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Advances in Coating Optimization and Thermal Noise Measurement for Interferometry

This report summarizes key findings from the Caltech LSC-VIRGO meeting held on March 19, 2007, focusing on optimized coatings for interferometers. Efforts led by Eric Black, Akira Villar, and others highlight a two-year research project addressing fabrication delays and the relationship between coating materials and thermal noise. Ongoing analyses aim to resolve discrepancies in thermal noise measurements, improve coating homogeneity, and explore the potential for direct measurement of thermo-optic noise. Future work includes coordination with international teams for advanced coating experimentation and data collection.

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Advances in Coating Optimization and Thermal Noise Measurement for Interferometry

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  1. Thermal Noise InterferometerAdvisory Board Review Eric Black, Akira Villar, Greg Ogin, Tara Chelermsongsak, Riccardo Desalvo, and Kenneth G. Libbrecht Caltech LSC-VIRGO Meeting March 19, 2007

  2. Recent • Optimized coatings • In coordination with Sannio, Italy group and LMA (Lyon, France) • End of 2yr project (expected 1yr, but met with fabrication delays (bubbles)) • Results consistent with Sannio models • Analysis in progress to determine material loss parameters (silica in question), whether or not thermooptic noise components add coherently or in quadrature • Coating-loss homogeneity • Addresses long-standing spot-size issue of relevance to big interferometers • Ring dampers • Calibration • Discrepancy of ~20% between direct (TNI) and indirect (Q) measurements of coating thermal noise needed to be resolved. • Akira will present in detail. • Lab move • Caltech division requested

  3. Current • Doped, optimized coating measurement • In coordination with Sannio, Italy and LMA (Lyon, France) • Coating design contingent upon analysis of undoped, optimized results (in progress) • Photothermal recommissioning after lab move (Tara) • Thermomechanical properties of coatings needed to model noise floor of AdvLIGO, AdVIRGO, etc. • Could take data with a few weeks notice. • Shot-noise analysis and mitigation • Necessary (and possibly sufficient) for a direct measurement of dn/dT in coatings. • Akira will talk about this in detail. • Analysis of SOPRA ellipsometry data on dn/dT in thin-film tantala • In coordination with Andri Gretarsson (Embry-Riddle) • Greg Ogin will talk about this in detail. • Updating BENCH to include coating optimization • In coordination with Innocenzo Pinto (Sannio, Italy), Clare Bayley and Gregg Harry (MIT) • Greg Ogin will talk about this in detail.

  4. Future • Gold barrel coatings • In coordination with David Blair’s group (UWA) • They measure parametric gain at Gin-gin. • We measure thermal noise and Q suppression at the TNI. • Direct measurement of thermo-optic noise • May be achievable with proposed shot-noise improvements (Akira) • Almost certainly achievable with new mirrors that increase arm-cavity finesse • Photothermal measurements of  in advanced coatings • Have already done periodic si-ta • Can measure in doped, optimized, or any combination • Other materials? • Photothermal measurement of thermo-optic response in coatings • Separate thermomechanical and thermorefractive components • Yields dn/dT with existing hardware • Charge noise measurement, mitigation-scheme test?

  5. Questions • Which size optics to use for barrel-coating measurements? • TNI size: 4” diameter by 4” thick right cylinders • Gin-gin size: 4”diameter by 2” thick right cylinders • Modify suspension to accept Gin-gin-sized optics, or use TNI optics? • Is a direct measurement of thermo-optic noise important enough to justify buying a new set of (high-finesse) mirrors, if shot-noise improvements do not uncover it with existing optics? • How soon can we expect samples for photothermal measurements of ? • Sample delivery now (probably) drives schedule on this experiment.

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