ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 1 IB in vacuum Faraday isolator E. Genin, S. Hebri, S. Hamdani, P. La Penna, J. Marque EGO

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 2 IB Faraday isolator in vacuum l l 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 l l Beam out of IMC: 5 mm waist: need of a large Faraday isolator l l Two candidates: EOTech (larger aperture) and Isowave (lower absorption) l l 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 l l 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 l l After the new IB installation, with the Faraday, it was possible to lock soon the recycling cavity with full power

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 3 Brewster dielectric polarizers Rotator February 2005

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 4 20W Pump HeNe Shack- Hartman Faraday 2f (F.Cleva)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 5 20 W Nd:YAG input power: l Two rods: Rod 1, –Beam waist 1.1 mm: focal length = 31 m –Rod 1, Beam waist 2 mm : focal lenght = 90 m  F th ~ 1/  2 –Rod 2, Beam waist 2 mm : focal length = 82 m  First rod less absorbing than second one l Optical isolation: larger than 42 dB (less than W backreflected)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 6

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 7 Where is the IB Faraday

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 8 Two main problems experienced: l Change of isolation from air to vacuum l Thermal lensing: Change of beam shape and beam/ITF matching

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 9 Faraday isolation measurement Power reflected measured on the Laser Bench (rejected by the last Laser Bench Faraday). LB Faraday Power meter SIB Faraday

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 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)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 11 Explanation 1) 1)This could be explained by a change in the magnetic field (by 10 mT, total field about 1 T) 2) 2)Explanation: the internal magnetic field was perturbed by the presence of an Allen key inside 3) 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

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 12 Explanation

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 13 Isolation tuning l l After Allen key removal the polarizer could be turned back close to the nominal position (defined by a marker) l l In this condition we tuned the isolation: measured on the laser bench it was more than 10,000 l l We blocked the Faraday and realigned optics and mounts in this condition l l Then the IB tower was closed, evacuated, and things changed: the isolation dropped to less than 1,000

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 14 Isolation: before (WS1) and after (WS2) Purple: WS1 Black: WS2

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 15 Isolation improvement Black: WS1 (before improvement) Purple: WS2 (after improvement)  More than factor 10 improvement (factor 15)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 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)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 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?

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 18 Finite elements Matlab model Gaussian beam conduction Vacuum or not air cooling heat radiation Shield or not radiation M. Punturo Matlab’s algorithm

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 19 Tuned in air at 40 dB: changes in vacuum l Thermal dependance of Verdet constant: Expected change of isolation from air (40 dB) to vacuum, depending on TGG rod absorption

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 20 LIGO measurements: isolation changes in vacuum (A. Lucianetti, LIGO)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 21 Tuning of Faraday isolation  /2 Crystal: about 43.7° rotation

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 22 Tuning of the Faraday isolation  /2 Crystal: about 43.7° rotation 2 nd polarizer (with respect to first pol.): about 45° + (45°-43.7°) rotated Crossed polariz.comp. reflected away

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 23 Tuning of the Faraday isolation  /2 Total rotation: 45°+1.3°+ 43.7° = 90° : good isolation (P2 reflects light) 43.7° ° °+0° °+43.7° 0° 2.6° rejection

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 24 If crystal rotation changes  /2 Total rotation: 45°+1.3°+43° = 89.3° : worse isolation 43° 45°+1.3° 0° 43°

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 25 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°

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 26 motorized waveplate

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 27 Two main problems experienced: l Change of isolation from air to vacuum l Thermal lensing: Change of beam shape and beam/ITF matching

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 28 RFC matching improvement after PR alignment PR alignment RFC trans RFC ref

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 29 Changes in shape with PR alignment B1p camera beam shape PR_ty -150  rad misaligned PR_ty 1500  rad misaligned

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 30 Beam/ITF matching: free swinging cavity TEM0 TEM4 TEM2 TEM3 TEM5 TEM1 +usb lsb 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)

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 31 Matching change with PR alignment When the PR is misaligned by 150  rad there is more light passing through the Faraday isolator (larger heating) l 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. l After power reduction (20%) the difference in mismatching became: 2.5% : PR misaligned 10 mrad 3.5% : PR misaligned 150  rad

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 32 Expected vs measured matching change l Expected focal length with W (PR aligned): 160 m, 1% mismatching l Measured with PR aligned: 3.5% PR misaligned: 2.5% 1%: expected with 160 m F th (20 W, 2.65 mm waist, obtained with good telescope alignment) 2.5%: PR misaligned 3.5%: PR aligned 360 m F th : 10 W, 2.65 mm waist

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 33 fit limits ~+- 1waist Circle fit Radius of curvature focal (S. Hamdani) Zemax Matlab + Zemax

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 34 VIRGO+ (50Watt) Thermal lens: f=66m Mismatch: 5% Advanced VIRGO (200Watt) Thermal lens: f=13m Mismatch: 37% (E. Genin, S. Hamdani) Virgo: - -10W + 3W - - mismaching:~0.5% Virgo +: W + 10W - - mismaching:~5% Advanced Virgo: W + 50W - - mismaching:~37% Thermal lensing: Virgo vs Virgo+ vs AdvVirgo

ILIAS WG1 meeting, 19/06/07 IB Faraday Isolator 35 Thermal induced focal length: compensation DKDP Compensator