Transcript Slides - Agenda INFN

INJ
High power optics for the AdV INJ system
Benjamin Canuel
European Gravitational Observatory
ASPERA Technology Forum 21/10/2011
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Scope of INJ subsystem
INJ
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The Injection system (INJ) of AdV takes care of the optics downstream of
the high power laser, and of the interface of these optics with the laser and
the Interferometer.
●
The whole system must deliver :
• A laser beam at the ITF input port with the required power and size → mode
matching>99% .
• A frequency stability of the laser sufficient to lock the interferometer and reach
AdV sensitivity goal (close interaction with Laser subsystem needed).
• A beam pointing stability sufficient to reach AdV sensitivity goal.
• An adjustable ITF input power (two orders of magnitude) keeping beam
properties unchanged.
• Various EOMs for phase modulation the laser beam used for Interferometer
Sensing and Control.
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AdV INJ design
INJ
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In-air optics:
– EOM system for IMC and ITF control
– IMC mode-matching telescope.
– Input Power Control system (IPC 1)
– Beam pointing control system (BPC)
– Steering optics
– Beam analysis system
– High power compliant optical components
•
In-vacuum optics:
– 144 m long Resonant Input Mode-Cleaner (IMC)
– HP Faraday isolator
– Mode-Matching Telescope (MMT)
– PSL intensity stabilization photodiode
– Reference cavity (RFC)
– Steering optics
– Input Power Control system (IPC 2)
– High power compliant optical components
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INJ
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Dealing with thermal effects in INJ
INJ
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When going from Virgo+ to AdV, one of the main change for
the Injection system is the increase of power: from 50W (20W
for Virgo) to 200W.
This can drive problems related to thermal effects which mainly
affect the following parts :
• The Faraday isolators and in particular the vacuum compatible Faraday
isolator (FI) which should be able to withstand high laser power (up to
about 250W)
• Beam dumps (should dump powerful beams ITF,IMC reflection ).
• Various optics on the benches polarizers, waveplates, Electro Optical
Modulators.
Most probably we will still have to live with a few of them which will
require dynamic control and monitoring of the thermal effects.
• Beam monitoring and a possible adaptive optics system.
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AdV Faraday Isolator
INJ
A Faraday isolator should be
used under vacuum between
PR miror and IMC cavity.
Isolates IMC from light back
reflected from ITF.
Three different spurious effects impact “classical”
Faraday isolator performances when going to high
power:
Thermal lensing
Thermal isolation change
Thermal depolarization
They are all linked to relatively high loss present in
magneto optic crystal (TGG)
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AdV Faraday Isolator: requirements
INJ
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The requirements for the AdV Faraday isolator (FI) are:
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•
•
•
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Isolation factor > 40 dB with 250W laser power going through the FI.
Residual thermal focal lensing > 100 m.
Transmission > 95 %.
Be insensitive to thermal conditions changes going from air to vacuum.
UHV compatible: residual pressure ≤10-6 mbar.
Built in collaboration with the Institute
of Applied Physics
(E. Khazanov group, Russia) [1] [2].
● All the optics of the Faraday isolator
except DKDP crystals have been coated
by LMA (to ensure a good transmission of
the FI).
●
[1]Khazanov et al.,“AdV Faraday isolator design study”, VIR-0245A-10
FI prototype on its breadboard
[2] E.A. Khazanov, O.V. Palashov, I.B. Mukhin, D.S. Zheleznov, A.V. Voitovich, B. Canuel and E. Genin , “AdV INJ: The Faraday
isolator prototype for AdV Description and assembling procedure.”, VIR-0283A-10.
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Thermal lensing compensation
INJ
Thermal lensing in TGG crystals:
•TGG1:2300ppm/cm,TGG2:2600ppm/cm
•Looking for a
compensation.
power
independant
DKDP as a compensation material:
Promising to make passive compensation for
thermal focusing in optical materials because of
its large negative dn/dT values.
dn/dTDKDP=-4.4.10-5K-1
(dn/dTTGG=1.9.10-5K-1)
At 180W: 7m to 35m.
The Virgo Collaboration, “In-vacuum optical isolation changes by heating in a Faraday isolator”, APPLIED OPTICS, Vol. 47, No. 31 (2008).
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Thermal isolation change
INJ

Thermal isolation change:
Is due to the modification of the rotation angle of the FI when the TGG crystal is heated.
Linked to the temperature dependence of the Verdet constant:
1 dV

