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Substrates, Polishing, Coatings and Metrology for the 2 nd generation of GW detector

This overview discusses the advancements in substrates, polishing, coatings, and metrology techniques for the second generation of gravitational wave detectors. Topics covered include the Virgo mirrors, flatness specifications, corrective coating, ion beam polishing, and metrology measurements. The goal is to improve the sensitivity and performance of future detectors.

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Substrates, Polishing, Coatings and Metrology for the 2 nd generation of GW detector

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  1. Substrates, Polishing, Coatings and Metrology for the 2nd generation of GW detector Laurent PINARD Laboratoire des Matériaux Avancés – Lyon - FRANCE

  2. Overview • Introduction – The Virgo mirrors • Advanced Virgo : New mirrors • Substrates : new type of fused silica • Polishing : Crucial point = Flatness (Round trip losses) • Cavity simulations, Spec definition, Foreseen polishing • solutions (Corrective coating, Ion beam polishing) • Metrology : Absorption, Defects, Scattering, Flatness • Coating : Thickness uniformity, Thermal noise • Scenario for the 3rd generation of GW detector (ET)

  3. Introduction : The Virgo mirrors Mirrors : 35 cm diameter, 10 cm thick 20 kg LMA : in charge of the mirror coating

  4. Introduction : The Virgo mirrors Silica substrate polishing (Gooch and Housego, ex General Optics (US)) • Microroughness : < 1 Å rms • ROC : 3450 +/- 100 m • Few point defects • Flatness : < 8 nm RMS on 150 mm achieved : # 3-4 nm RMS

  5. Introduction : The Virgo mirrors Coating deposited by Ion Beam Sputtering

  6. Advanced Virgo • LIGO/VIRGO allow verification of the upper limit predictions • Event rate too small • Detector improvement needed • Sensitivity x 10 = rate x 1000 • Advanced Virgo approved in Dec 2009 • Main impovement: • - High power laser (200 W) • - New optical configuration • - Heavier mirrors (40 kg) • - Monolithic suspension • LMA : Mirror responsible • ~ 5 M€ investment (25% of the total budget)

  7. In Virgo : Round-Trip Losses in the cavity = 400 - 500 ppm Main Origin : Flatness defects having a period of 1 cm or more • For Advanced Virgo : Round-Trip Losses = 75 ppm (specif.) 50 ppm : Flatness defects specification very severe 25 ppm : Abs+Diff+T Advanced Virgo : The Mirrors AdV IM Substrate : • Low absorption Silica • (Suprasil 3002 - Heraeus) • Diameter = 35 cm, • Thickness = 20 cm, Weight = 40 kg • Unit Cost 130 k€ (without polishing)

  8. Advanced Virgo : Mirror polishing spec. Goal : Define a spec. on Flatness for the IM and EM (losses 50 ppm) PSD(1D) Extraction Measured maps at LMA or by the polisher Artificial maps generated with the PSD obtained, with different RMS flatness values Cavity simulation PSD shape depends on the polisher ( f-n, n[0;2])

  9. Advanced Virgo : Mirror polishing spec. ROC = 1420 m ROC = 1683 m 0.5 nm RMS = spécification

  10. Advanced Virgo : Polishing - Solutions 2 solutions identified « Classical Polishing » (best as possible) + Corrective Coating (LMA) AdV Baseline Microroughness preserved « Classical Polishing » + Ion Beam Figuring LIGO solution Drawbacks : More expensive (factor 2-3) Microroughness # 1.5 Å rms

  11. Ion Source Robot Substrate in translation mask Sputtered Atoms Silica Target Interferometer Advanced Virgo : Polishing - Solutions Corrective Coating Developed in 2005/2006 at LMA Use of the IBS deposition chamber developed for Virgo in 2000 Add material (silica) to suppress holes and aim at a «perfect» plane

  12. Advanced Virgo : Polishing - Solutions Corrective Coating Substrate 156 mm VIRGO type Before correction (120 mm) 3.3 nm R.M.S. 16 nm P.V. After correction (120 mm) 0.98 nm R.M.S. 10 nm P.V. Microroughness preserved (0,5 Å RMS) Defect at the center linked to the robot (mechanics)

