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.