Interferometry Experiments at Goddard Space Flight Center

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Interferometry Experiments at Goddard Space
Flight Center

• Kenji Numata, Jordan Camp
• Gravitational Astrophysics Laboratory
• Code 663
• Astrophysics Science Division
• NASA/GSFC (Univ. of Maryland)

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1. Iodine laser stabilization

• Iodine meets LISA frequency noise requirements
• Provides absolute reference frequency (/- 300
  Hz)
• System does not need temperature stabilization
• Frequency tunable as is
• System simplification is possible 1 or no
  modulators
• Concern raised at last WG meeting distortion of
  main beam through frequency doubling crystal

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Iodine laser stabilization

• New type of doubling crystal now available from
  AdvR Periodically poled lithium niobate
  waveguide
• High doubling efficiency 1 mW green output for
  100 mW IR input
• So we can use 100 mW picked off from main beam to
  go through waveguide for generation of green
  light
• Main beam suffers no distortion!
• Iodine has many attractive features as back-up
  technology
• Raise your hand if you believe.

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2. Suspension Point Interferometer

• Concept active low-frequency environment
  stabilization
• Stabilized metrology
• Stabilization interferometer measures
  environmental relative motion.
• Signal is fed back to actuator (Hexapod
  controlled by PZTs).
• Measurement interferometer measures stability of
  components (interferometer itself)
• Outside-of-loop measurement
• Both interferometers use molecular iodine as
  frequency reference

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Optical Configuration

• Metrology interferometer
• 2 stabilization beams
• Length yaw (rotation)
• 1 measurement beam
• Homodyne Michelson
• Retro-reflectors for easier alignment robustness

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Photographs

• Main components
• Laser with iodine frequency reference
• Monolithic silicate-bonded optics
• Hexapod
• Distance between hexapods 1m
• Digital control system
• 2-DoF control (length yaw)

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Stabilization Result

• Current system performance in frequency spectrum
• Free-running
• 1um/rtHz _at_1mHz level (over 1m / identical
  optical table / vacuum)
• Stabilized
• Gain 500 achieved
• Satisfies 30pm/rtHz above 10mHz

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Identifying Noise Sources

• In progress
• Interferometer (sensor) noise
• Evaluated on single optical bench
• Electronic noise
• Small contributions

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Monolithic Test Interferometer

• Test on single optical bench
• Same readout scheme to evaluate noises
• 0.1nm/rtHz at 1mHz
• Limited by unidentified noises --- beam pointing
  noise?
• Upper limit of stability of ULE bench, mirrors
• 1nm/rtHz at 1mHz with two controlled optical
  benches
• Further improvements should be possible in
  2-bench configuration.

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Other Possible Noises

• Beam pointing
• Stable fiber launcher required
• Coupling from uncontrolled degree of freedom
• Other DoFs to be controlled

SIM beam launcher
LISA pathfinder beam launcher
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DMI Test

• Calibration of commercial interferometer (DMI by
  Zygo)
• Planned to be used in JWST mirror testing
• Question
• How stable/sensitive is DMI over long path length
  time scales?
• Our system provides stable environment
• To evaluate DMI

Work done with Dr. Anthony Yu (Code 554)
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DMI Experiment

• DMI mounting to ULE base
• No relative drift allowed
• DMI requires screw threads for mounting
• Polished invar piece as a spacer
• ltLambda/4 level polish (done by Cumberland
  Optics)
• Silicate-bonded to ULE
• Most stable glass-metal bonding method
• Displacement noise level ltpm/rtHz
• No alignment possible once fixed
• Vacuum
• DMI was put inside vacuum tank.
• Eliminates noises associated with air

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Results

• Still under progress
• In frequency domain
• 10 times worse than our system
• 10nm/rtHz _at_ 1mHz
• In time domain
• Drift rate 4nm/hour (temperature dependent)

These data were taken with 1m path length.
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5. Future Works

• Upgrades (short term)
• Achieving LISA requirements and better DMI
  evaluations
• Several remaining issues
• Direct measurement of DMIs frequency noise
  (drift) using 2 DMIs.
• Adding other DoF control
• Pitch might be coupled to DMI measurement
  through beam height difference.
• Compact monolithic fiber launcher
• Removal of unstable commercial mounts
• Heterodyne system
• Provides unlimited locking points, actuation
  range, robustness, and flexibility
• Current homodyne system actuation range lt 0.5um
• Longer path length with 2 separated tables
• Imitates realistic measurement environments

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Other Application

• Phasemeter testing for LISA
• LISAs main output heterodyne interferometer
  with phasemeter
• Microradian phase variation due to gravitational
  wave
• Response, stability, and linearity of phasemeter
• Space-like stable environment
• Known signal injection
• Stability test of other LISA optical components
• Ex.) Angle actuators

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Summary

• Testbed to enable stabilized metrology
• Stability between optical benches (components) is
  crucial.
• Thermal/seismic drifts obscure main measurement.
• LISA pm level stability between two benches
• Other missions main mirror metrology, secondary
  mirror adjustment, etc.
• Suppression of environmental motion
• Sub-nanometer stability over 1000 seconds
  (gain500)
• Reaching LISA requirements
• Scalable applicable to other advanced telescope
  mission
• Application example of stable environment DMI
  stability test
• Few nm/hour drift in a standard lab environment
  over 1m.
• Suppression of near DC drift to nm level is very
  hard!