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.