Laser System for Atom Interferometry
Andrew Chew
Content
• Overview of related Theory
• Experimental Setup:
– Laser System
– Frequency/Phase Stabilization
• 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.
Cooling of Atoms
• Atoms behave mostly as
matter waves when they are
cooled below the subdoppler
limit. (subkelvin)
• The uncertainty of the
momentum space of an
ensemble of atoms is
reduced.
• Atoms as matter waves
behave similarly to light
waves and can interfere and
produce interference effects.
• E.g. Atoms interfere when
they pass through bragg
diffraction gratings
Magneto-Optical Trap
• First demonstrated by Raab et. al in 1987.
• The trap relies on the effect of a magnetic field gradient has on
the energy levels of an atom, and optical transition rules, and the
radiative force.
• To generate such a trap, a magnetic field gradient is applied to a
region of space, typically with a pair of anti-helmholtz coils.
• The magnetic field at the center of a pair of anti-helmholtz coils
follows the relation: B(z) = A z. (B ~ a few Gauss)
• The gradient A along the axial and the vertical direction of the
anti-helmholtz coils have opposite sign.
• The energy levels of the atom are shifted as appropriate
depending on the sign of the magnetic field.
Magneto-Optical Trap
• At low field relative strengths, the
zeeman levels shift according to:
• A pair of counter-propagating σpolarized beams are lined along the
vertical direction.
• Due to the magnetic quantization
axis, the polarization of the σ- beam
is σ+ where the B field is negative.
• Atoms experience a force F ± due to
the σ ± polarization.
• Where
and
Rubidium-87
• To create a Rb-87 MOT, the
lasers will be detuned off the
F=2 -> F’=3 transition.
• A repumper laser is tuned to
the F=1->F’=2 transition to
repopulate the F=2 state.
• The frequency separation
between the hyperfine ground
states is 6.84GHz
• Raman lasers are tuned with a
~3GHz detuning from the
Cooling laser.
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 two for Raman lasers.
• Repumper laser consisting of one DFB laser diode.
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
Cooling & Repumper Lasers
Cooling & Repumper Lasers
• The Cooling laser and the Repumper laser are both on the same
optical breadboard.
• The Cooling laser is frequency shifted by 250MHz using an AOM,
then frequency locked to the crossover transition between the Rb87 F=2 -> F’=3 and F=2->F’=1 transition.
• The Repumper laser is steered to the F=1->F=2’ transition
• The Cooling laser and the Repumper laser are phased locked using
a Trombone lock or Microwave interferometer to keep the frequency
separation constant, i.e. the relative frequency stability of the
Cooling laser is transferred to the Repumper laser via the
Trombone PLL.
• The Cooling laser and Repumper laser are overlapped and passed
into a PM fiber.
• A fiber table is used to split the beams into 6 beams and then
launched into PM fibers.
• The PM fibers are aimed at the glass cell of the Vacuum chamber.
Trombone PLL
• The Laser beat signal goes through a series of RF amplifiers, and is
split off once with a RF Power splitter where the signal is diverted to
a spectrum analyzer for analysis.
• The signal is split off again with a RF Power splitter and one signal
is goes through a phase shifter which can be thought of as a phase
delay line that is several wavelengths long.
• The phase delayed signal and the non-phase delayed signal are
then mixed with a RF mixer. The error signal is then passed into a
PID then to the Repumper laser current to stabilize the Repumper
laser frequency.
Raman Lasers
Raman Lasers
• The Raman lasers must be stabilized to stable frequency
references to ensure that the frequency separation between them
is kept at 6.84GHz.
• The Raman lasers are overlapped to produce the laser beat note.
• The laser beat note is amplified and mixed with a 7GHz reference
oscillator then filtered with a low-pass filter to produce a 160MHz
signal.
Raman Lasers
• The beat note is then passed into a PLL board where the frequency
divided by 2 and then is compared against a 80MHz frequency
reference using a digital phase-frequency detector.
• The signal is then filtered, integrated and two outputs are produced:
one fast and one slow for the laser current and the laser piezo
feedback.
Vacuum System
• Vacuum Chamber consists of 2 glass cells and a central metallic
vacuum chamber.
• A Titanium Ion-Getter Pump and A Titanium Sublimation pump is
attached to the Vacuum chamber
• The Ion Getter pump operates continuously, while the Titanium
Sublimation pump is operated initially during baking and then
switched off.
• There are dispensers to introduce the Rubidium and Cesium
atoms into the vacuum system.
• Prior to use, the vacuum system is baked with a rotary vane pump
and a turbomolecular pump running together with other two
pumps.
• A Mass Spectrometer is used to monitor the gas pressure levels.
• We need a vacuum pressure of 10-10 mbar.