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Faraday isolator on the IB E. Genin, P. La Penna EGO Amsterdam, 03/07/06 Commissioning Meeting 1 It was decided to put a Faraday isolator between the IMC and the PR because the ITF reflected light (with PR aligned) induced noise in the IMC (converted in frequency noise) 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 (http://wwwcascina.virgo.infn.it/collmeetings/presentations/2005/200503/DetectorMeeting/Cleva_28Feb05_Faraday.ppt) It has been reassembled and tuned in clean room with about 300 mW Yag beam: same isolation factor (about 40 db, surely better than 30 db) Amsterdam, 03/07/06 Commissioning Meeting 2 Amsterdam, 03/07/06 Commissioning Meeting 3 How the IF was aligned L/2 Amsterdam, 03/07/06 photodiode Commissioning Meeting 4 Check of the Faraday isolation L/2 Amsterdam, 03/07/06 Commissioning Meeting 5 Check of the Faraday isolation L/2 Crystal: about 43.7° rotation Amsterdam, 03/07/06 Commissioning Meeting 6 Check of the Faraday isolation photodiode L/2 Crystal: about 43.7° rotation Amsterdam, 03/07/06 Commissioning Meeting 7 Check of the Faraday isolation Photodiode < 10-3 L/2 2nd polarizer: about 2×(45°-43.7°) rotation Crystal: about 43.7° rotation Total rotation back-and-forth: 90° Amsterdam, 03/07/06 Commissioning Meeting 8 Faraday isolator Amsterdam, 03/07/06 Commissioning Meeting 9 Faraday isolator Amsterdam, 03/07/06 Commissioning Meeting 10 Faraday isolation July 2004: before beam attenuation March 2006 New IB 9% fringes 0.6% fringes More than factor 10 improvement (corresponding to more than 100 attenuation in power) Amsterdam, 03/07/06 Commissioning Meeting 11 Faraday isolation Evidence of fringes in the IMC when the PR is aligned since the RFC was locked (March 2006) 0.5% fringes 3% fringes Amsterdam, 03/07/06 Commissioning Meeting 12 Faraday isolation Light reflected towards the laser and filtered by the last LB Faraday (March 2006) Power meter PR aligned: P change100 mW Amsterdam, 03/07/06 Commissioning Meeting 13 Check of Faraday isolation in the tower L/2 About 30 microwatts 12 – 6 mW •6 mW entering 30 mW reflected power (about a factor 100 attenuation) (100 times worse than expected) •In order to optimize the attenuation the first polarizer has to be rotated by about 5 degrees L/2 Amsterdam, 03/07/06 Commissioning Meeting 14 Check of Faraday isolation 1) The polarizers seem to be in the same position (rotator markers are aligned), polarization in the ITF is vertical, dampers and mirrors are centered; 2) In order to optimize the attenuation the first polarizer has to be rotated by about 5 degrees: there is much more light reflected by the second polarizer (more than 10 %), M9 and M10 (mirrors for the Faraday reflection) have to be moved a lot (mounts to be modified, etc.) 3) It’s not clear whether something has moved in the Faraday when mounting the bench, damage occurred, external magnetic field influence (50 mT are needed to rotate the polarization bay about 2.5 degrees). Amsterdam, 03/07/06 Commissioning Meeting 15 Possible explanations for less isolation than expected: Mechanical damage or movement of the Faraday polarizer during IB mounting (unlikely, no evidence after visual inspection, to be checked more thoroughfully) Misalignment of the isolator (doesn’t seem to explain the effect) Thermal damages either of the crystal or of the polarizers Action of external magnetic fields: already evaluated before, should be negligible: a more accurate computation and simulation is being done Possible rotation of the TGG crystal: this can be induced by an external magnetic field Thermal effects (thermal dependence of Verdet constant, photoelastic birefringece, thermal lensing, …): the Faraday has been tuned in air with low power (0.5 W) whereas now about 7W (in vacuum) are going through it test in tower have been performed with less than 50 mW: the Faraday should be tuned again with effective 7 W (but it is impossible to tune in vacuum) Amsterdam, 03/07/06 Commissioning Meeting 16 Thermal effects A Faraday isolator is influenced by parasitic effect since absorption coefficient of magneto-optic media are relatively high This property induces a non-uniform cross-section distribution of temperature which can influence the laser beam in 3 ways: • Thermal lensing (due to refractive index temperature dependence) • A non-uniform distribution of rotation angle of the polarization plane in the TGG is induced by Verdet constant temperature dependence • Simultaneous appearance of circular birefringence (Faraday effect) and linear birefringence due to mechanical straints (photoelastic effect) caused by temperature gradient. Amsterdam, 03/07/06 Commissioning Meeting 17 Faraday Isolator normal behaviour Polarizer Polarizer Rotation of 45° of the linear polarization Reflected light from the ITF Magneto optic rod All the polarized light is stopped by the polarizer The isolation ratio is maximized. Amsterdam, 03/07/06 Commissioning