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GW Interferometry at Goddard Space Flight Center

Jordan CampNASA / Goddard Space Flight Center

Jan. 20, 2005

LIGO-G050038-00-Z

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This TalkThis Talk

•Frequency stabilization of lasers– Optical cavity– Molecular iodine

•Suspension point interferometer (SPI) for testing of low-frequency interferometry

•A few “politically correct” slides– New science direction for NASA

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Interferometry in SpaceInterferometry in Space

• Space interferometry a strategic direction in astronomy

• NASA and ESA are planning a significant number of space interferometry missions

– SIM 2010 Stellar interferometry – LISA 2014 Gravitational Waves– TPF-C 2015

TPF 2020 Extra-solar terrestrial planetsDarwin 2020

– MAXIM 2025 Black Hole imager (x-rays)– Etc…

• Need frequency stabilized lasers for all of these…

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LISA – Search for Gravitational WavesLISA – Search for Gravitational Waves

• A variety of astrophysical phenomena produce low-frequency gravitational waves– Massive BH binary

coalescence– Massive BH capture of

stellar mass BH– Galactic compact

binaries

•LISA will measure strain from GW’s of 10-21

•Measure position to 10-12 m, spacecraft separation of 5 x 109 m

• / ~ 10-21 for stabilized laser

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Methods of Laser StabilizationMethods of Laser Stabilization

• Frequency stabilization is only as good as the stability of the reference

• Optical resonator – Low-loss mirrors held fixed by ULE cavity– Length of cavity determines resonant

frequency– Variable DC frequency, temperature sensitive

• Atomic or molecular gas transition– Gas held in transparent cell– Transition provides absolute frequency

reference– Better at low frequencies, worse at high (~ 1

mHz crossover)

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Optical Cavities: experimental set-up Optical Cavities: experimental set-up

LaserIsolator

EOM

Cavities in 5 layers of gold Cavities in 5 layers of gold coated stainless steel in coated stainless steel in

vacuum chambervacuum chamber

Macor standoffs

Cavities manufactured from ULE Cavities manufactured from ULE cylinders with fused silica mirrors cylinders with fused silica mirrors optically contacted to end facesoptically contacted to end faces

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Results (Mueller, McNamara)Results (Mueller, McNamara)

ServoServoCrossoversCrossovers

LISA frequency noise requirementLISA frequency noise requirement

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Thermal noise limit to cavity frequency stabilization (K. Numata)

Thermal noise limit to cavity frequency stabilization (K. Numata)

•Thermal noise is fundamental limit to cavity stability

• Mechanical loss of spacer, mirror, coatings causes thermal noiseLimit ~ 3 Hz / Hz1/2 1 mHz

10-2 Hz / Hz1/2 100 Hz

NIST and VIRGO data both limited by ULE mirror substrates (Q ~ 6 x 104)

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Iodine laser stabilization layoutIodine laser stabilization layout

Laser

Laser

FI

FI

PP-MgO:LN

PP-MgO:LN

AOM

AOM

Iodine

Iodine

19 MHZ EOM

21 MHZ EOM

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Iodine stabilization laboratory setupIodine stabilization laboratory setup

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Iodine noise performance(20 cm length cell) V. LeonhardtIodine noise performance(20 cm length cell) V. Leonhardt

Intensity

Temperature

Electronic

0.0001 0.001 0.01 0.1Frequency [Hz]

1

10

100

1000

10000

lock-in amplifiert-noise sys1t-noise sys2ampl. sys2 probeampl. sys1 probefrequency noise

Absolute frequency

Insensitive to alignment and temperature

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Low-Frequency Interferometry TestbedLow-Frequency Interferometry Testbed

Suspension Point Interferometer testing platform

•goal: lock platforms at picometer, nanoradian level•stable platforms will allow study of interferometry, noise

Iodine stabilized laser

Iodine stabilized laser

Hexapod PZT actuator

IFO output

IFO output

“Stabilization (locking)” beam

“Evaluation (measurement)” beam

Stability information

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Suspension Point InterferometerSuspension Point Interferometer

Hexapod: 6 PZT’s for 6 DOF controlres ~ 230 Hz, Q ~ 62 iodine stabilized lasers:

sense/control hexapod, and measure residual noise

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SPI Performance so farSPI Performance so far

10-15

10-14

10-13

10-12

10-11

10-10

RM

S S

ensi

ng

No

ise

[m]

0.0001 0.001 0.01 0.1 1

Frequency[Hz]

10-13

10-12

10-11

10-10

10-9

10-8

Dis

pla

cem

ent

No

ise

[m/r

tHz]

0.01 0.1 1 10 100 1000

Frequency[Hz]

Measurement system noise

Actuator noise

Sensor noise

Closed loop stability

Next: use ULE and bonded optics

Stability limited by CTE of mechanical mounts

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New direction for NASA scienceNew direction for NASA science

• “We support NASA’s Vision for Space Exploration….”– Moon, mars, infinity and beyond….

• Science activities shifting in this direction– Earth science measurements to support

planetary science of Mars, Saturn, etc.– Astrophysics must also show relevance to this

(not easy…)

• Terrestrial planet finding now a hot subject

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TPF – C : Search for Terrestrial PlanetsTPF – C : Search for Terrestrial Planets

•Coronagraph will look for reflected light from planet by blocking direct star light from nearby stars (< 15 pc)

•Spectroscopy of signal will give information on composition of planetary atmosphere

•Water, CO2, methane

•Contrast ratio of 1 : 109

•Telescope stability is very important

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Interferometry in TPF-CInterferometry in TPF-C

Stabilized laser and hexapod will control secondary mirror to10-9 m, 10-9 radian over hour timescaleMetrology and control scheme will be developed on SPI

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SummarySummary

• Stabilized lasers will fly!– LISA 2013– TPF-C2014

• Optical resonator and molecular transition under study– Noise requirements– Space qualification

• SPI testbed for low-frequency space interferometry – Iodine stabilization most useful