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Absolute calibration of the LIGO test mass displacement actuators

Absolute calibration of the LIGO test mass displacement actuators. Rick Savage LIGO Hanford Observatory E. Goetz , J. Garofoli, G. Gonzalez, E. Hirose, P. Kalmus, K. Kawabe, J. Kissel, M. Landry, B. O'Reilly, A. Stuver, M. Sung. Overview.

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Absolute calibration of the LIGO test mass displacement actuators

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  1. Absolute calibration of the LIGO test mass displacement actuators Rick Savage LIGO Hanford Observatory E. Goetz, J. Garofoli, G. Gonzalez, E. Hirose, P. Kalmus, K. Kawabe, J. Kissel, M. Landry, B. O'Reilly, A. Stuver, M. Sung

  2. R. Savage Amaldi 8 Columbia Univ. June 2009 Overview • Three methods for calibration of the displacement actuators • Free-swinging Michelson technique • Photon calibrator • Laser frequency modulation • Comparison of three methods • Bottom line: Three methods agree at the 5% level.

  3. R. Savage Amaldi 8 Columbia Univ. June 2009 Calibration in LIGO • Strives to calibrate the detector’s response to differential length variations • Goal for initial LIGO (S5 science run) • 10% in amplitude and 10 deg. in phase • Over frequency band from 40 Hz to 2 kHz • Over all times with a 1 second resolution (update interval) • Similar calibration goals for S6 • AdvLigo and beyond • Motivated by better source localization, waveform reconstruction, etc. (from analysis groups), potentially down to a few or even 1% level in amplitude

  4. R. Savage Amaldi 8 Columbia Univ. June 2009 Differential arm length control loop • Closed-loop response to differential length variations • Accurate measurement of the length actuation function is essential for DARM ERR calibration DARM ERR Actuation Path

  5. R. Savage Amaldi 8 Columbia Univ. June 2009 Free-swinging Michelson method Use laser wavelength (1064 nm) as a length reference and bootstrap ETM calibration via a series of measurements Record free-swinging (short) Michelson signal at AS port. Lock short Michelson config. and measure OLTF Measure ITM to AS signal TF Lock single arm and measure ETM/ITM TF ITM AS Now using asymmetric Michelson technique to simplify this method ITM ETM AS

  6. R. Savage Amaldi 8 Columbia Univ. June 2009 Free-swinging Michelson results • Swept sine measurements over whole frequency band • ~ 5% variation due to actuation path electronics • Model corrected for “residuals”

  7. R. Savage Amaldi 8 Columbia Univ. June 2009 Photon Calibrator method Use a power-modulated auxiliary laser to displace the mass via the recoil of the photons (in “Science mode” configuration).

  8. R. Savage Amaldi 8 Columbia Univ. June 2009 Pcal overview • Have been (are being) used at Glasgow, GEO, Virgo as well • Under development at LIGO since about 2001(D. Sigg et al.) • During and after S5, several upgrades were implemented to improve accuracy and precision (Goetz, Kalmus, Erickson, Savage) • Two-beam configuration to minimize influence of elastic deformation by the Pcal beams (recognized by S. Hild et al. at GEO) • Accounting for rotation-induced apparent length variations • Absolute power calibration via temperature-stabilized photodetectors mounted on integrating spheres with a “Gold Standard” calibrated at NIST • Simultaneous coil and Pcal actuation at closely separated freqs.

  9. R. Savage Amaldi 8 Columbia Univ. June 2009 Pcal hardware LLO eLigo Pcal (with M. West and R. DeRosa) Plan to run Pcal lines continuously on both interferometers during S6 Transfer of Gold Standard calibration to Working Standard

  10. R. Savage Amaldi 8 Columbia Univ. June 2009 2-beam vs 1-beam Pcals Single, centeredPcal beam Two-beam Pcal

  11. R. Savage Amaldi 8 Columbia Univ. June 2009 Pcal results • Dominant sources of errors • Rotation (beam position uncertainty) ~ 1% • Laser power measurement ~ 0.7% • Statistical ~ .25% • Overall ~1.3%(one sigma)

  12. R. Savage Amaldi 8 Columbia Univ. June 2009 Frequency modulation method Modulate the laser frequency to create an “effective” length modulation (in single arm lock configuration). • Frequency-to-length transfer function • For f < few kHz

  13. R. Savage Amaldi 8 Columbia Univ. June 2009 Frequency modulation via VCO

  14. R. Savage Amaldi 8 Columbia Univ. June 2009 VCO method Error point Vco Coil Calibration of VCO by measuring sideband to carrier ratio Simultaneous excitation of coils and VCO

  15. R. Savage Amaldi 8 Columbia Univ. June 2009 VCO results • Dominant sources of errors • VCO calibration~0.5% • Measurement statistical variations~ 1% • Overall ~1.2%(one sigma)

  16. R. Savage Amaldi 8 Columbia Univ. June 2009 Comparison of results +,- 5%

  17. R. Savage Amaldi 8 Columbia Univ. June 2009 Conclusions • Free-swinging Michelson • Michelson and single-arm locks – multiple measurements required • Swept sine over full frequency range • Photon calibrator • Operates in “science mode” configuration • Available laser power limits max. displacement, especially at high frequency – S/N dropping as f^3 • Frequency modulation • Simple, “force-free” approach • Single-arm locks • Swept sine possible • Three methods agree at the 5% level • No systematic discrepancies observed • More extensive use of both Pcal and VCO methods planned for S6.

  18. R. Savage Amaldi 8 Columbia Univ. June 2009 LIGO Pcal enhancements

  19. R. Savage Amaldi 8 Columbia Univ. June 2009 Absolute power calibration Transfer of Gold Standardto Working Standard In-chamber measurement of optical efficiency

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