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 ω 1 and ω 2 are used to couple the |g,p› and |i,p+ ħk 1 › states, and the |e, p + ħ(k 1 -k 2 )› and |i› states respectively. A large detuning Δ suppresses spontaneous emission from the intermediate |i,p+ ħk 1 › state. The ground states are effectively stable.

Ramsey-Bordé Interferometer A sequence of π/2, π and π/2 Raman pulses 1 st π/2 pulse acts a beam splitter: Places the atomic wave in a superposition of |g,p› and |e, p + ħk eff › states π pulse acts a mirror: Flips the |g,p› to the |e, p + ħk eff › states and vice versa 2 nd 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 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: , 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

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

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 mbar.