1. LIGO-G060191-00-W Washington State University, Pullman April, 2006 1 Lasers and Optics of Gravitational Wave Detectors Rick Savage LIGO Hanford Observatory

2. LIGO-G060191-00-W Washington State University, Pullman April, 2006 2 Overview l LIGO laser requirements/performance l LIGO core optics requirements/performance l Optics thermal compensation system »Excess absorption in H1 interferometer optics l New measurement technique –Dynamic response of 4-km-long optical cavity

3. LIGO-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 4 Closer look - more lasers and optics

5. LIGO-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 13 Concept for Advanced LIGO PSL

14. LIGO-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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 ~ 3 times more surface distortion than same power absorbed in bulk coating substrate coating substrate

20. LIGO-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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 determine which optics responsible of excess absorption

23. LIGO-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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-G060191-00-W Washington State University, Pullman April, 2006 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 was tested l Eventually ITMX was replaced and ITMY was cleaned in-situ ITM bulk ITM surface ETM surface From analysis by K. Kawabe

26. LIGO-G060191-00-W Washington State University, Pullman April, 2006 26 l Simple question: “For a resonant optical cavity, can the Pound-Drever-Hall locking signal distinguish between frequency and length variations?” l i.e. does l Of course! Or does it? Origin of G-factor measurement technique

27. LIGO-G060191-00-W Washington State University, Pullman April, 2006 27 High-frequency response of optical cavities l Dynamic resonance of light in Fabry-Perot cavities ( Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, 305 239 ).

28. LIGO-G060191-00-W Washington State University, Pullman April, 2006 28 High frequency length response 1FSR2FSR LIGO band l Peaks in length response at multiples of FSR suggest searches for GWs at high frequencies. l HF response to GWs different than length response l Different antenna pattern, but still enhancement in sensitivity

29. LIGO-G060191-00-W Washington State University, Pullman April, 2006 29 High frequency response to GWs l Long wavelength approximation not valid in this regime l Antenna pattern becomes a function of source frequency as well as sky location and polarization l All-sky-averaged response about a factor of 5 lower than at low freq. l Significant sensitivity near multiples of 37.5 kHz (arm cavity FSR) Movie (by H. Elliott): Antenna pattern for one source polarization as source frequency sweeps from 22 to 36 kHz

30. LIGO-G060191-00-W Washington State University, Pullman April, 2006 30 G-factor 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.

31. LIGO-G060191-00-W Washington State University, Pullman April, 2006 31 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.

32. LIGO-G060191-00-W Washington State University, Pullman April, 2006 32 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

33. LIGO-G060191-00-W Washington State University, Pullman April, 2006 33 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

34. LIGO-G060191-00-W Washington State University, Pullman April, 2006 34 Summary l LIGO utilized 10-W solid state lasers »Relative frequency stability ~ 10 -21 /rtHz »Relative power stability ~ 10 -8 /rtHz »Advanced LIGO lasers: similar requirements at 200 watt power level l LIGO test masses (mirrors) 25 cm dia., 10 cm thick fused silica »Surface uniformity ~ /100 p-v (1 nm rms) over 20 cm diameter »Coating absorption < 1 ppm, bulk absorption ~ few ppm/cm »Active thermal compensation required to match curvatures of optics »Non-invasive measurement techniques required for characterizing optics performance l High frequency responses to length and frequency modulation not the same. l Length sensitivity peaks at multiples of arm cavity FSR l Antenna pattern is frequency dependent at high frequencies l Increased sensitivity to HF gravitational waves.