T
 V dT
Change of isolation when power sent inside the FI is modified or when thermal
condition is changed such as going from air to vacuum.
Effect compensable by introducing a
half wave plate inside the FI [1]
7dB
Under vacuum at 250W by turning
the wave plate we can recover an
optimum isolation
[1]The Virgo Collaboration, “In-vacuum Faraday isolation remote tuning”, Applied Optics, 49, 4780-4790 (2010).
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Thermal depolarisation inside TGG
INJ
Thermal depolarization inside TGG crystal is a limiting effect for Faraday isolation:
Measured by placing TGG without housing between crossed polarizers
depolarisation  (dB)
-25
At high Power[1]:
-30
 P
-35
2
-40
-45
-50
1
10
Power (W)
100
1000
We could show that the limiting “depolarization” effect at high power is in reality a
self induced Spin to Orbit angular momentum conversion .
[1 ]Efim Khazanov et al., APPLIED OPTICS, 41-3, 483-492 (2002)
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Depolarization as mode conversion
INJ
Similar to the birefringence geometry of a q-plate[1],
device developped for the creation/manipulation of
OAM states.
Geometry of birefringence in
heated TGG
SAM
s+
Input
Converted part
 1,0
 1,2
OAM
m=0
SAM
s-
OAM
m=+2
OAM content of converted field were measured by interference and tomography
Compensation…
Left polarization input
[1] S. Mosca, B. Canuel, E. Karimi, B. Piccirillo, L. Marrucci, R. De Rosa, E. Genin, L. Milano and E. Santamato,” Photon selfinduced spin-to-orbital conversion in a terbium-gallium-garnet crystal at high laser power”, Phys. Rev. A 82, 043806 (2010)
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Faraday isolator for Advanced Virgo
INJ
The FI prototype under development includes compensation of all effects
previously mentioned
Nearly fulfills all requirements for AdV:
• Isolation factor 38dB with 250W
• Residual thermal focal lensing > 35 m.
• Transmission ≥ 95 %.
• Almost insensitive to thermal conditions
changes going from air to vacuum
(temperature increase ≈ 6 ⁰C).
• UHV compatible: residual pressure
reached≈10-6 mbar.
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High Power beam dumps
INJ
Mandatory for INJ to dump ITF and IMC reflections.
Diffused light on optics of external benches can spoil the ITF sensitivity
Creates direct and up-converted noise (DL phase modulated by seismic noise).
For AdV, we need high power, low diffusing beam dumps
Home made beam trap
BEST RESULTS:
obtained with
absorbing glass (KG5
type from Schott)
15-30ppm
Damage threshold
1.8W@w0=1mm
5.6W@w0=2.5mm
MEDIUM POWER...
Razor blades (Up to 5W)
Opto sigma (Up to 30W)
3 orders of magnitude worse
4 orders of magnitude worse
HIGH
POWER
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High Power beam dump
INJ
We tested the possibility of using Silicon Plates
•Very good thermal conductivity (150 W.m-1.K-1, comparable with good metal)
•Optical quality surface and diffusion comparable to absorbing glass.
• Large absorption at 1064 nm
Silicon Carbide looks even more promising.
•Even better thermal conductivity.
• Larger absorption at 1064 nm (only a few hundred of microns of thickness necessary)
• Pending : polishing?
T (°K)
Absorbing
glass > 200 ⁰C
Si; ΔT=25⁰C
Damage threshold of SiC and Si has been
measured:
- SiC: 30kW/cm2
- Si: 6kW/cm2
→ SiC is the
best material.
SiC; ΔT=10⁰C
Thermal behaviour of Si, SiC (with 10W) and
absorbing glass (2W).
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High power beam dumps: in-air and in-vacuum
INJ
IN-AIR
DT =37°C at 100W
UNDER-VACUUM
10W
Time constant (to) is 74min.
DT at equilibrium (B) is 96°C
Harder under vacuum:
Material and design
optimized for heat
transmission to the
walls
Successfully installed
on Virgo.
Adapted to limited
Power
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A new idea: viewport beam dump
INJ
Vacuum side
air side
Designed by T. Zelenova
Absorbing element (Si or SiC plate)
→Heat is mainly removed by conduction and use the convection (out of the vacuum) to
remove heat more efficiently than removing it by radiation.
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High Power compatible components
INJ
Polarizers:
•thermal beam distortion
•polarization quality.
Tested Thin Film Plates at Brewster and normal angles
HP cubes, with different bonding technologies.
30 dB of polarization quality easy to obtain with standard Brewster TFPP and cubes
Waveplates: Standard quartz contacted optical plates from CVI 29 to 50 dB.
EO Material: Should stand about 25 kW/cm2, CW. Measurement of
absorption of 3 possible candidates: KTP, RTP and MgO-LiNbO3.
Losses on RTP are <50ppm/cm.
Prototype ready, most critical point RF chain to ensure compliance
with specs in terms of phase/amplitude noise.
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Monitoring thermal effects
INJ
Beam monitoring is an essential part of INJ:
• Monitor residual thermal effects.
• Assess quality of beam at all stages of injection system.
• Provide error signals for tuning and active compensation systems.
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•
•
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Output of EIB → Thermal effects on EIB, Beam quality entering IMC
Transmission of IMC → Thermal effects in IMC, Error signal for active compensation
Input of ITF → Thermal effects in Faraday, Beam quality entering ITF
Reflection of ITF → Thermal effects inside ITF
Beam monitoring system:
•NF camera & FF camera
•Wavefront sensor (Phasics SID4)
•Scanning Fabry-Perot (confocal FP from Coherent)
•Phase camera (scanning heterodyne interferometer)
Measures simultaneously wavefronts of different
fields carrier and sidebands
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ASPERA Technology Forum 21/10/2011
Thermally Deformable Mirror (TDM)
INJ
● Correction of the main wavefront aberrations on the injection bench:
Control of the thermally induced distortions
to optimize the beam coupling at the input of the ITF
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Working principle
• Beam reflection after going through
the mirror substrate
• Modification of the optical path by
conduction with an array of micro-resistors
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Advantages
• Simple and cheap
• No noise introduced
• Ultra high vacuum compatible
Micro-heater array
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TDM: First results
INJ
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Linear response to power change: max~160nm
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Reproducibility better than l/100
Superposition (phase images)
R23 @ 4
mA
R21 @ 6 mA
R39 @ 8 mA
R41 @ 10 mA
the 4 resistors
switched on
together
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AdV INJ status and perspectives
INJ
Activity related to the design/development and the selection of appropriate
components for INJ is almost completed.
● We are currently working on the Final design of INJ : working on the details of
optical design and definition of optics parameters.
● We are almost ready to order parts for the Faraday isolators, the EOMs, IMC
cavity mirrors.
● More work is needed before ordering other optical components: waveplates,
polarizers, super-polished optics,…
→ installation of INJ parts for AdV expected to start in Spring 2013.
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