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 σ- polarized 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 Rb-87 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.