ilias wg1 meeting, 19/06/07 ib faraday isolator 1 ib in vacuum faraday isolator e. genin, s. hebri,...

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 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


Brewster dielectric polarizers

Rotator

February 2005


20W Pump

HeNe

Shack-Hartman

Faraday

2f2f

(F.Cleva)(F.Cleva)


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)


Where is the IB Faraday


Two main problems experienced:

Change of isolation from air to vacuum

Thermal lensing:• Change of beam shape and beam/ITF matching


Faraday isolation measurement

Power reflected measured on the Laser Bench (rejected by the last Laser Bench Faraday).

LB Faraday

Power meter

SIB Faraday


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)


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


Explanation


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


Isolation: before (WS1) and after (WS2)

Purple: WS1Black: WS2


Isolation improvement

0.05

0.75

Black: WS1 (before improvement)Purple: WS2 (after improvement)

More than factor 10 improvement (factor 15)


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)


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?


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


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


LIGO measurements: isolation changes in vacuum

(A. Lucianetti, LIGO)(A. Lucianetti, LIGO)


Tuning of Faraday isolation

/2

Crystal: about 43.7° rotation


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


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


If crystal rotation changes

/2

Total rotation: 45°+1.3°+43° = 89.3° : worse isolation

43° 45°+1.3°

0°43°


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°


motorizedwaveplate


Two main problems experienced:

Change of isolation from air to vacuum

Thermal lensing:• Change of beam shape and beam/ITF matching


RFC matching improvement after PR alignment

PR alignment

RFC trans RFC ref


Changes in shape with PR alignment

B1p camera beam shape

PR_ty -150 rad misaligned PR_ty 1500 rad misaligned


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)


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


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


fit limits~+- 1waist

Circle fit

Radius of curvature

focal

(S. Hamdani)

Zemax

Matlab + Zemax


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


Thermal induced focal length: compensation

DKDPCompensator

ilias wg1 meeting, 19/06/07 ib faraday isolator 1 ib in vacuum faraday isolator e. genin, s. hebri,...

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