A Sagnac interferometer with frequency modulation for sensitive saturated absorption and application

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A Sagnac interferometer with frequency modulation
for sensitive saturated absorption(and
applications for LISA!)
Glenn de Vine, Matthieu Vangeleyn, Alain
Brillet, C. Nary Man David McClelland, Malcolm
Gray
Observatoire de la Côte d'Azur Département
ARTEMIS NICE glenn.devine_at_obs-nice.fr
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Talk Outline

• LISA - lasers and frequency noise
• Sagnac interferometer basics
• Saturation spectroscopy basics
• Sagnac interferometer for noise-rejection
• Details of the technique
• Theoretical modeling
• Experimental results
• The Future

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• The LISA Interferometer
• Arm lengths 5 million km
• Arm length difference 50,000 km (1)
• Frequency noise now couples in due to unequal arm
  length
• Equal arm length Michelson
• freq noise is common and
• not a concern
• white light interferometer

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Frequency Noise Coupling
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Measurement Sensitivity

• In order to measure a relative arm length
  difference, dx 2 pm/?Hz, using
• we require a detector (laser) frequency
  sensitivity (stability), d?, of
• 6x10-6 Hz/?Hz

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LISA Lasers

• LISA will employ the most stable CW lasers
  currently available
• NdYAG lasers at 1064 nm
• Intensity noise requirements should be met with
  noise-eaters
• Laser frequency noise needs to be overcome
• Typical free running laser frequency noise
• 104/f Hz/?Hz
• LISA detection band is 100 ?Hz to 1 Hz
• At 100 ?Hz we require a stability improvement of
  over 13 orders of magnitude

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Frequency Stabilisation Methods

• Arm locking - stable reference, well established
  in ground-based GWDs
• Time-delay interferometry - new technique,
  currently being tested
• Mechanical reference (cavity) - ULE, ZeroDur, etc
• Atomic or molecular reference
• No method alone will achieve the 13 orders of
  magnitude improvement required
• Solution will be a combination

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Atomic vs Mechanical (Cavity)

• Atomic -
• for
• absolute reference, best long term stability
• against
• not space-rated, absorptions typically very weak
  at 1064 nm
• Cavity -
• for
• simple, space-rated, best short term stability
• against
• not absolute, aging, long term stability is
  susceptible to thermal variations

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Iodine Spectroscopy for LISA Laser Frequency
Stabilisation

• develop high performance frequency stability by
  locking a laser using Doppler-free saturated
  absorption spectroscopy of iodine at 532 nm for
  1064 nm absolute stability
• achieve LISA laser frequency stability
  requirement of lt 1 Hz/vHz from 100 ?Hz to 1 Hz

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Iodine

• Sufficient absorption from hyperfine resonances
  at 532 nm (the harmonic of 1064 nm - weak
  absorptions Cs2,CO2,C2H2)
• Commercially available lasers with doubled (532
  nm) and fundamental (1064 nm) outputs
• The spectroscopy (and thus, frequency stability)
  can benefit from improved techniques to enhance
  the signal and/or reduce the noise

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Sagnac Interferometry
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Saturation Spectroscopy

• Energy levels of I2 1. electronic 2.
  vibrational 3. rotational

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Saturation Spectroscopy

• Energy levels of I2 1. electronic
  2. vibrational (1 GHz) 3. rotational (1 MHz)

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Saturation Spectroscopy

• Energy levels of I2 1. electronic
  2. vibrational (1 GHz) 3. rotational (1 MHz)
• Boltzmann thermal distribution - Doppler shifts
  transition frequencies relative to laser
  frequency
• Doppler shifting is greater than hyperfine
  linewidth
• Counter-propagating pump and probe fields - both
  interact only with molecules of zero longitudinal
  velocity (to first order)

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Saturation Spectroscopy

• Pump saturates vibrational transition, allows
  probe to interact with hyperfine (rotational)
  transitions
• When pump and probe frequency are coincident with
  hyperfine transition, the transparency from the
  hole burnt by the pump produces the inverted Lamb
  dip

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A new spectroscopy technique
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3rd Harmonic Sagnac Spectroscopy
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Experimental Results
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Applications for LISA

• Laser frequency stabilisation
• Initial phase-locking of LISA lasers
• Could use Cs2 at 1064 nm

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Further Work

• Optimise error signal fringe visibility, show
  1st harmonic. Then stabilise laser
• Complete 2nd identical system
• Independent long-term laser frequency stability
  measurement against LISA requirements
• Compare with modulation transfer results
• Simple, yet powerful (potentially
  shot-noise-limited) technique can be used for any
  spectroscopic application