1. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 1 High-Power Stabilized Lasers and Optics of GW Detectors Rick Savage LIGO Hanford Observatory

2. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 2 Overview l In general, 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 l Lasers »Initial LIGO - ~10 watts –Requirements, performance »Advanced LIGO ~ 200 watts –Concept, status l Optics »Initial and advanced LIGO core optics – test masses –Requirements, performance »Excess absorption in H1 interferometer optics –Efforts to identify absorption site

3. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 3 GW detector – laser and optics Laser end test mass 4 km (2 km) Fabry-Perot arm cavity recycling mirror input test mass beam splitter Power Recycled Michelson Interferometer with Fabry-Perot Arm Cavities Power Recycled Michelson Interferometer with Fabry-Perot Arm Cavities signal

4. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 4 Closer look - more lasers and optics

5. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 5 Pre-Stabilized Laser System l Laser source l Frequency pre-stabilization and actuator for further stab. l Compensation for Earth tides l Power stab. in GW band l Power stab. at modulation freq. (~ 25 MHz)

6. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 6 Initial LIGO 10-W laser l Master Oscillator Power Amplifier configuration (vs. injection-locked oscillator) l Lightwave Model 126 non-planar ring oscillator (Innolight) l Double-pass, four-stage amplifier »Four rods - 160 watts of laser diode pump power l 10 watts in TEM 00 mode

7. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 7 LIGO I PSL performance l Running continuously since Dec. 1998 on Hanford 2k interferometer l Maximum output power has dropped to ~ 6 watts l Replacement of amplifier pump diode bars had restored performance in other units l Servo systems maintain lock indefinitely (weeks - months)

8. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 8 Frequency stabilization l Three nested control loops »20-cm fixed reference cavity »12-m suspended modecleaner »4-km suspended arm cavity l Ultimate goal:  f/f ~ 3 x 10 -22

9. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 9 Power stabilization l In-band (40 Hz – 7 kHz) RIN »Sensors located before and after suspended modecleaner »Current shunt actuator - amp. pump diode current 3e-8/rtHz l RIN at 25 MHz mod. freq. »Passive filtering in 3-mirror triangular ring cavity (PMC) »Bandwidth (FWHM) ~ 3.2 MHz

10. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 10 Earth Tide Compensation Up to 200  m over 4 km l Prediction applied to ref. cav. temp. (open loop) l End test mass stack fine actuators relieve uncompensated residual 100  m predictionresidual

11. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 11 Concept for Advanced LIGO laser l Being developed by GEO/LZH l Injection-locked, end- pumped slave lasers l 180 W output with 1200 W of pump light

12. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 12 Brassboard Performance l LZH/MPI Hannover l Integrated front end based on GEO 600 laser – 12-14 watts l High-power slave – 195 watts M 2 < 1.15

13. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 13 Concept for Advanced LIGO PSL

14. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 14 Core Optics – Test Masses l Low-absorption fused silica substrates »25 cm dia. x 10 cm thick, 20 kg l Low-loss ion beam coatings l Suspended from single loop of music wire (0.3 mm) l Rare-earth magnets glued to face and side for orientation actuation l Internal mode Qs > 2e6

15. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 15 LIGO I core optics Caltech data R ITM ~ 14 km (sagitta ~ 0.6 ) ; R ETM ~ 8 km Surface uniformity ~ /100 over 20 cm. dia. (~ 1 nm rms) l “Super-polished” – micro-roughness < 1 Angstrom l Scatter (diffuse and aperture diffraction) < 30 ppm l Substrate absorption < 4 ppm/cm l Coating absorption < 0.5 ppm

16. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 16 Adv. LIGO Core Optics l LIGO recently chose fused silica over sapphire »Familiarity and experience with polishing, coating, suspending, thermally compensating, etc. – less perceived risk l Other projects (e.g. LCGT) still pursuing sapphire test masses l Thermal noise in coatings expected to be greatest challenge sapphire fused silica 38 cm dia., 15.4 cm thick, 38 kg

17. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 17 Processing, Installation and Alignment Experience indicates that processing and handling may be source of optical loss gluing vacuum baking wet cleaning suspending balancing transporting

18. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 18 Thermal Issues l Circulating power in arm cavities »~ 25 kW for inital LIGO »~ 600 kW for adv. LIGO l Substrate bulk absorption »~ 4 ppm/cm for initial LIGO »~ 0.5 ppm/cm ($) for adv. LIGO l Coating absorption »~ 0.5 ppm for initial & adv. LIGO l Thermo-optic coefficient » dn/dT ~ 8.7 ppm/degK l Thermal expansion coefficient »0.55 ppm/degK l “Cold” radius of curvature of optics adjusted for expected “hot” state radius depth Surface absorption Bulk absorption

19. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 19 Coating vs. substrate absorption Optical path difference Surface distortion l OPD almost same for same amount of power absorbed in coating or substrate l Power absorbed in coating causes more surface distortion than same power absorbed in bulk

20. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 20 Thermal compensation system CO 2 Laser ? Over-heat Correction Inhomogeneous Correction Under-heat Correction ZnSe Viewport ITM PRM SRM ITM Compensation Plates Adv. LIGO concept

21. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 21 Anomalous absorption in H1 ifo. ITMY ITMX l Negative values imply annulus heating l Significantly more absorption in BS/ITMX than in ITMY l How to identify absorption site? TCS power is absorbed in HR coatings of ITMs

22. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 22 Need for remote diagnostics l Water absorption in viton spring seats makes vacuum incursions very costly. »Even with dry air purge, experience indicates that 1-2 weeks pumping required per 8 hours vented before beam tubes can be exposed to chambers l Development of remote diagnostics to develop which optics responsible of excess absorption

23. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 23 Spot size measurements ITMX ITMY l BeamView CCD cameras in ghost beams from AR coatings l Lock ifo. w/o TCS heating l Measure spot size changes as ifo. cools from full lock state l Curvature change in ITMX path about twice that in ITMY path

24. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 24 Arm cavity g factor changes l Again, lock full ifo. w/o TCS heating, break lock, lock single arm and measure arm cavity g factor at precise intervals after breaking lock l g factor change in Xarm larger than Yarm by factor of ~ 1.6 l Calibrate with TCS (ITM-only surface absorption)

25. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 25 Results and options l Beamsplitter not significant absorber l ITMX is a significant absorber ~ 25 mW/watt incident l ITMY absorption also high ~ 10 mW/watt incident » Factor of ~5 greater than absorption in H2 or L1 ITMs l Options »Try to clean ITMX in situ »Replace ITMX »Higher power TCS system l 30-watt TCS laser presently being tested ITM bulk ITM surface ETM surface

26. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 26 Measurement Technique l 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, H  (s), has cusps at multiples of FSR and features at freqs. related to the phase modulation sidebands.

27. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 27 Misaligned cavity l Features appear at frequencies related to higher-order transverse modes. Transverse mode spacing: f tm = f 01 - f 00 = (f fsr /  acos (g 1 g 2 ) 1/2 l g 1,2 = 1 - L/R 1,2 l Infer mirror curvature changes from transverse mode spacing freq. changes. l This technique proposed by F. Bondu, Aug. 2002. Rakhmanov, Debieu, Bondu, Savage, Class. Quantum Grav. 21 (2004) S487-S492.

28. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 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 Q mon or I mon signal Focus on phase of feature near 63 kHz 2f fsr - f tm

29. LIGO-G050251-00-W 2005 CLEO/QELS Joint Symposium on Gravitational Wave Detection 29 Data and (lsqcurvefit) fits. Assume metrology value for R ETMx = 7260 m Metrology value for ITMx = 14240 m ITMx TCS annulus heating  decrease in ROC (increase in curvature) R = 14337 mR = 14096 m