Lasers and Optics of Gravitational Wave Detectors Rick Savage LIGO Hanford Observatory

GW detector – laser and optics 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 signal

Closer look - more lasers and optics

Pre-Stabilized Laser System Laser source Frequency pre-stabilization and actuator for further stab. Compensation for Earth tides Power stab. in GW band Power stab. at modulation freq. (~ 25 MHz)

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

LIGO I PSL performance 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 Servo systems maintain lock indefinitely (weeks - months)

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

Power stabilization 3e-8/rtHz In-band (40 Hz – 7 kHz) RIN Sensors located before and after suspended modecleaner Current shunt actuator - amp. pump diode current RIN at 25 MHz mod. freq. Passive filtering in 3-mirror triangular ring cavity (PMC) Bandwidth (FWHM) ~ 3.2 MHz 3e-8/rtHz

Earth Tide Compensation Up to 200 mm over 4 km Prediction applied to ref. cav. temp. (open loop) End test mass stack fine actuators relieve uncompensated residual 100mm prediction residual

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

Brassboard Performance LZH/MPI Hannover Integrated front end based on GEO 600 laser – 12-14 watts High-power slave – 195 watts M2 < 1.15

Concept for Advanced LIGO PSL

Core Optics – Test Masses Low-absorption fused silica substrates 25 cm dia. x 10 cm thick, 10 kg Low-loss ion beam coatings Suspended from single loop of music wire (0.3 mm) Rare-earth magnets glued to face and side for orientation actuation Internal mode Qs > 2e6

LIGO I core optics Caltech data RITM ~ 14 km (sagitta ~ 0.6 l) ; RETM ~ 8 km Surface uniformity ~ l/100 over 20 cm. dia. (~ 1 nm rms) “Super-polished” – micro-roughness < 1 Angstrom Scatter (diffuse and aperture diffraction) < 30 ppm Substrate absorption < 4 ppm/cm Coating absorption < 0.5 ppm

Adv. LIGO Core Optics fused silica sapphire LIGO recently chose fused silica over sapphire Familiarity and experience with polishing, coating, suspending, thermally compensating, etc. – less perceived risk Other projects (e.g. LCGT) still pursuing sapphire test masses Thermal noise in coatings expected to be greatest challenge fused silica sapphire 38 cm dia., 15.4 cm thick, 38 kg

Processing, Installation and Alignment Experience indicates that processing and handling may be source of optical loss gluing vacuum baking wet cleaning suspending balancing transporting

Thermal Issues Surface absorption depth radius Bulk absorption Circulating power in arm cavities ~ 25 kW for initial LIGO ~ 600 kW for adv. LIGO Substrate bulk absorption ~ 4 ppm/cm for initial LIGO ~ 0.5 ppm/cm ($) for adv. LIGO Coating absorption ~ 0.5 ppm for initial & adv. LIGO Thermo-optic coefficient dn/dT ~ 8.7 ppm/degK Thermal expansion coefficient 0.55 ppm/degK “Cold” radius of curvature of optics adjusted for expected “hot” state depth radius Bulk absorption

Thermal compensation system CO2 Laser ? ZnSe Viewport ITM PRM SRM ITM Compensation Plates Over-heat Correction Under-heat Correction Inhomogeneous Correction Adv. LIGO concept

Coating vs. substrate absorption Optical path difference Surface distortion substrate coating coating substrate OPD almost same for same amount of power absorbed in coating or substrate Power absorbed in coating causes ~ 3 times more surface distortion than same power absorbed in bulk

Summary LIGO utilizes 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 LIGO test masses (mirrors) 25 cm dia., 10 cm thick fused silica Surface uniformity ~ l/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 performance of optics

Anomalous absorption in H1 ifo. ITMY ITMX Negative values imply annulus heating Significantly more absorption in BS/ITMX than in ITMY How to identify absorption site? TCS power is absorbed in HR coatings of ITMs

Need for remote diagnostics 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 Development of remote diagnostics to determine which optics responsible of excess absorption

Spot size measurements ITMX BeamView CCD cameras in ghost beams from AR coatings Lock ifo. w/o TCS heating Measure spot size changes as ifo. cools from full lock state Curvature change in ITMX path about twice that in ITMY path ITMY

Arm cavity g factor changes 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 g factor change in Xarm larger than Yarm by factor of ~ 1.6 Calibrate with TCS (ITM-only surface absorption)

Results and options Beamsplitter not significant absorber ITMX is a significant absorber ~ 25 mW/watt incident ITMY absorption also high ~ 10 mW/watt incident Factor of ~5 greater than absorption in H2 or L1 ITMs Options Try to clean ITMX in situ Replace ITMX Higher power TCS system 30-watt TCS laser was tested Eventually ITMX was replaced and ITMY was cleaned in-situ ETM surface ITM surface ITM bulk From analysis by K. Kawabe

Origin of G-factor measurement technique Simple question: “For a resonant optical cavity, can the Pound-Drever-Hall locking signal distinguish between frequency and length variations?” i.e. does Of course! Or does it?

High-frequency response of optical cavities Dynamic resonance of light in Fabry-Perot cavities (Rakhmanov, Savage, Reitze, Tanner 2002 Phys. Lett. A, 305 239).

High frequency length response 1FSR 2FSR LIGO band Peaks in length response at multiples of FSR suggest searches for GWs at high frequencies. HF response to GWs different than length response Different antenna pattern, but still enhancement in sensitivity

High frequency response to GWs Long wavelength approximation not valid in this regime Antenna pattern becomes a function of source frequency as well as sky location and polarization All-sky-averaged response about a factor of 5 lower than at low freq. 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

G-factor 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.

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.

H1 data – Sept. 23, 2003 2ffsr- ftm 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

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