Commissioning the LIGO detectors Stability and sensitivity improvements October 21, 2004 Nergis Mavalvala

LIGO : Laser Interferometer Gravitational-wave Observatory 3 k m ( ± 1 s ) 4 km 2 km LA 4 km

Initial LIGO

H1 noise history (science design) S3 S2 S1

Interferometer Layout laser

L1 Estimated Noise Limits for S2 (expected)

H1 Noise Model

Recent commissioning efforts Reliability & Stability Seismic retrofit at LLO: Hydraulic External Pre-Isolator (HEPI) Sensitivity Operate at high power: achieve designed optical gain Laser power increase Thermal compensation system (TCS)

Seismic Noise Motion of the earth is a few mm at low frequencies Passive and active seismic isolation Amplify mechanical resonances Get isolation above a few Hz

Daily Variability of Seismic Noise RMS motion in 1-3 Hz band day night Livingston Displacement (m) Hanford PRE-ISOLATOR REQUIREMENT Time (GPS seconds)

Hydraulic External Pre-Isolation: HEPI at LLO

X-arm length disturbance on a noisy afternoon

HEPI summary Remaining tasks Summary Complete commissioning of final six chambers Sensor and whitening filter optimization Scripting, safeties, man-machine interface software Summary All hardware installed Crucial one-arm test completed successfully LIGO should soon have two sites capable of night and day operation with reasonable duty cycle Pre-Isolator is first Advanced LIGO subsystem shown to work at required specification in an Observatory setting

H1 noise history (science design) S3 S2 S1

Acoustic Noise Coupling Peaks occur in 80-1000 Hz band 10 to 100x the design sensitivity Source of H1-H2 correlations Acoustic Excitations loud quiet

Acoustic Mitigation Acoustic enclosures around output tables Reduce couplings Float optics tables Simplify beam path New stiffer periscope Reduce source Muffle fan noise at electronics crates Racks on isolation legs Move racks Reduce HVAC noise Insulate mechanical room 18

H2 Faraday Isolator Replacement Larger aperture to reduce beam clipping Reduce thermally induced drift at REFL

H1 noise history (science design) S3 S2 S1

What is shot noise? Laser light comes in discrete packets of photons Poisson statistics of light arrival times at the gravitational-wave port photodiodes Statistical fluctuation in detected photons appears as phase (i.e. length or strain) noise in interferometer Signal-to-noise ratio increases as the square-root of laser power in the ifo

What can we do about shot noise? Increase laser power Lasers refurbishing, now running at ~8W Approximately 4W incident on interferometers Input optics-train modified and aligned for better throughput Additional photodiodes added to antisymmetric port Additional power produces “thermal lensing” in interferometer optics Need Thermal Compensation System (TCS) Ensure other high-frequency noise sources are reduced accordingly E.g. “oscillator phase noise”

Thermal Compensation System (TCS) Mirrors of the interferometer absorb laser light and heat up  thermal lensing Mirror profile (shape) distorted according to the laser beam and absorption profiles Use external laser beam shaped to match the input beam to the mode supported in the arm cavities CO2 Laser ITM ZnSe Viewport Over-heat pattern Inner radius = 4cm Outer radius =11cm Over-heat Correction Under-heat Correction Inhomogeneous Correction

Two CO2 lasers installed on H1 To input test mass (ITM) High Reflectivity surface

TCS on the power recycled Michelson Beam images at antisymmetric port No Heating 30 mW 60 mW 90 mW Best match 120 mW 150 mW 180 mW Input beam

Output Mode Cleaner -- The Good Carrier contrast defect improves by 20x With OMC: carrier 2% of total power Makes it possible to reduce modulation depth Removes offset corresponding to 10-12 m Reduced AM noise coupling: factor of 60 at 3 kHz Reduced oscillator phase noise coupling: factor of 2 at 3 kHz ASI signal decreases by a factor of 7 “ASI locking” symmetrizes RF sidebands Would be able to operate with a single PD at AS port!

Output Mode Cleaner -- The Bad

Output Mode Cleaner -- The Ugly Higher order modes and beam jitter generate a PDH-like signal Elliptical beam is a problem Triangular cavity geometry is a problem beam jitter 5o

High-frequency noise improvements Recent high-frequency noise improvement by factor of 2 to 4 Thermal compensation required to support this gain in sensitivity (otherwise thermal lensing would limit such gains)

Many other key commissioning efforts underway, including Parallel efforts Many other key commissioning efforts underway, including Control of angular degrees-of-freedom (d.o.f.) All 14 angular d.o.f. controlled on Hanford 4km interferometer Input pointing into the interferometers controlled Acoustic mitigation Reducing control noise from other feedback systems in the interferometers

Time Line 1999 2000 2001 2002 2003 2004 Inauguration First Lock 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q 1Q 2Q 3Q 4Q Inauguration First Lock Full Lock all IFO Now Engineering E1 E2 E3 E4 E5 E6 E7 E8 E9 Runs S1 S2 S3 S4 (expected) Science First Science Data

Looking ahead L1 interferometer: H1 interferometer: H2 interferometer: Finish Hydraulic external pre-Isolator (HEPI) Get good spectrum back Implement improvements from LHO 4km interferometer H1 interferometer: High power operations “output mode cleaner” test version 2 New frequency and intensity stabilization, “common mode” boards, non-resonant sideband photodiodes on interferometer reflected output port Optimize dewhitening for new low-noise digital-to-analog converters H2 interferometer: Implement improvements from the Hanford 4km interferometer Expect 4-6 week science run beginning Jan 2005 On track to reach design sensitivity and begin an extended science run summer 2005