IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 1
IB in vacuum Faraday isolator
E. Genin, S. Hebri, S. Hamdani, P. La Penna, J. MarqueEGO
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 2
IB Faraday isolator in vacuum
Why a Faraday isolator between the IMC and the PR: the ITF reflected light (with PR aligned) induced noise in the IMC (converted in frequency noise): impossible to lock the recycling cavity at full power
Beam out of IMC: 5 mm waist: need of a large Faraday isolator
Two candidates: EOTech (larger aperture) and Isowave (lower absorption)
The Faraday isolator:▪ Electro Optics Technology (EOTech), with 20 mm aperture, vacuum compatible:▪ Crystal: TGG (1.8 mm length), ▪ ~1 T magnetic field ▪ two Brewster thin film polarizers (Karl Lambrecht) ▪ about 20 cm long ▪ 3.5 kg weight ▪ 10,000 $ cost
The Faraday isolator had been tested in Nice with 20W Yag beam in air: more than 40 db isolation (factor 10,000 in power), no significant thermal lensing effect
After the new IB installation, with the Faraday, it was possible to lock soon the recycling cavity with full power
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 3
Brewster dielectric polarizers
Rotator
February 2005
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 4
20W Pump
HeNe
Shack-Hartman
Faraday
2f2f
(F.Cleva)(F.Cleva)
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 5
20 W Nd:YAG input power:
Two rods:
• Rod 1, – Beam waist 1.1 mm: focal length = 31 m– Rod 1, Beam waist 2 mm : focal lenght = 90 m
Fth ~ 1/2
– Rod 2, Beam waist 2 mm : focal length = 82 m
First rod less absorbing than second one
Optical isolation: larger than 42 dB (less than 0.008 W backreflected)
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 6
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 7
Where is the IB Faraday
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 8
Two main problems experienced:
Change of isolation from air to vacuum
Thermal lensing:• Change of beam shape and beam/ITF matching
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 9
Faraday isolation measurement
Power reflected measured on the Laser Bench (rejected by the last Laser Bench Faraday).
LB Faraday
Power meter
SIB Faraday
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 10
Isolation was less than 100
In order to optimize the attenuation (40 dB) the first polarizer had to be rotated by about 5 degrees:
Cleary some problem there
Input light (7 W)
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 11
Explanation
1) This could be explained by a change in the magnetic field (by 10 mT, total field about 1 T)
2) Explanation: the internal magnetic field was perturbed by the presence of an Allen key inside
3) After removal the first polarizer was turned back, isolation on Laser Bench (in air) was better than 40 dB, second polarizer reflection of the order of 100 mW
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 12
Explanation
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 13
Isolation tuning
After Allen key removal the polarizer could be turned back close to the nominal position (defined by a marker)
In this condition we tuned the isolation: measured on the laser bench it was more than 10,000
We blocked the Faraday and realigned optics and mounts in this condition
Then the IB tower was closed, evacuated, and things changed: the isolation dropped to less than 1,000
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Isolation: before (WS1) and after (WS2)
Purple: WS1Black: WS2
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Isolation improvement
0.05
0.75
Black: WS1 (before improvement)Purple: WS2 (after improvement)
More than factor 10 improvement (factor 15)
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 16
Absolute power measurement (power meter)
1V = 30 mW
12 mW
Isolation: 9.5 W /12 mW > 800
(it was 10,000 in air: more than factor 10 lost)
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 17
Isolation improved by more than a factor 10 (but 800 now)
No explanation why it is not 10,000, as measured with the IB tower open:
Different alignment?Temperature?
Remote tuning necessary?
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 18
Finite elements Matlab model
Gaussian beam
conduction
Vacuum or not
air c
oolin
g
air c
oolin
g
heat radiation
Shield or not
radiationM. Punturo Matlab’s algorithm
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10
100
1000
FI tuned in air (b=10 W m-2 K-1)
FI tuned in air (b=30 W m-2 K-1)
Isola
tion fa
ctor
Losses [cm-1]
Tuned in air at 40 dB: changes in vacuum
Thermal dependance of Verdet constant: Expected change of isolation from air (40 dB) to vacuum, depending on
TGG rod absorption
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LIGO measurements: isolation changes in vacuum
(A. Lucianetti, LIGO)(A. Lucianetti, LIGO)
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Tuning of Faraday isolation
/2
Crystal: about 43.7° rotation
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Tuning of the Faraday isolation
/2
Crystal: about 43.7° rotation
2nd polarizer (with respect to first pol.): about 45° + (45°-43.7°) rotated
Crossed polariz.comp. reflected away
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Tuning of the Faraday isolation
/2
Total rotation: 45°+1.3°+ 43.7° = 90° : good isolation (P2 reflects light)
43.7° 45+1.3°
45+1.3°+0°45+1.3°+43.7°
0°
2.6° rejection
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 24
If crystal rotation changes
/2
Total rotation: 45°+1.3°+43° = 89.3° : worse isolation
43° 45°+1.3°
0°43°
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Compensation
/2
Total rotation: 43°+45°+1.3°+0.7° = 90° : good isolation
43°-0.7° 45°+1.3°
45°+1.3°+0°45°+1.3°+43
/2
-0.7°
Larger rejection: 4°
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motorizedwaveplate
IB Faraday IsolatorILIAS WG1 meeting, 19/06/07 27
Two main problems experienced:
Change of isolation from air to vacuum
Thermal lensing:• Change of beam shape and beam/ITF matching
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RFC matching improvement after PR alignment
PR alignment
RFC trans RFC ref
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Changes in shape with PR alignment
B1p camera beam shape
PR_ty -150 rad misaligned PR_ty 1500 rad misaligned
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Beam/ITF matching: free swinging cavity
TEM0
TEM4
TEM2 TEM3
TEM5
TEM1+usblsb
TEM0
TEMX means sum of TEMnm with n+m=X
Amplitudes of SB is 3% modulation index = 0.3
TEM2 is 2.5% of TEM0
(J.Marque, November 2006)(J.Marque, November 2006)
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Matching change with PR alignment
When the PR is misaligned by 150 rad there is more light passing through the Faraday isolator (larger heating)
Matching depends on the alignement of PR mirror: • 3-4% when PR is misaligned by 150 µrad with respect to PR
misaligned by more than 1mrad.
After power reduction (20%) the difference in mismatching became:• 2.5% : PR misaligned 10 mrad
• 3.5% : PR misaligned 150 rad
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Expected vs measured matching change
Expected focal length with 10+10 W (PR aligned): 160 m,
1% mismatching
Measured with • PR aligned:
3.5%• PR
misaligned: 2.5%
1%: expected with 160 m Fth
(20 W, 2.65 mm waist,obtained with good telescope alignment)
2.5%: PR misaligned
3.5%: PR aligned
360 m Fth: 10 W, 2.65 mm waist
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fit limits~+- 1waist
Circle fit
Radius of curvature
focal
(S. Hamdani)
Zemax
Matlab + Zemax
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VIRGO+ (50Watt)Thermal lens: f=66mMismatch: 5%
Advanced VIRGO (200Watt)Thermal lens: f=13mMismatch: 37%
(E. Genin, S. Hamdani)
Virgo:-10W + 3W- mismaching:~0.5%
Virgo +:- 40W + 10W- mismaching:~5%
Advanced Virgo:- 150W + 50W- mismaching:~37%
Thermal lensing: Virgo vs Virgo+ vs AdvVirgo
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Thermal induced focal length: compensation
DKDPCompensator