Phase stabilization of laser beams in a cold atom accelerometer

Abstract

A cold atom accelerometer measures the displacement of a proof mass of laser cooled atoms with respect to an instrument reference frame. The cold atom interferometer's reference frame is defined by a pair of specially prepared, counter-propagating laser beams, that measure inertially induced atom displacements with nm scale resolution. This corresponds to acceleration sensitivities comparable to state of the art electro-mechanical accelerometers. In dynamic environments, sensitivity is limited by the stability of the relative laser phase of the two interrogation laser beams, which is adversely affected by vibrations and temperature fluctuations of the interrogation beam optics. Without an independent measurement, the cold atom interferometer cannot distinguish platform acceleration from laser phase fluctuations, which thus are a potentially serious source of error. In this thesis, a Michelson optical interferometer and an optical feedback loop were used to stabilize the relative phase of the interrogation laser beams in a cold atom accelerometer. A digital controller stabilized the relative phase via an electro-optic phase modulator. This control loop's bandwidth encompasses 98.8% of the noise power as determined from the power spectral density of the open loop 795nm Michelson signal. Increasing the controller bandwidth would gain the system marginal improvement in noise reduction. At an atom interferometer dwell time of 1 msec, active laser phase stabilization improved the atom interferometer sensitivity; at an atom interferometer dwell time of 8msec, an improvement was no longer evident. Improvements to the laser phase stabilization system are proposed to increase atom interferometer stability at longer dwell times.