Status of the Laser Interferometer Gravitational-Wave Observatory

LIGO-G020518-00-W

Status of the Laser Interferometer Gravitational-Wave Observatory

Reported on behalf of LIGO colleagues by

Fred Raab,

LIGO Hanford Observatory

Status of LIGO 2LIGO-G020518-00-W

Laser Interferometer Gravitational-wave Observatory

LIGO Mission: To sense the distortions of spacetime, known as gravitational waves, created by cosmic cataclysms and to exploit this new sense for astrophysical studies

Over nearly 400 years, astronomy has developed many clever ways to “see” and study the universe; LIGO hopes to add a “sound track” using interferometers that function like large terrestrial microphones, listening for vibrations of space.


Gravitational Collapse and Its Outcomes Present LIGO Opportunities

fGW > few Hz accessible from earth

fGW < several kHz interesting for compact objects


Potential LIGO Sources

Supernovae, with strength depending on asymmetry of collapse

Inspirals and mergers of compact stars, like black holes, neutron stars

Starquakes and wobbles of neutron stars and black holes

Stochastic waves from the early universe, cosmic strings, etc.

Unknown phenomena


Gravitational Waves

Gravitational waves are ripples in space when it is stirred up by rapid motions of large concentrations of matter or energy

Rendering of space stirred by two orbiting black holes:


Catching WavesFrom Black Holes

Sketches courtesy of Kip Thorne


Energy Loss Caused By Gravitational Radiation Confirmed

In 1974, J. Taylor and R. Hulse discovered a pulsar orbiting a companion neutron star. This “binary pulsar” provides some of the best tests of General Relativity. Theory predicts the orbital period of 8 hours should change as energy is carried away by gravitational waves.

Taylor and Hulse were awarded the 1993 Nobel Prize for Physics for this work.

LIGO-G020518-00-W

How does LIGO detect spacetime vibrations?


Important Signature of Gravitational Waves

Gravitational waves shrink space along one axis perpendicular to the wave direction as they stretch space along another axis perpendicular both to the shrink axis and to the wave direction.


Laser

Beam Splitter

End Mirror End Mirror

ScreenViewing

Sketch of a Michelson Interferometer


LIGO (Washington) LIGO (Louisiana)

The Laser InterferometerGravitational-Wave Observatory

Supported by the U.S. National Science Foundation; operated by Caltech and MIT; the research focus for more than 400 LIGO Scientific Collaboration members

worldwide.


2998 km

(+/- 10 ms)

CIT

MIT

The Four Corners of the LIGO Laboratory

Observatories at Hanford, WA (LHO) & Livingston, LA (LLO)

Support Facilities @ Caltech & MIT campuses

LHO

LLO


Part of Future International Detector Network

LIGO

Simultaneously detect signal (within msec)

detection confidence locate the sources

decompose the polarization of gravitational waves

GEO VirgoTAMA

AIGO


Spacetime is Stiff!

=> Wave can carry huge energy with miniscule amplitude!

h ~ (G/c4) (ENS/r)


Some of the Technical Challenges

Typical Strains less than ~ 10-21 at Earth ~ 1 hair’s width at 4 light years

Understand displacement fluctuations of 4-km arms at the millifermi level (1/1000th of a proton diameter)

Control arm lengths to 10-13 meters RMS Detect optical phase changes of ~ 10-10 radians Engineer structures to mitigate recoil from atomic

vibrations in suspended mirrors Provide clear optical paths within 4-km UHV beam

lines


Recycling Mirror

Optical

Cavity

4 km or2-1/2

miles

Beam Splitter

Laser

Photodetector

Fabry-Perot-Michelson with Power Recycling


Vacuum Chambers Provide Quiet Homes for Mirrors

View inside Corner Station

Standing at vertex beam splitter

P < 10-6 Torr in chambers to reduce acoustics & molecular Brownian motion


Beam Tube Provides Distortion-Free 4-km Pathway for Light

Requires P ~ 10-9 Torr H2 Equivalent Baked out by insulating tube and driving ~2000 amps

from end to end


What Limits Sensitivityof Interferometers?