Meeting 18 Effects on Faraday isolation ratio Verdet constant temperature dependence. Photo-elastic effect Due to depolarization effects we can decompose the polarization in 2 orthogonal polarization Polarizer Polarizer Reflected light from the ITF Magneto optic rod The « depolarization » of the light induces a decrease of the isolation ratio because one part of the light is transmitted by the second polarizer. Amsterdam, 03/07/06 Commissioning Meeting 19 Theoretical description Self-induced depolarization in Terbium Gallium Garnet (magneto-optic medium used in our Faraday rotator): studied by Khazanov et al. They proposed a model describing self-induced depolarization for TGG without magnetic-field and verified it experimentally. Moreover they studied the self-induced depolarization phenomenon (isolation ratio decrease) in the Faraday isolator experimentally. Model: estimate: • depolarization ratio due to Verdet constant temperature dependence • photo-elastic effect = f(P0, Q, , , , V) Amsterdam, 03/07/06 P0 is the incident power Q, TGG thermo-optical constant, , the wavelength , TGG thermal conductivity , optical losses in TGG V, Verdet constant Commissioning Meeting 20 Theoretical results Estimated the depolarization with respect to the incident laser power P0 (without magnetic field): between 0.5 W and 7W, about 20 dB of isolation lost Amsterdam, 03/07/06 Commissioning Meeting 21 Theoretical results Khazanov et al. , “Investigation of self-induced dpolarization fo laser radiation in Terbium Gallium Garnet”, IEEE journal of quantum electronics, Vol. 35, No. 8, August 1999. Dependence of depolarization parameter on P0 without magnetic field ▪ experimental result - Theoretical model - with magnetic field experimental result (FI tuning at 0.5 W) □ experimental result (FI tuning at 8 W) Amsterdam, 03/07/06 Dependence of depolarization parameter on θ in a magnetic field for diffrent transverse location of the beam (experimental results). Faraday Isolation depends on the power used to perform the tuning: Should be retuned with 7-8 W incident power. Moreover, it depends also of the beam transverse location in the TGG. Commissioning Meeting 22 Check of Faraday isolation 1) Modelization of the Faraday to be developed (we have the optical model: • magnetic field mapping and simulation could be useful. • EOT refuses to give the description of the magnets, it’s proprietary) 2) Measurements in the tower (low power, lack of space, short time) are not very reliable; 3) Better measurements should be made if the attenuation is not sufficient: • more time (days), • preparation (send part of the 20 W beam directly inside the tower without passing through the dihedron). 4) Open the tower, check and retune the Faraday at full power (proper setup and optical simulation being prepared) Amsterdam, 03/07/06 Commissioning Meeting 23 Faraday isolation measurement Isolation 7W Amsterdam, 03/07/06 Commissioning Meeting 24 Faraday isolator measurement Amsterdam, 03/07/06 Commissioning Meeting 25 Faraday isolation measurement Isolation 7W Amsterdam, 03/07/06 Commissioning Meeting 26 Faraday isolator measurement Amsterdam, 03/07/06 Commissioning Meeting 27 Zemax simulation : Consequences of first brewster rotation on beam position in the ITF. M6 M4 M5 Brewster #2 Faraday y z Input Waist = 2.65 mm x Brewster #1 Turning the first polarizer (Brewster #1) of several degrees improves the isolation ratio Input beam Amsterdam, 03/07/06 We computed the horizontal and vertical shift of the beam in order to check if rotation induces alignment problems in the ITF. Commissioning Meeting 28 Vertical and horizontal beam shifts Amsterdam, 03/07/06 Commissioning Meeting 29 Mounting modification Amsterdam, 03/07/06 Commissioning Meeting 30 Possible beam dump modification Amsterdam, 03/07/06 Commissioning Meeting 31 Future upgrades Future actions: Thermal problems could be always there and could change with the time Different problems will arise with higher power laser (Virgo +) It would be interesting to have the possibility to tune the Faraday isolation Design change of the Faraday setup (CRE in preparation): possibility to remotely rotate one polarizer or place a remotely controlled waveplate between the crystal and the polarizer (this will allow isolation tuning at the expense of power losses). Amsterdam, 03/07/06 Commissioning Meeting 32 Remotely controlled waveplate L/2 L/2 Amsterdam, 03/07/06 Commissioning Meeting 33 Remotely controlled polarizer L/2 Amsterdam, 03/07/06 L/2 Commissioning Meeting 34 Conclusion • The Faraday isolation (about 100, 20 dB) is sufficient at the moment • It could and should be better (10,000, 40 dB) • Unless major mechanical problems, it’s probably due to the tuning with too low power • An intervention in tower could improve the situation Rotation of the Faraday Modification of some mount • A system for remotely control the isolation could be designed Amsterdam, 03/07/06 Commissioning Meeting 35