  13. Advanced Virgo : Polishing - Solutions Corrective Coating New robot developed

  14. fc Advanced Virgo : Polishing - Solutions Corrective Coating • CC effect on the PSD : Flatten the spectrum in the low frequency domain up to a cutoff frequency fc

  15. Advanced Virgo : Polishing - Solutions Corrective Coating • Cutoff frequency : 50 m-1 (correction of defects with a period  2 cm) , reasonably achievable with the CC • To reach 0.5 nm rms after CC, we can not start from any flatness • Simulations (1000) done with artificial maps obtained with the PSD for different RMS flatness (4 nm like Virgo, 1.5 nm)

  16. Advanced Virgo : Polishing - Solutions Corrective Coating 1% of simulations give losses < 50 ppm CC not sufficiently powerful

  17. Advanced Virgo : Polishing - Solutions Corrective Coating 96% of simulations give losses < 50 ppm Conclusion : Before CC, flatness at least lower than 1.5 nm rms, Obtained by « classical » polishing : challenge Other constraints for the polishers : - accuracy on the ROC (+/- 10m) - points defects

  18. Advanced Virgo : Polishing - Solutions Ion Beam Figuring • Always a “classical” polishing phase • ROC et flatness corrected with a small ion beam Opposite to the Corrective Coating : Remove material (silica) instead of adding material.

  19. Advanced Virgo : Polishing - Solutions Ion Beam Figuring ROC accuracy +/- 10 m on 2 km Polisher : CoastLine Optics +ASML (US) LIGO Substrate : 0.21 nm RMS - 2 nm PV (150 mm diam.)

  20. Advanced Virgo : Flatness Metrology • Based on a phase shifting interferometer (ADE Phase Shift) • MIRAU type interferometer • Working wavelength 1064 nm, Aperture 150 mm, Distortion correction • References flats known by “three flat test” measurement

  21. Mirror Advanced Virgo : Flatness Metrology • Measurement on larger diameter (320 mm) with stitching interferometry • Mirror on motorized sample holder • Measurement of Sub-pupils, total wavefront reconstructed mathematically Y X

  22. Advanced Virgo : Flatness Metrology Flat surface Advanced Ligo Optic 0.57 nm RMS, 4,7 nm PV (Tinsley) 0.62 nm RMS, 6 nm PV (LMA)

  23. Advanced Virgo : Flatness Metrology • Wavefront measurement has to be improved to measure sub-nm surface flatness on curved surface and large diameter (stitching) • Reference sphere necessary • Vibration isolation improvement • Protection from turbulences • Improve the way to support the mirror : more stable, no deformation

  24. Advanced Virgo : Coating Uniformity • Identical interferometer cavities : Necessary to coat two large substrates at the same time • Coating uniformity needed on 80 cm diameter 35 cm IBS Virgo coating machine Twin mirrors

  25. Advanced Virgo : Coating Uniformity • Masks between targets and substrates • Mask shape calculated with a home-made software • Two different masks for H and L layers, several iterations

  26. Advanced Virgo : Coating Uniformity Ti: Ta2O5 0.2% rms on 160 mm SiO2 0.08% rms on 160 mm

  27. Mirror n°1 Mirror n°2 Advanced Virgo : Coatings Uniformity • Mirrors performances matched : same T, same wavefront • Flatness consistent with thickness profile of monolayers • Not enough for Advanced detector (0.5 nm RMS) 1.6 nm rms on 160 mm

  28. Advanced Virgo : Coatings Uniformity Recent result (July 2011) an a 34 cm LIGO mirror (single rotation) 0.56 nm RMS on 160 mm Next development : planetary motion to coat two substrates at the same time

  29. Scenario for the 3rd generation of mirrors • Einstein Telescope (ET) design study : final document May 2011 • Strategy for the next generation of mirrors ET – High Frequency ET – Low Frequency (cryogenic) • Fused Silica, 1064 nm • 62 cm diameter, 30 cm thick • 200 kg • Polishing spec.: same as Advanced Virgo • Same coating as AdV • Silicon : new material, 1550 nm • > 45 cm diameter, #50 cm thick, • 211 kg • Polishing spec.: same as Advanced Virgo • Same coating as AdV BUT study of the perf. at 1.55 µm and 10°K necessary

  30. Scenario for the 3rd generation of mirrors Cryogenic setup developed to measure at 10°K the coating quality factor (thermal noise) and the substrate/coating absorption at 1.55 µm

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