LIGO-G W Washington State University, Pullman April, Lasers and Optics of Gravitational Wave Detectors Rick Savage LIGO Hanford Observatory

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, Closer look - more lasers and optics

LIGO-G W Washington State University, Pullman April, 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)

LIGO-G W Washington State University, Pullman April, 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 watts of laser diode pump power l 10 watts in TEM 00 mode

LIGO-G W Washington State University, Pullman April, LIGO I PSL performance l Running continuously since Dec 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)

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, Brassboard Performance l LZH/MPI Hannover l Integrated front end based on GEO 600 laser – watts l High-power slave – 195 watts M 2 < 1.15

LIGO-G W Washington State University, Pullman April, Concept for Advanced LIGO PSL

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, Processing, Installation and Alignment Experience indicates that processing and handling may be source of optical loss gluing vacuum baking wet cleaning suspending balancing transporting

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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)

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, G-factor Measurement Technique l Dynamic resonance of light in Fabry-Perot cavities ( Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, ). 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.

LIGO-G W Washington State University, Pullman April, 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 Rakhmanov, Debieu, Bondu, Savage, Class. Quantum Grav. 21 (2004) S487-S492.

LIGO-G W Washington State University, Pullman April, 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

LIGO-G W Washington State University, Pullman April, Data and (lsqcurvefit) fits. Assume metrology value for R ETMx = 7260 m Metrology value for ITMx = m ITMx TCS annulus heating  decrease in ROC (increase in curvature) R = mR = m

LIGO-G W Washington State University, Pullman April, Summary l LIGO utilized 10-W solid state lasers »Relative frequency stability ~ /rtHz »Relative power stability ~ /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.