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High-Power Stabilized Lasers and Optics of GW Detectors

High-Power Stabilized Lasers and Optics of GW Detectors. Rick Savage LIGO Hanford Observatory. Overview. In general, I will discuss issues and hardware solutions from a LIGO perspective because of familiarity.

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High-Power Stabilized Lasers and Optics of GW Detectors

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  1. High-Power Stabilized Lasers and Optics of GW Detectors Rick Savage LIGO Hanford Observatory

  2. Overview • In general, I will discuss issues and hardware solutions from a LIGO perspective because of familiarity. • Other GW interferometers (GEO, LCGT, TAMA, Virgo) face similar issues and have developed their own solutions as will be seen in subsequent talks in this session. • Lasers • Initial LIGO - ~10 watts • Requirements, performance, technical issues • Advanced LIGO ~ 200 watts • Concept, status • Optics • Initial LIGO core optics – test masses • Requirements, performance, technical issues • Advanced LIGO • Plans

  3. Power Recycled Michelson Interferometer with Fabry-Perot Arm Cavities end test mass 4 km (2 km) Fabry-Perotarm cavity recycling mirror input test mass Laser beam splitter signal GW detector – laser and optics

  4. Closer look - more lasers and optics

  5. Pre-Stabilized Laser System • Laser source • Frequencypre-stabilizationand actuator forfurther stab. • Compensation for Earth tides • Power stab. inGW band • Power stab. at modulation freq.(~ 25 MHz)

  6. Initial LIGO 10-W laser • Master Oscillator Power Amplifier configuration (vs. injection-locked oscillator) • Lightwave Model 126 non-planar ring oscillator (Innolight) • Double-pass, four-stage amplifier • Four rods - 160 watts of laser diode pump power • 10 watts in TEM00 mode

  7. Running continuously since Dec. 1998 on Hanford 2k interferometer Maximum output power has dropped to ~ 6 watts Replacement of amplifier pump diode bars had restored performance in other units LIGO PSL hardware

  8. Concept for Advanced LIGO laser • Being developed by GEO/LZH • Injection-locked, end-pumped slave lasers • 180 W output with 1200 W of pump light

  9. Frequency stabilization • Three nested control loops • 20-cm fixed reference cavity • 12-m suspended modecleaner • 4-km suspended arm cavity • Ultimate goal: Df/f ~ 3 x 10-22

  10. Power stabilization • Sensors located before and after suspended modecleaner • Current shunt actuator controlling amplifier pump diode current • Pre-modecleaner for RIN measured upstream of MC

  11. RIN at 20-30 MHz • Describe requirement • Give formula for filtering by PMC ala T. Ralph (from old CCD) • PMC parameters • Photo of optically contacted PMC

  12. Tidal Compensation

  13. Overall experience with LIGO I PSL • Reliability • Long locks • Pmc problems • Laser problems • Ref cav performance

  14. Core Optics – Test Masses • Core optics requirements for initial and advanced ligo • Coating requirements • Q factor • Scattering/ absorption, etc. • Thermal noise – internal modes; noise due to coatings

  15. Surface uniformity < 1 nm rms Scatter < 50 ppm Absorption < 2 ppm ROC matched < 3% Internal mode Q’s > 2 x 106 LIGO I core optics Caltech data CSIRO data

  16. Advanced LIGO core optics

  17. Preparation and installation challenges • Photos of cleaning and installation • Description of problems with etching coatings during cleaning.

  18. Practical issues • Anamolous absorption • Vacuum incursions very costly – time and risky. • Need to make remote measuements due to water absorption in spring seats

  19. CO2 Laser ZnSe Viewport Over-heat pattern Inner radius = 4cm Outer radius =11cm Over-heat Correction Under-heat Correction Inhomogeneous Correction ? Thermal compensation system

  20. ITM Compensation Plates PRM ITM SRM Next-generation TCS • Design utilizes a fused silica suspended compensation plate • Actuation by a scanned CO2 laser (Small scale asymmetric correction) and nichrome heater ring (Large scale symmetric correction) • No direct actuation on ITMs for improved noise reduction, simplicity and lower power (Sapphire)

  21. Kilometer-scale Fabry-Perot cavities • Free spectral range ~ 37.5 kHz • Plot of H_w(f) and H_L(f)

  22. G-factor measurements

  23. Pre-stabilized laser • MOPA source • Frequency reference cavity • Pre-modecleaner cavity • Electro-optics modulators for stabilization and locking to cavities • Core optics • Optical levers • Wavefront sensors • Output beams • modematching telescopes • Periscopes • Photodetectors • 15-m modecleaner cavity • Wavefront sensors and piezo-controlled input pointing • Faraday isolator • Mode-matching telescope

