Towards a Laser System for Atom Interferometry

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Towards a Laser System for Atom Interferometry

• Andrew Chew

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Content

• Overview of related Theory
• Experimental Setup
• Laser System
• Frequency Stabilization
• Characterisation of realized Lasers
• Outlook

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Atom Interferometry

• Similar to Light Interferometry
• Atoms replace role of the light.
• Atom-optical elements replace mirrors and beam
  splitters

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Motivation

• Light Interferometry is used to make inertial
  sensors but the long wavelength limits the
  resolution of the phase measurement.
• The atomic de Broglie wavelength is much shorter
  and thus allows for greater resolution of the
  phase measurement.
• Atoms have mass and thus we can make measurements
  of the forces exerted on them.
• An example would be the measurement of the
  gravitation force.

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Raman Transitions

• Stimulated Raman Transitions result in the super
  position of e and g states
• Two phase-locked Lasers of frequency ?1 and ?2
  are used to couple the g,p and i,p hk1
  states, and the e, p h(k1-k2) and i states
  respectively.
• A large detuning ? suppresses spontaneous
  emission from the intermediate i,p hk1 state.
• The ground states are effectively stable.

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Ramsey-Bordé Interferometer

• A sequence of p/2, p and p/2 Raman pulses
• 1st p/2 pulse acts a beam splitter Places the
  atomic wave in a superposition of g,p and e, p
  hkeff states
• p pulse acts a mirror Flips the g,p to the
  e, p hkeff states and vice versa
• 2nd p/2 pulse acts a beam splitter Projecting
  the atoms onto the initial state.

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Laser System

• Extended Cavity Diode Laser (ECDL) design used by
  Gilowski et. al in Narrow bandwidth interference
  filter-stabilized diode laser systems for the
  manipulation of neutral atoms. Optics
  Communications, 280443-447, 2007.
• 3 Master Oscillator Power Amplifier (MOPA)
  systems for each wavelength, each consisting of
  an ECDL as the seeder and a Tapered Amplifier as
  the amplifier. One MOPA is for cooling, another
  for Raman lasers and last for the repumper beam

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Experimental Setup

• Laser system for Rubidium consisting of cooling
  and repumper lasers for preparation of atomic
  cloud.
• Raman laser system for atom interferometry.
• Laser system for imaging and detection of
  internal atomic states.
• 1 set of laser systems for each individual
  species of atoms used for interferometry

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ECDL Design

• Cavity Length Defined by the distance between the
  laser diode and the cavity mirror/output coupler.
• Output coupler mounted on a piezo-electric
  transducer which is partially transmitting and
  reflecting.
• Inside the cavity, the emitted laser beam is
  collimated using a collimating lens, and then
  focused onto the output coupler, forming a very
  stable angular insensitive cavity.
• DFB laser diode which promises narrow linewidth
  is used

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Laser Operation

• Tuning of wavelength by changing
• Laser diode current (Fast MHz time scale)
• Cavity length (acoustic time scale, kHz)
• Temperature (Hz time scale)

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Lasers
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Fabry Perot ECDL
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Littrow ECDL
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Laser Characterization

• Heterodyne 2 lasers to obtain their beat note in
  a optical setup shown below
• Linewidth of the beat note corresponds to
• We need 3 lasers and beat each one with each
  other to obtain a system of 3 simultaneous
  equations

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Laser Characterization

• We will beat 3 lasers 1 ECDL laser using a DFB
  ECDL, an Edge Emitting ECDL and a Littrow ECDL
  laser

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Beat Note

• DFB ECDL and Edge Emitting ECDL Beat Linewidth
  0.4775 /- 0.0300 MHz
• Sweep Rate 30ms
• Bandwidth 30KHz

• DFB ECDL and Littrow ECDL Beat Linewidth 0.4910
  /- 0.0276 MHz
• Sweep Rate 30ms
• Bandwidth 30KHz

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Beat Note

• Edge Emitting Diode and Littrow ECDL Beat
  Linewidth 0.5295 /- 0.0356 MHz
• Sweep Rate 30ms
• Bandwidth 30KHz

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Results
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Analysis

• The Spectrum Analyzer was set to have a fast
  sweep rate setting of 30ms as the free running
  DFB and Fabry Perot ECDL have a slow frequency
  drift of a few MHz within 100ms timescale.
• A more ideal setup would require all 3 lasers
  locked to an atomic reference during the
  measurement.
• The DFB ECDL, as expected, has the narrowest
  linewidth of all the 3 lasers

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Outlook

• The Laser system is characterized and we will now
  proceed to build the tapered amplifier to form
  the MOPA system. 2 other MOPAs will also be
  constructed
• Vacuum system for experiment will be constructed.
• We want to do inertial measurements by year-end.
• Laser system for the second atomic species will
  also need to be set up and characterized.