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Saint Petersburg International Conference on Integrated Navigation Systems

Angular Alignment of Separated Raman Beams Based on Optical Interference in Atom-Interferometer Gyroscopes

Jian Liu, Xiangxiang Lu, Dongliang Pei, Junfeng Zhang, Weiren Liu, Xiaoming Zhao

Published May 30, 2022• 0 citations
Publisher

Abstract

The angular alignment of the effective wave vectors of spatially separated Raman beams is crucial for the closed interferometric paths in atom-interferometer gyroscopes. A sensitive and reliable method based on the optical interference is proposed for the preliminary alignment of Raman beams in a four-pulse atom interferometer. The distribution of the optical interference fringe is perpendicular to the angular deviation, while the fringe width reveals the size of the misaligned angle. The alignment method can also be extended in the atom-interferometer gyroscopes with three pairs of separated Raman beams.

DOI
10.23919/icins51784.2022.9815448

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The angular alignment of the effective wave vectors of spatially separated Raman beams is crucial for the closed interferometric paths in atom-interferometer gyroscopes. A sensitive and reliable method based on the optical interference is proposed for the preliminary alignment of Raman beams in a four-pulse atom interferometer. The distribution of the optical interference fringe is perpendicular to the angular deviation, while the fringe width reveals the size of the misaligned angle. The alignment method can also be extended in the atom-interferometer gyroscopes with three pairs of separated Raman beams.
Cold-atom interferometers commonly face systematic effects originating from the coupling between the trajectory of the atomic wave packet and the wavefront of the laser beams driving the interferometer. Detrimental for the accuracy and the stability of such inertial sensors, these systematics are particularly enhanced in architectures based on spatially separated laser beams. Here we analyze the effect of a coupling between the relative alignment of two separated laser beams and the trajectory of the atomic wave packet in a four-light-pulse cold-atom gyroscope operated in fountain configuration. We present a method to align the two laser beams at the $0.2\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{rad}$ level and to determine the optimal mean velocity of the atomic wave packet with an accuracy of $0.2\phantom{\rule{4pt}{0ex}}\mathrm{mm}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. Such fine tuning constrains the associated gyroscope bias to a level of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{3.33333pt}{0ex}}\mathrm{rad}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. In addition, we reveal this coupling using the point-source interferometry technique by analyzing single-shot time-of-flight fluorescence traces, which allows us to measure large angular misalignments between the interrogation beams. The alignment method which we present here can be employed in other sensor configurations and is particularly relevant to emerging gravitational wave detector concepts based on cold-atom interferometry.
An atom interferometer based on Doppler-insensitive Raman transition is proposed, which has sharply peaked interference fringes for multi-wave interference. We show that two sets of counter-propagating Doppler-insensitive Raman beam pairs can be used to construct a new type of multi-wave beam splitter, which can be used to construct an atom interferometer. Although the spacing between adjacent diffraction orders of the interferometer is small, they can be distinguished by the internal state of the atom. Our analysis shows that the width of the fringes of this atom interferometer is inversely proportional to the width (duration) of the beam splitter and the Rabi frequency of the Raman beams, that is, the interferometer can achieve high resolution at high light intensity and long pulse width.
