems proven to be efficient and reliable in initial detectors: recovered 85% of sidebands recycling gain.

In advanced detectors, thermal effects must be corrected with higher precision: More power in the cavities; Larger spot size on ITMs; One more effect to take care of: TMs RoC.

Good experimental results from the DAS: Good agreement with simulations; Easy to assemble; High degree of flexibility.

Ring heater design is progressing. High sensitivity wave-front sensor identified for advanced

detectors.

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thermal effects and their compensation in advanced virgo

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