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ILIAS 5-6/11/2004 WG T2

ILIAS 5-6/11/2004 WG T2. Task T2 (WG 11). AIM: exact definition (theoretical and experimental) of photo-thermal noise. PARTICIPANTS INFN (AURIGA group; also LENS Florence, INOA Florence and Naple) CNRS (LKB Paris) IGR (expressed interest in the task). ILIAS 5-6/11/2004 WG T2.

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ILIAS 5-6/11/2004 WG T2

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  1. ILIAS 5-6/11/2004 WG T2 Task T2 (WG 11) AIM: exact definition (theoretical and experimental) of photo-thermal noise • PARTICIPANTS • INFN (AURIGA group; also LENS Florence, INOA Florence and Naple) • CNRS (LKB Paris) • IGR (expressed interest in the task)

  2. ILIAS 5-6/11/2004 WG T2 • Photo-thermal effect • Physical origin: heating of mirrors by absorbed laser light • by thermal expansion: deformation/displacement • due to shot noise in the absorbed radiation: displacement noise Depends on: • laser power impinging on the mirrors • absorption coefficient • material: - thermal expansion • - thermal conductivity and capacitance • temperature (through the above parameters) • mirror size and shape/suspension • beam waist • detection frequency

  3. Photo-thermal effect • Frequency dependence (theory): • Half-space approximation: • - linear decay above wc = 2k/csw2where • k: thermal conductivity • cs: volumetric thermal capacitance • w: beam waist • - logarithmic below wc • nc (Hz) • Fused silica Sapphire • w/2 300K 1K 300K 1K • 10mm 0.0015 4.8 0.02 19000 • 0.1mm 15 48000 200 1.9·108 • Cut-off depending on the mirror shape and suspension (heat dispersion) ILIAS 5-6/11/2004 WG T2

  4. Mirror  half space approximation Braginsky et al., Phys. Lett. A 264, 1 (1999) Cerdonio et al., Phys. Rev. D 63, 082003 (2001) dL = L0 K(w/wc)  1/W ILIAS 5-6/11/2004 WG T2

  5. Fitting curve: L0/Pabs = 1.5·10-4mm/mW M. De Rosaet al.: “Experimental measurement of photothermal effect in Fabry-Perot cavities ”, Phys. Rev. Lett. 89, 237402 (2002)

  6. ILIAS 5-6/11/2004 WG T2 Cryogenic operation, high Finesse cavity nc: 2.8 Hz  10 kHz (fused silica) 50 MHz (sapphire) L0/Pabs : 1.5·10-4mm/mW 0.5·10-5mm/mW F: 40 000  1000 000

  7. ILIAS 5-6/11/2004 WG T2 Work Plan a) Years 1-2: Room temperature meas. of photo-th. noise;waist dependence b) Years 2-3: Different substrates and coatings. c) Years 1-3: Test and setup of high Finesse cavities at low T d) Years 2-4: Measurements at low T

  8. ILIAS 5-6/11/2004 WG T2 a) Years 1-2: Room temperature meas. of photo-th. noise;waist dependence • Work performed: • Optimized the measurement apparatus: • - AOM for wider and faster tuning on both cavities • - EOM for amplitude modulation on one single cavity • Designed and built specific mounts for using short ( small waist) cavities on the present acoustically-isolated, thermally stabilized environment • Next step: • Check of the previous measurements, with better accuracy (fall of half-space approximation??) • Independent measurement of absorption??

  9. ILIAS 5-6/11/2004 WG T2 b) Years 2-3: Different substrates and coatings Depends on the availability of different mirrors (at present: fused-silica substrates and coatings by REO

  10. Setup and test of high-finesse cavities Mirrors made by J.M. Mackowski Input mirror T = 20 ppm, total losses < 10 ppm Compact cavity: L = 0.2 mm • Cavity finesse = 230 000, input power > 3 mW Tests at cryogenic temperature in progress

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