1. INJ ASPERA Technology Forum 21/10/2011 1 High power optics for the AdV INJ system Benjamin Canuel European Gravitational Observatory

2. INJ ASPERA Technology Forum 21/10/2011 2 Scope of INJ subsystem ● 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.

3. INJ ASPERA Technology Forum 21/10/2011 AdV INJ design 3 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

4. INJ ASPERA Technology Forum 21/10/2011 4

5. INJ Dealing with thermal effects in INJ 5 ● 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.

6. INJ ASPERA Technology Forum 21/10/2011 6 AdV Faraday Isolator 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)

7. INJ ASPERA Technology Forum 21/10/2011 ● ● The requirements for the AdV Faraday isolator (FI) are: 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. 7 [ 1]Khazanov et al.,“AdV Faraday isolator design study”, VIR-0245A-10VIR-0245A-10. [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.VIR-0283A-10 ● ● 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). FI prototype on its breadboard AdV Faraday Isolator: requirements

8. INJ ASPERA Technology Forum 21/10/2011 8 Thermal lensing in TGG crystals: TGG1:2300ppm/cm,TGG2:2600ppm/cm Looking for a power independant compensation. The Virgo Collaboration, “In-vacuum optical isolation changes by heating in a Faraday isolator”, APPLIED OPTICS, Vol. 47, No. 31 (2008). 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/dT DKDP =-4.4.10 -5 K -1 (dn/dT TGG =1.9.10 -5 K -1 ) At 180W: 7m to 35m. Thermal lensing compensation

9. INJ ASPERA Technology Forum 21/10/2011 Thermal isolation change Effect compensable by introducing a half wave plate inside the FI [1] Under vacuum at 250W by turning the wave plate we can recover an optimum isolation 9 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]The Virgo Collaboration, “In-vacuum Faraday isolation remote tuning”, Applied Optics, 49, 4780-4790 (2010). Change of isolation when power sent inside the FI is modified or when thermal condition is changed such as going from air to vacuum. 7dB

10. INJ ASPERA Technology Forum 21/10/2011 Thermal depolarisation inside TGG Thermal depolarization inside TGG crystal is a limiting effect for Faraday isolation: Measured by placing TGG without housing between crossed polarizers At high Power[1]: 10 [1 ]Efim Khazanov et al., APPLIED OPTICS, 41-3, 483-492 (2002) We could show that the limiting “depolarization” effect at high power is in reality a self induced Spin to Orbit angular momentum conversion.

11. INJ ASPERA Technology Forum 21/10/2011 Depolarization as mode conversion 11 Geometry of birefringence in heated TGG Similar to the birefringence geometry of a q-plate[1], device developped for the creation/manipulation of OAM states. OAM content of converted field were measured by interference and tomography OAM m=0 Converted part Input SAM  + OAM m=+2 SAM  - Left polarization input Compensation… [1] S. Mosca, B. Canuel, E. Karimi, B. Piccirillo, L. Marrucci, R. De Rosa, E. Genin, L. Milano and E. Santamato,” Photon self- induced spin-to-orbital conversion in a terbium-gallium-garnet crystal at high laser power”, Phys. Rev. A 82, 043806 (2010)

12. INJ ASPERA Technology Forum 21/10/2011 Faraday isolator for Advanced Virgo 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. 12 The FI prototype under development includes compensation of all effects previously mentioned

13. INJ ASPERA Technology Forum 21/10/2011 BEST RESULTS: obtained with absorbing glass (KG5 type from Schott) 15-30ppm Home made beam trap Damage threshold 1.8W@w 0 =1mm 5.6W@w 0 =2.5mm High Power beam dumps 3 orders of magnitude worse 4 orders of magnitude worse Razor blades (Up to 5W) Opto sigma (Up to 30W) HIGH POWER MEDIUM POWER... 13 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

14. INJ ASPERA Technology Forum 21/10/2011 14 High Power beam dump SiC; ΔT=10⁰C Si; ΔT=25⁰C Thermal behaviour of Si, SiC (with 10W) and absorbing glass (2W). Absorbing glass > 200 ⁰C Damage threshold of SiC and Si has been measured: - - SiC: 30kW/cm 2 - - Si: 6kW/cm 2 → SiC is the best material. 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 We tested the possibility of using Silicon Plates 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)

15. INJ ASPERA Technology Forum 21/10/2011 15 10W Time constant (to) is 74min. DT at equilibrium (B) is 96°C High power beam dumps: in-air and in-vacuum Harder under vacuum: Material and design optimized for heat transmission to the walls Successfully installed on Virgo. IN-AIR UNDER-VACUUM Adapted to limited Power DT =37°C at 100W

16. INJ ASPERA Technology Forum 21/10/2011 A new idea: viewport beam dump 16 Designed by T. Zelenova → Heat is mainly removed by conduction and use the convection (out of the vacuum) to remove heat more efficiently than removing it by radiation. Vacuum side air side Absorbing element (Si or SiC plate)

17. INJ ASPERA Technology Forum 21/10/2011 17 High Power compatible components 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/cm 2, 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.

18. INJ ASPERA Technology Forum 21/10/2011 Monitoring thermal effects 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. 18 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 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 Beam monitoring system:

19. INJ ASPERA Technology Forum 21/10/2011 Thermally Deformable Mirror (TDM) ● 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 ● Working principle Beam reflection after going through the mirror substrate Modification of the optical path by conduction with an array of micro-resistors ● Advantages Simple and cheap No noise introduced Ultra high vacuum compatible 19 Micro-heater array

20. INJ ASPERA Technology Forum 21/10/2011 TDM: First results ● Linear response to power change: max~160nm ● Reproducibility better than l/100 20 R23 @ 4 mA R21 @ 6 mA R39 @ 8 mA R41 @ 10 mA the 4 resistors the 4 resistors switched on together ● ● Superposition (phase images)

21. INJ ASPERA Technology Forum 21/10/2011 21 AdV INJ status and perspectives ● 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.