LIGOs Thermal Noise Interferometer

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LIGOs Thermal Noise Interferometer

• Eric D. Black
• LIGO/VIRGO Thermal Noise Meeting
• 7 October, 2006

Kenneth Libbrecht, Akira Villar, Greg Ogin With
Dennis Coyne, Riccardo Desalvo
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Direct Measurement of Mirror Thermal Noise

• Short arm cavities, long mode cleaner (frequency
  reference) reduce laser frequency noise, relative
  to test cavity length noise.
• Measurement made as relevant to LIGO, AdLIGO as
  possible.
• Want to measure thermal noise at as low a level
  as possible in a small interferometer.
• Low-mechanical-loss substrates Fused Silica,
  Sapphire
• Silica-Tantala coatings
• Largest practical spot size

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TNI direct measurement of coating thermal noise

• Silica-tantala coatings on fused silica
  substrates
• Multiple calibrations performed.
• Noise source (in thermal noise band) localized
  inside cavities
• Assuming isotropic model, coating loss angle
  agrees with Penn, et al. ringdown measurement

• Assuming anisotropic model,

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Sapphire

• Noise floor in Sapphire dominated by Substrate
  Thermoelastic noise.
• Parameters
• a 2.7e-6 K-1
• k 44 W/mK
• Braginskys model validated in Sapphire - First
  measurement in AdLIGO candidate substrate material

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Doped Tantala (Ti-Ta2O5/SiO2) Coatings

• Doping the high-index layers (tantalum pentoxide)
  with titanium reduces the noise floor.
• Upper blue line noise floor with undoped
  coatings
• Lower blue line noise floor with titanium-doped
  coatings
• Reduction in noise floor is slightly greater than
  expected from Q measurements (end of range).

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Parametric Instability
Ju, et al. G050325-00
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Condition for Instability
Mechanical Q
Anti-Stokes mode
Stokes Mode
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Possible Solutions

• Active feedback
• Thermal detuning
• Q reduction

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Buna O-Rings

• Buna, 1/8 thick, undersized diameter (3.25)
• Q reduction is too large to measure by ringdown
  cant excite modes
• Substantial increase in the noise floor from
  200Hz to 4kHz
• Still thermal noise limited above 5kHz

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Kapton Tape

• Essentially massless, to avoid waving modes
  seen with buna.
• Lossy strip closely approximates models of Gras,
  Blair, et al.
• No observed increase in the noise floor
• Q reductions range from factor of 2 to none at
  all

No ring damper Buna o-ring Kapton tape
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Waving modes of rings
Free end (anti-node)
Shear wave equation (transverse waves)
wave speed
Mirror substrate (node)
Predicts fundamental f 145 kHz for 4 mm copper
ring - Well out of thermal noise region
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Copper rings

• Buna, kapton results tell us
• Need mass to damp mirror Qs
• Need waving mode well above the measurement
  band
• Ultimate plan was to use monolithic rings,
  expanded by heating for installation
• Use screw tension for prototypes
• Qs were reduced below our ability to excite the
  modes, as with buna.
• Noise floor increases, showing both new modes and
  additional broadband noise.

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Aperiodic Coatings

• Goal reduce high-loss material in HR coating by
  varying layer thickness, while preserving phase
  relationship for reflection
• Collaboration with
• Vincenzo Galdi and Innocenzo Pinto at Benevento
  (coating design),
• LMA at Lyon (fabrication and loss measurements),
  and
• Sheila Rowan at Glasgow (loss measurements)
• Gains from and design of optimization depend on
  how different the loss angles are between high-
  and low-index materials.
• We have well-confirmed measurements of
  , but is more subtle.
• Best measurements now
• Coatings are being fabricated at LMA. We will
  measure their noise floors at the TNI when they
  are done.