High Power Input Opticsfor Advanced Virgo

Julien Marque, Benjamin Canuel, Richard Day,Eric Génin, Flavio Nocera, Federico Paoletti

The European Gravitational Observatory is a consortium of:

Outline

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1 – The Advanced Virgo (AdV) injection system2 – The Faraday Isolator3 – The Electro Optical Modulation System4 – Beam Dumps5 – Thermally Deformable Mirror

The AdV Injection System

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In air optics:- Electro Optical Modulation (EOM) system for Input Mode Cleaner (IMC) and Interferometer control- IMC mode matching telescope- Input Power Control system (IPC)- Beam pointing control system- Beam analysis system (wavefront sensor, phase camera)

Input power = 25W (Virgo+), 125W (Advanced Virgo)

In vacuum optics:- 144m long triangular IMC cavity- Faraday Isolator- ITF mode matching telescope- 32cm long triangular Reference Cavity (RFC)- Input Power Control system (IPC)

The Faraday Isolator

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Thermal issue: TGG crystal absorbs (typically 2000ppm), creates change of mean temperature and a radial temperature gradient.

As a consequence, 3 effects can limit the performances:1) Refractive index of TGG temperature dependant (2.10-5 K-1), thermal expansion is not negligible (1.10-5 K-1): induces thermal lensing2) The Verdet constant is temperature dependant: induces variation of mean rotation angle3) Thermal expansion results in mechanical stress: radial birefringence leads to “depolarisation”

Requirements:- 40dB isolation with 200W passing through the Faraday- Residual focal thermal lensing > 100m- Throughput > 95%- UHV compatible, 20mm aperture

Magneto optic medium = TGG crystal (Terbium Gallium Garnet)large Verdet constant, low absorption, high thermal conductivity@ 1064nm

The Faraday Isolator: lensing

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Thermal lensing due to heating of the TGG crystal:Absorption(TGG1) = 2300 +/-100 ppm/cmAbsorption(TGG2) = 2600 +/-100 ppm/cmWithout compensation, thermal lensing = 10m @100W

Solution: add an element on the optical path with negative thermo-optic coefficient.Selected DKDP crystal (Deuterated Potassium Phosphate)Thermo-optic coefficient = -4.10-5 K-1

Absorption = 900ppm/cmFine compensation over large dynamics is obtained by cutting DKDP at the right length.

Pump/Probe beams setup

The Faraday Isolator: rotation angle

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Mean rotation angle of TGG crystal is temperature dependant:

Temperature increase with 250W in vacuum (residual pressure = 2.5 10-6 mbar): 6° (copper holders are used to extract heat).Leads to 7dB drop without compensation.

Solution: add a remotely tuneable half waveplate in the optical path to turn polarisation by 1°.Drawback: 2% of light is lost.

TdT

dV

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1

The Virgo collaboration, “In-vacuum Faraday isolation remote tuning” Appl. Opt 49, 4780 (2010)

The Faraday Isolator: “depolarisation”

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1 10 100 1000Power (W)

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2PgAt high Power[1]:

[1 ]Efim Khazanov et al., APPLIED OPTICS, 41-3, 483-492 (2002)

Most critical problem is the depolarisation at high power:

The gradients of temperature introduce some mechanical stress which creates radial birefringence. Heated TGG acts like complicated waveplate: direction of birefringence axis depends on φ, phase retardation depends on r.

The Faraday Isolator: “depolarisation”

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This problem can be treated using Jones’ matrix formalism:

Good agreement in term of amplitude! What about the phase?

Intensity of the converted beam for 100W circular impinging polarization: measured computed

Conclusion: some part of the beam is acquiring Orbital Angular Momentum (OAM) that is responsible for the non common orbital phase dependence of the beam.Depolarisation was due to a self conversion of Spin to Orbital Angular Momentum.

Intensity of the converted beam for 100W circular impinging polarisation interfered with a reference beam:

S. Mosca, B. Canuel, E. Karimi, B. Piccirillo, L. Marrucci, R. De Rosa, E. Genin, L. Milano and E. Santamato, "Photon self-induced-spin-to-orbital conversion in a terbium-gallium-garnet crystal at high laser power," Phys. Rev. A 82, 043806 (2010)

The Faraday Isolator: “depolarisation”

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Solution: the 2 TGG crystal design [1] (Institute of Applied Physics, Nizhny Novgorod, Russia). The second TGG converts back into the gaussian mode the light that was “depolarized” by the first TGG.

Measurement of Faraday Isolation in final configuration:45dB at low power38dB in vacuum for 240W

[1] E. Khazanov et al, “Compensation of Thermally Induced Modal Distortions in Faraday Isolators”, IEEE Journ. Quant. Electr., 40 (10), (2004)

The Electro Optical Modulation System

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Electro Optical material selected: RTP from Raicol2 sections of modulations (10 MHz and 65MHz) designed to get the highest modulation index with the lowest possible RF power.

Modulation depth measurement (0.5W RF power):m10MHz =0.163m65MHz =0.117

Beam Dumps

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3 materials under tests for making beam dumps: KG5, Si and SiC.Thermal conduction: KG5 (1 W/m/K), Si (150 W/m/K), SiC (490 W/m/K).Damage threshold: KG5 (25W/cm2), Si (6kW/cm2), SiC (30kW/cm2).

KG5, 2W

SiC, 10W

Requirement for scattering is fine with superpolished surface (TIS of 10ppm).How to extract the heat then? A problem in particular for vacuum beam dumps.By radiation towards the tank. Beam dump mount made of sanded copper or pre-baked stainless steel to optimize emissivity (70%).

Si, 10WSiC (30kW/cm2)

Thermally Deformable Mirror

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Slow thermally induced beam wavefront distortions can be compensated using deformable mirrors driven by thermal actuators. The set of heating actuators is placed in direct contact with the reflecting surface of the mirror, enabling an efficient control of its refractive index and shape (vacuum compatible, noise free).

Best suitable material: SF57 (88nm/K) with compared to BK7 (47nm/K)

printed circuit board with thin film resistive layers

Thermally Deformable Mirror

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Simulation of correction of a low order wavefront aberration

Efficiency

Measurement of astigmatism correction (color scale in waves)Measurement setup

Conclusion

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Prototype of Faraday Isolator fulfills almost all requirements up to 250WAll defect mechanisms well understood (thermal lensing passively compensated by a DKDP crystal, Verdet constant change actively compensated by adding an extra half waveplate, “depolarisation” passively compensated using the 2 TGG design)

EOM prototype is satisfactory. Waiting for final requirements (depending on IFO optical scheme) for production and characterisation

High power beam dump material selected and validated

Developed novel adaptive optics for matching input beam into interferometer

Beam analysis system

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