High Power Input Optics
for Advanced Virgo
Julien Marque, Benjamin Canuel, Richard Day,
Eric Génin, Flavio Nocera, Federico Paoletti
The European Gravitational Observatory is a consortium of:
Outline
1 – The Advanced Virgo (AdV) injection system
2 – The Faraday Isolator
3 – The Electro Optical Modulation System
4 – Beam Dumps
5 – Thermally Deformable Mirror
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The AdV Injection System
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)
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)
Input power = 25W (Virgo+), 125W (Advanced Virgo)
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The Faraday Isolator
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
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 lensing
2) The Verdet constant is temperature dependant: 1 dV
= 3.510-3 K -1
induces variation of mean rotation angle
V dT
3) Thermal expansion results in mechanical stress: radial birefringence leads to
“depolarisation”
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The Faraday Isolator: lensing
Thermal lensing due to heating of the TGG crystal:
Absorption(TGG1) = 2300 +/-100 ppm/cm
Absorption(TGG2) = 2600 +/-100 ppm/cm
Without compensation, thermal lensing = 10m @100W
Solution: add an element on the optical path with negative thermooptic coefficient.
Selected DKDP crystal (Deuterated Potassium Phosphate)
Thermo-optic coefficient = -4.10-5 K-1
Absorption = 900ppm/cm
Fine compensation over large dynamics is obtained by cutting
DKDP at the right length.
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Pump/Probe beams setup
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The Faraday Isolator: rotation angle
Mean rotation angle of TGG crystal is temperature
dependant:



1 dV
T
V dT
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.
The Virgo collaboration, “In-vacuum Faraday isolation remote tuning” Appl. Opt 49, 4780 (2010)
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The Faraday Isolator: “depolarisation”
Most critical problem is the depolarisation at high power:
depolarisation g (dB)
-25
-30
-35
At high Power[1]:
-40
g P
-45
-50
1
10
Power (W)
100
1000
2
[1 ]Efim Khazanov et al., APPLIED OPTICS, 41-3, 483-492 (2002)
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.
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The Faraday Isolator: “depolarisation”
This problem can be treated using Jones’ matrix formalism:
Intensity of the converted beam for 100W circular impinging polarization:
measured
computed
Good agreement in term of amplitude! What about the phase?
Intensity of the converted beam for 100W circular impinging
polarisation interfered with a reference beam:
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.
S. Mosca, B. Canuel, E. Karimi, B. Piccirillo, L. Marrucci, R. De Rosa, E. Genin, L. Milano and E. Santamato, "Photon self-inducedspin-to-orbital conversion in a terbium-gallium-garnet crystal at high laser power," Phys. Rev. A 82, 043806 (2010)
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The Faraday Isolator: “depolarisation”
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 power
38dB 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)
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The Electro Optical Modulation System
Electro Optical material selected: RTP from Raicol
2 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 =0.163
m65MHz =0.117
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Beam Dumps
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
Si, 10W
SiC (30kW/cm2)
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%).
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Thermally Deformable Mirror
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).
printed circuit board with thin film resistive layers
Best suitable material: SF57 (88nm/K) with compared to BK7 (47nm/K)
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Thermally Deformable Mirror
Efficiency
Simulation of correction of a low order wavefront aberration
Measurement setup
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Measurement of astigmatism correction (color scale in waves)
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Conclusion
Prototype of Faraday Isolator fulfills almost all requirements up
to 250W
All 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
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Beam analysis system
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