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Physical Review A

Determination of the tilt of Raman lasers using the tilt-scanned fringe for atom gravimeters

Xiao‐Chun Duan, Wenxin Geng, Hua-Qing Luo, Yaoyao Xu, Zhong-Kun Hu

Published Jul 25, 2025• 2 citations
Publisher

Abstract

The sensitive axis of an atom gravimeter is defined by the direction of the Raman lasers. Any tilt of the Raman lasers with respect to the vertical direction introduces systematic errors in gravity measurements. In this work, we report a determination of the tilt of Raman lasers, where the fringe of the atom interferometer is scanned by varying the tilt, rather than the phase, of the Raman lasers. Unlike the periodic cosine fringes typically used in atom interferometers, the fringe obtained by changing the tilt, referred to as the tilt-scanned fringe, is aperiodic in tilt and exhibits a shift due to the presence of a nonzero tilt. The tilt-scanned fringe is highly sensitive to the tilt-caused shift, enabling convenient and precise determination of the Raman laser tilt in atom gravimeters. We demonstrate that one tilt-scanned fringe, corresponding to a tilt measurement sampling time of 13 s, can determine the tilt with a typical precision of about $30\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{rad}$ in our developed atom gravimeter. The short-term sensitivity and measurement accuracy of the tilt-scanned fringe method were evaluated, demonstrating performance comparable to conventional tilt determination techniques. We expect this method to offer a convenient solution for Raman laser alignment in atom gravimeters and enhance the efficiency of absolute gravity surveys.

DOI
10.1103/tqyc-kmf3

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Conference on Precision Electromagnetic Measurements•Jul 1, 2016•0 citations
The sensitive axis of an atom gravimeter is defined by the direction of the Raman lasers. Any tilt of the Raman lasers with respect to the vertical direction introduces systematic errors in gravity measurements. In this work, we report a determination of the tilt of Raman lasers, where the fringe of the atom interferometer is scanned by varying the tilt, rather than the phase, of the Raman lasers. Unlike the periodic cosine fringes typically used in atom interferometers, the fringe obtained by changing the tilt, referred to as the tilt-scanned fringe, is aperiodic in tilt and exhibits a shift due to the presence of a nonzero tilt. The tilt-scanned fringe is highly sensitive to the tilt-caused shift, enabling convenient and precise determination of the Raman laser tilt in atom gravimeters. We demonstrate that one tilt-scanned fringe, corresponding to a tilt measurement sampling time of 13 s, can determine the tilt with a typical precision of about 30\textmurad30\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{rad}30\textmurad in our developed atom gravimeter. The short-term sensitivity and measurement accuracy of the tilt-scanned fringe method were evaluated, demonstrating performance comparable to conventional tilt determination techniques. We expect this method to offer a convenient solution for Raman laser alignment in atom gravimeters and enhance the efficiency of absolute gravity surveys.
The sensitive axes of atom gravimeters are defined by the directions of the respective Raman lasers. Any tilt of the Raman lasers with respect to the vertical direction introduces errors in gravity measurements. In this work, we report a fast determination of the tilt of Raman lasers, where the fringe of the atom interferometer is scanned by varying the tilt, rather than the phase, of the Raman lasers. Unlike the periodic cosine fringes typically used in atom interferometers, the fringe obtained by changing the tilt, referred to as the tilt-scanned fringe, is aperiodic and symmetric with respect to zero tilt. The tilt-scanned fringe is highly sensitive to asymmetries caused by non-zero tilt, enabling fast and precise determination of the Raman laser tilt in atom gravimeters. We demonstrate that one tilt-scanned fringe, corresponding to a measurement cycle time of 13 s, can determine the tilt with a typical precision of about 30 μμμrad in our developed atom gravimeter. Further investigation proves that the tilt-scanned fringe approach shortens the measurement cycle time by over an order of magnitude while keeping comparable precision with conventional tilt determination techniques. The fast tilt determination presented here is significant for the application of atom gravimeters, particularly in absolute gravity surveys.
Light-pulse atom gravimetry has emerged as a powerful technique for precisely measuring absolute gravitational acceleration. Raman lasers are commonly used in this technique to coherently manipulate the atomic wave packet, of which the effective frequency is chirped continuously to compensate for the Doppler shift. In this study we investigate an additional phase shift in the Raman lasers that arises due to the frequency chirp. We directly measure this phase shift by recording the beat signal of the Raman lasers with a high-speed oscilloscope, providing an independent evaluation of the resultant error in atom gravimeters caused by this phase shift. Concurrently, we detect the influence of the additional phase shift on gravity measurements using our atom gravimeter, and the results are in good agreement with the independent evaluation. Notably, there may still be a residual error at the microgal level when performing differential measurements by reversing the direction of the effective wave vector of the Raman lasers. We propose that this residual error can in principle be eliminated by selecting the same chirp frequency range for both directions in the differential measurement.
We demonstrate that the shear phase introduced by a tilting Raman beam in a Mach-Zehnder atom interferometer actually shifts the absolute phase, and propose how this shift can be compensated for by reference to a special point of the interference fringe. In the experiment, we obtain shear interference fringes and achieve the extraction of absolute phase corresponding to gravity acceleration. To suppress the impact of position drift on measurement accuracy, we adopt a positive and negative tilt angle alternating measurement (PNTAM) method. After using this PNTAM method, when the position drift leads to a change of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{g}$, the impact of this drift on the absolute measurement value is less than $1.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{g}$. This work expands the application range of the phase shear readout based atom interferometer from the current relative or differential gravity measurement to the field of absolute gravity measurement.
As the existence of the gravity gradient, the output of gravimeters is actually the gravitational acceleration at the reference instrumental height. Precise knowledge of the reference height is indispensable in the utilization of gravity measurements, especially for absolute gravimeters. Here, we present an interferometric method to measure the distance between the atomic cloud and a reflecting mirror directly, which consequently determines the reference height of our atom gravimeter. This interferometric method is based on a frequency jump of Raman lasers applied at the π pulse of the atom interferometer, which induces an additional phase shift proportional to the interested distance. An uncertainty of 2 mm is achieved here for the distance measurement, and the effect of the gravity gradient on absolute gravity measurements can thus be constrained within 1 μGal. This work provides a concrete-object-based measurement of the reference height for atom gravimeters.
The Coriolis effect can introduce significant systematic errors in atom interferometers. This study examines the coupling effect between the Coriolis effect and the Raman beams–orientation in a fountain atomic gravimeter. Theoretical analysis indicates that the Coriolis effect is correlated with the orientation of the Raman beams, and tilt modulation experiments were conducted to substantiate this correlation. Furthermore, by aligning the orientations of the atomic fountain, the vacuum chamber, and the Raman mirror, we achieved simultaneous suppression of both the Coriolis effect and the tilt effect to sub- μ Gal levels in a high-precision atomic gravimeter. This work offers guidance on assessing systematic errors in high-precision atomic gravimeters.
This paper introduces an atomic gravimeter under development at KRISS for precise measurement of gravitational acceleration. The gravimeter based on atom interferometer uses two counter-propagating Raman lasers of π/2-π-π/2 pulse sequence and laser cooled87Rb atoms in free fall. Atomic interference fringes caused by Raman lasers were observed at various time intervals between the three Raman pulses. The frequency sweep rate α which compensates Doppler shift due to gravitational acceleration g was obtained from the interference fringes. In the presentation, the current status and recent results of the KRISS atomic gravimeter will be discussed.