In the presence of Earth gravity and gravity-gradient forces, centrifugal and Coriolis forces caused by the Earth rotation, the phase of the time-domain atom interferometers is calculated with accuracy up to the terms proportional to the fourth degree of the time separation between pulses. We considered double-loop atom interferometers and found appropriate condition to eliminate their sensitivity to acceleration to get atomic gyroscope, or to eliminate the sensitivity to rotation to increase accuracy of the atomic gravimeter. Consequent use of these interferometers allows one to measure all components of the acceleration and rotation frequency projection on the plane perpendicular to gravity acceleration. Atom interference on the Raman transition driving by noncounterpropagating optical fields is proposed to exclude stimulated echo processes which can affect the accuracy of the atomic gyroscopes. Using noncounterpropagating optical fields allows one to get a new type of the Ramsey fringes arising in the unidirectional Raman pulses and therefore centered at the two-quantum line center. Density matrix in the Wigner representation is used to perform calculations. It is shown that in the time between pulses, in the noninertial frame, for atoms with fully quantized spatial degrees of freedom, this density matrix obeys classical Liouville equations.
Atom-interferometer-based gyroscopes are expected to have a wide range of applications due to their high sensitivity; however, their dynamic range is limited by dephasing from the velocity-dependent Sagnac phase shift and the longitudinal velocity distribution of the atoms, restricting measurements of large angular velocities. In this study, we present a method for restoring the contrast deterioration in angular velocity measurements with interferometer gyroscopes using atomic beams. We show that by introducing a pseudorotation effect with appropriate two-photon detunings for Raman light in the interferometer, it is possible to effectively cancel the Sagnac phase shift for all atoms in the velocity distribution of the beam. Consequently, the contrast is unaffected by the rotation. Furthermore, we apply this method to an interferometer gyroscope with counterpropagating atomic beams sharing the same Raman light. It is found that the angular velocity of the system can be estimated through the detuning point where the phase difference between the two interferometers is zero. This approach ensures that the scale factor of the atom interferometer gyroscope is independent of the change in the longitudinal velocity distribution of the atomic beam. We demonstrate our technique using an interferometer gyroscope with thermal atomic beams of rubidium-87, achieving a measurement of angular velocity of $1.0{\phantom{\rule{0.2em}{0ex}}}^{\ensuremath{\circ}}/\mathrm{s}$ even with an acceleration of 0.68m/s20.68\phantom{\rule{0.2em}{0ex}}\mathrm{m}/{\mathrm{s}}^{2}0.68m/s2 on a three-axis rotation table. This simple and robust dispersion-compensation method with Raman-light detuning will benefit dynamic angular velocity measurements in field applications such as the inertial navigation of vehicles.
We present a precision atom interferometry gyroscope with a Sagnac area of 6.0 cm 2 . To drive the cold atom interferometry gyroscope, we utilize two sets of Raman beams with a pulse sequence of π /2- π - π - π /2. We present a procedure for aligning the two sets of Raman beams at the microradian level by making use of the atom interferometer itself in different configurations. Additionally, we measured the atomic cloud trajectory and implemented a spatial symmetry alignment for the atom interferometer. With these improvements, we construct a sensitive atom gyroscope with a stability of 1 nrad/s at an integration time of 10000 s. We analyzed the influence of Raman beams and the limitations of the system, and present the technical limits to the sensitivity and long-term stability. Finally, we point out the primary sources of noise in the atom interferometry gyroscope, clearing the way for the construction of further large-area atom gyroscopes.
We report a Mach-Zehnder-type dual-atom-interferometer gyroscope with an interrogation arm of 40-cm length and the interference area up to 1.2cm21.2\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}1.2cm2. The precise angular alignment of the large-scale separated Raman lasers is demonstrated by seeking the phase intersection of Ramsey-$\mathrm{Bord}\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}$ interferometers after the gravity effect is compensated and by decoupling the velocity dependent cross-talk phase shifts, and applied to build the Mach-Zehnder atom interferometer. Then a compact inertial rotation sensor is realized based on dual large-area Mach-Zehnder atom interferometers by precisely aligning the large-scale separated Raman lasers, in which the coherence is well preserved and the common noise is differentially suppressed. The sensor presents a sensitivity of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ rad/s/Hz1/2\mathrm{rad}/\mathrm{s}/{\mathrm{Hz}}^{1/2}rad/s/Hz1/2, and a stability of $9.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{rad}/\mathrm{s}$ at 23000 s. The absolute rotation measurement is carried out by adjusting the atomic velocity which corresponds to modulating the scale factor.