• Seismic noise & vibration limit at low frequencies

• Atomic vibrations (Thermal Noise) inside components limit at mid frequencies

• Quantum nature of light (Shot Noise) limits at high frequencies

• Myriad details of the lasers, electronics, etc., can make problems above these levels


Design for Low Background Spec’d From Prototype Operation

For Example: Noise-Equivalent Displacement of 40-meter Interferometer (ca1994)


Vibration Isolation Systems

» Reduce in-band seismic motion by 4 - 6 orders of magnitude» Little or no attenuation below 10Hz» Large range actuation for initial alignment and drift compensation» Quiet actuation to correct for Earth tides and microseism at 0.15 Hz during

observation

HAM Chamber BSC Chamber


Seismic Isolation – Springs and Masses

damped springcross section


Seismic System Performance

102

100

10-2

10-4

10-6

10-8

10-10

Horizontal

Vertical

10-6

HAM stackin air

BSC stackin vacuum


Core Optics Suspension and Control

Local sensors/actuators provide damping and control forces

Mirror is balanced on 1/100th inchdiameter wire to 1/100th degree of arc

Optics suspended as simple pendulums


Suspended Mirror Approximates a Free Mass Above Resonance


IO

Frequency Stabilization of the Light Employs Three Stages

Pre-stabilized laser delivers light to the long mode cleaner

• Start with high-quality, custom-built Nd:YAG laser

• Improve frequency, amplitude and spatial purity of beam

Actuator inputs provide for further laser stabilization

• Wideband• Tidal

10-WattLaser

PSL Interferometer

15m4 km


Interferometer Control System •Multiple Input / Multiple Output

•Three tightly coupled cavities

•Ill-conditioned (off-diagonal) plant matrix

•Highly nonlinear response over most of phase space

•Transition to stable, linear regime takes plant through singularity

•Employs adaptive control system that evaluates plant evolution and reconfigures feedback paths and gains during lock acquisition

•But it works!


Steps to Locking an Interferometer

signal

LaserX Arm

Y Arm

Composite Video


Watching the Interferometer Lock

signal

X Arm

Y Arm

Laser


Why is Locking Difficult?

One meter, about 40 inches

Human hair, about 100 microns000,10

Wavelength of light, about 1 micron100

LIGO sensitivity, 10-18 meter000,1

Nuclear diameter, 10-15 meter000,100

Atomic diameter, 10-10 meter000,10

Earthtides, about 100 microns

Microseismic motion, about 1 micron

Precision required to lock, about 10-10 meter


Tidal Compensation Data

Tidal evaluation on 21-hour locked section of S1 data

Residual signal on voice coils

Predicted tides

Residual signal on laser

Feedforward

Feedback


Microseism

Trended data (courtesy of Gladstone High School) shows large variability of microseism, on several-day- and annual- cycles

Reduction by feed-forward derived from seismometers

Microseism at 0.12 Hz dominates ground velocity


Background Forces in GW Band = Thermal Noise ~ kBT/mode

Strategy: Compress energy into narrow resonance outside band of interest require high mechanical Q, low friction

xrms 10-11 mf < 1 Hz

xrms 210-17 mf ~ 350 Hz

xrms 510-16 mf 10 kHz


Thermal Noise Checked in 1st Violins on H2, L1 During S1

Almost good enough for tracking calibration.


Chronology of Detector Installation & Commissioning

7/98 Begin detector installation 6/99 Lock first mode cleaner 11/99 Laser spot on first end mirror 12/99 First lock of a 2-km Fabry-Perot arm 4/00 Engineering Run 1 (E1) 10/00 Recombined LHO-2km interferometer in E2 run 10/00 First lock of LHO-2km power-recycled interferometer 2/01 Nisqually earthquake damages LHO interferometers 4/01 Recombined 4-km interferometer at LLO 5/01 Earthquake repairs completed at LHO 6/01 Last LIGO-1 mirror installed 12/01 Power recycling achieved for LLO-4km 1/02 E7: First triple coincidence run; first on-site data analysis 1/02 Power recycling achieved for LHO-4km 9/02 First Science Run (S1) completed 2/03 Second Science Run (S2) begun


LIGO SensitivityLivingston 4km Interferometer

May 01

Jan 03


S1 Analysis Working Groups

Data from S1 is being analyzed by LSC working groups for:» Detector Characterization – monitors of data quality, instrument

diagnostics, tuning displays

» Binary Inspirals – modeled chirps from BH’s, NS’s, MACHO’s

» Bursts – unmodeled deviations from stationarity; independent searches; searches triggered on GRB’s, SN’s

» Periodic Sources – known pulsar search; work toward all-sky search for unknown pulsars

» Stochastic Background – early universe


Summary

First triple coincidence run completed (17 days with ~23% triple coincidence duty factor)

On-line data analysis systems (Beowulf parallel supercomputer) functional at LHO and LLO

S1 coincidence analyses with GEO & TAMA are first science analyses with international laser-GW network

First science data analysis ongoing Interferometer control system still being commissioned and

tuned Working to increase immunity to high seismic noise periods

(especially important at LLO) S2 running 14Feb03 – 14Apr03


Future Plans

Improve reach of initial LIGO to run 1 yr at design sensitivity

Advanced LIGO technology under development with intent to install by 2008

Planning underway for space-based detector, LISA, to open up a lower frequency band ~ 2015


Despite a few difficulties, science runs started in 2002.

Status of the Laser Interferometer Gravitational-Wave Observatory

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