  24. Pre-stabilized laser • Laser source and ancillary optical components and feedback control loops necessary to provide frequency and amplitude stabilized light to the interferometers (input optics subsystem). • Requirements • ~ 10 watts of stabilized light (first generation) • Frequency pre-stabilization to the ?? Level (10 Hz to 100 kHz) • Power stabilization to the ?? level (10 Hz to 100 kHz). • Power stabilization at GW detection modulation freq. (20-30 MHz). • Availability – long (10s to 100s of hours continuous operation without loss of lock). • Insert schematic of PSL/photos

  25. LIGO 10-W laser • Master oscillator power amplifier configuration • Developed under contract with Lightwave Electronics (model 126MOPA) • Oscillator – Non-planar ring oscillator • Monolithic design • Free-running frequency stability • Free-running RIN • Power amplifier • Four-rod, double-pass

  26. Measurement Technique • Dynamic resonance of light in Fabry-Perot cavities (Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, 305 239). • Laser frequency to PDH signal transfer function, Hw(s), has cusps at multiples of FSR and features at freqs. related to the phase modulation sidebands.

  27. Misaligned cavity • Features appear at frequencies related to higher-order transverse modes. • Transverse mode spacing:ftm = f01- f00 = (ffsr/p) acos (g1g2)1/2 • g1,2 = 1 - L/R1,2 • Infer mirror curvature changes from transverse mode spacing freq. changes. • This technique proposed by F. Bondu, Aug. 2002.Rakhmanov, Debieu, Bondu, Savage, Class. Quantum Grav.21 (2004) S487-S492.

  28. H1 data – Sept. 23, 2003 • Lock a single arm • Mis-align input beam (MMT3) in yaw • Drive VCO test input (laser freq.) • Measure TF to ASPD Qmon or Imon signal • Focus on phase of feature near 63 kHz 2ffsr- ftm

  29. Data and (lsqcurvefit) fits. ITMx TCS annulus heating  decrease in ROC (increase in curvature) R = 14337 m R = 14096 m Assume metrology value for RETMx = 7260 m Metrology value for ITMx = 14240 m

  30. To investigate heating via 1 mm light … • Lock ifo. for > 2 hours w/o TCS; Plaser= 2 W • Break full lock (t = 0) and quickly lock a single arm. • Misalign input beam (MMT3) in yaw • Measure temporal evolution of Hw(s) • Note: 1mm light heats both ETM and ITM • H1 Xarm dataFeb. 18, 2005

  31. Yarm measurement Feb. 19, 2005

  32. Time-dependent model based on Hello-Vinet formalism (J. Phys. France 51(1990) 2243-2261) Free parameters: “cold” radius of curvature and power absorbed Fits by eye (+,- 20%) Comparison with model – Phil Willems Xarmbulk absorption76 mW Xarmsurface absorption33 mW DR ~ 370 m DR ~ 320 m

  33. Comparison with model - Yarm • Phil Willems – time-dependent Hello-Vinet model Yarmsurface absorption25 mW Yarmbulk absorption50 mW DR ~ 250 m DR ~ 190 m

  34. Calibration using TCS heaing results • TCS calibrationXarm: 220m / 37mW = 5.9 m/mWYarm: 190m / 45mW = 4.2 m/mW • Surface (not bulk) absorption • 1064 nm heatingXarm: 293m / 5.9 m/mW = 49mWYarm: 177m / 4.2 m/mW = 42 mWAssumes all heating on surface and no absorption in ETMs • Surface-equivalent, ITM-onlyabsorption calibration 14.5 km D ~ 220 m 14.28 km 13.9 km D ~ 190 m 13.71 km

  35. “Cold” values from 1064 nm meas.ITMX: 14.226 km difference ~ 50 mITMY: 13.615 km difference ~ 100m Systematic errors? Alignment drifts – sampling different areas of TM surfaces More complex, time-dependent behavior of surface distortions? Phil Willems studying with time-dependent model of surface distortions g factor measurements and reduced data available inLIGO-T050030-00-W 14.5 km D ~ 220 m 14.28 km 13.9 km D ~ 190 m 13.71 km Issues – “cold” curvature differences

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