Transcript Presentation4

Towards a Laser System for Atom
Interferometry
Andrew Chew
Content
• Overview of related Theory
• Experimental Setup:
– Laser System
– Frequency Stabilization
– Characterisation of realized Lasers
• Outlook
Atom Interferometry
• Similar to Light Interferometry
• Atoms replace role of the light.
• Atom-optical elements replace mirrors and beam splitters
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.
Raman Transitions
• Stimulated Raman
Transitions result in the
super position of |e› and
|g› states
• Two phase-locked Lasers
of frequency ω1 and ω2
are used to couple the
|g,p› and |i,p+ ħk1› states,
and the |e, p + ħ(k1-k2)›
and |i› states
respectively.
• A large detuning Δ
suppresses spontaneous
emission from the
intermediate |i,p+ ħk1›
state.
• The ground states are
effectively stable.
Ramsey-Bordé Interferometer
• A sequence of π/2, π
and π/2 Raman pulses
• 1st π/2 pulse acts a
beam splitter: Places
the atomic wave in a
superposition of |g,p›
and |e, p + ħkeff› states
• π pulse acts a mirror:
Flips the |g,p› to the
|e, p + ħkeff› states and
vice versa
• 2nd π/2 pulse acts a
beam splitter:
Projecting the atoms
onto the initial state.
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, 280:443-447, 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
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
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
Laser Operation
• Tuning of wavelength by changing
– Laser diode current (Fast MHz time scale)
– Cavity length (acoustic time scale, kHz)
– Temperature (Hz time scale)
Lasers
Fabry Perot ECDL
Littrow ECDL
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
Laser Characterization
• We will beat 3 lasers: 1 ECDL laser using a DFB ECDL, an Edge
Emitting ECDL and a Littrow ECDL laser
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
Beat Note
• Edge Emitting Diode and
Littrow ECDL Beat Linewidth:
0.5295 +/- 0.0356 MHz
• Sweep Rate: 30ms
• Bandwidth: 30KHz
Results
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
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.