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Sensors

Active Suppression of Differential Light Shift Drift in an Atom Gravimeter

Weihao Xu, Xi Chen, J. F. Li, Danfang Zhang, Wenzhang Wang, Jiaju Wei, Jiaqi Zhong, Biao Tang, Lin Zhou, Runbing Li, Jin Wang, Min-Sheng Zhan

Published Mar 4, 2026• 0 citations
Publisher

Abstract

Differential light shift (DLS) is an important error term that limits the atom interferometer's measurement precision, especially for the case of the electro-optic modulator (EOM)-based scheme, where multiple laser sidebands exist, and their ratios are hard to control synchronously. This article carried out an experimental and theoretical study on this subject. By conducting long-term gravity measurement, we find that the gravity exhibits drifts of about 13.13 μGal, and is strongly correlated to the Raman laser's sidebands. A model of the DLS-induced gravity error is established and a DLS compensation method is proposed to suppress the gravity drift to 2.54 μGal. Besides the compensation method, we propose a Dual-Sideband Ratio Locking scheme to more robustly eliminate the gravity measurement drift. By feeding back to both the EOM microwave power and the tapered amplifier's temperature, this method locks both the ±1 order sideband to a stability level of 10-5, which corresponds to a gravity error of less than 0.1 μGal. Long-term gravity measurement is carried out after the locking method, showing a long-term stability of 1.6 μGal. The proposed methods will benefit the suppression of the DLS effect for high-precision atom interference measurement.

DOI
10.3390/s26051620

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Differential light shift (DLS) is an important error term that limits the atom interferometer's measurement precision, especially for the case of the electro-optic modulator (EOM)-based scheme, where multiple laser sidebands exist, and their ratios are hard to control synchronously. This article carried out an experimental and theoretical study on this subject. By conducting long-term gravity measurement, we find that the gravity exhibits drifts of about 13.13 μGal, and is strongly correlated to the Raman laser's sidebands. A model of the DLS-induced gravity error is established and a DLS compensation method is proposed to suppress the gravity drift to 2.54 μGal. Besides the compensation method, we propose a Dual-Sideband Ratio Locking scheme to more robustly eliminate the gravity measurement drift. By feeding back to both the EOM microwave power and the tapered amplifier's temperature, this method locks both the ±1 order sideband to a stability level of 10-5, which corresponds to a gravity error of less than 0.1 μGal. Long-term gravity measurement is carried out after the locking method, showing a long-term stability of 1.6 μGal. The proposed methods will benefit the suppression of the DLS effect for high-precision atom interference measurement.
The additional sidebands (ASBs) in a Raman laser will have a significant effect on the performance of atom gravimeters (AGs) based on phase-modulated Raman lasers. We propose a method of modulating the sideband-to-carrier ratio in Raman lasers to determine the magic time intervals where the phase shift induced by the ASB effect is minimized, and this method is demonstrated by experiments. Among these magic time intervals, some noise-immunity points are predicted. Based on the prediction and the result of the ASB effect changing with the interval time T between adjacent Raman pulses, an optimal magic time interval is selected. Therefore, the uncertainty to the gravity measurement induced by the ASB effect when the AG works at the magic time interval is reduced to 0.5 μGal. Furthermore, the ASB effect and its zero-phase points in four-pulse atom interferometers are also discussed. This work provides a clear way to eliminate the phase shift induced by the ASB effect in high-precision AGs employing phase-modulated Raman lasers.
Using the electro-optical modulation method to generate Raman beams for cold atom interference is one of the better methods for constructing a more compact and robust laser system. But this way will generate some residual sidebands resulting in the additional interference phase shift, which can affect the measurement accuracy of cold atom interferometer. In order to weaken the effect of laser modulation sidebands on the phase shift of cold atom interference, a double-sideband suppressed-carrier modulation laser system for cold atom interference is constructed. Based on the designed laser system, the principle of double-sideband generation and suppression is analyzed in detail, and some residual sidebands are adjusted and controlled. Moreover, some important optical parameters that affect the phase shift of cold atomic interference, such as the initial distance between the Raman retro-reflection mirror and the atomic cloud, the interrogation time between two adjacent Raman pulses, the laser modulation depth and the initial velocity of the atomic cloud, are discussed and optimized. By optimizing these relevant parameters, the influence of residual modulation sidebands on the phase shift of cold atomic interference is weakened drastically. The research results indicate, making use of the method of double-sideband suppression, the phase shift of cold atomic interference can be optimized to 0.7 mrad when the initial distance between the Raman retro-reflection mirror and the atomic cloud is 105 mm, and the interrogation time between two adjacent Raman pulses is 82 ms. More importantly, this work can provide a method for weakening the influence of Raman sideband effect on the phase shift of cold atom interferometer, and the corresponding laser system can be applied to other inertial sensors such as atomic gravimeter or atomic gravity gradiometer.
Phase-modulator-generated laser for Raman transition is widely used in atom gravimeters to simplify a system and improve robustness. However, the induced additional sidebands (ASBs) lead to systematic errors in gravity measurement. This work presents a novel, to our knowledge, method to generate an optical single-sideband (OSSB) laser for Raman transition through phase modulation based on a Faraday anomalous dispersion atomic filter (FADOF). The experimental result indicates that it can reduce the additional sidebands with a signal-to-noise ratio (SNR, the intensity ratio of carrier and unwanted sidebands) of better than 50 dB, and the phase shift caused by ASBs is demonstrated to be effectively suppressed from 358.8 to 2.2 mrad. Furthermore, this system has already been applied on an atom gravimeter to achieve a primary gravity measurement. It shows that the FADOF-based Raman laser system is a new scheme for a compact atom absolute gravimeter.
Phase-modulator-generated Raman laser is widely used in atom gravimeters to simplify system and improve robustness. However, the induced additional sidebands (ASBs) lead to systematic errors in gravity measurement. This work presents a novel method to generate optical single-sideband (OSSB) Raman laser based on Faraday anomalous dispersion atomic filter (FADOF). The experimental result indicates that it can reduce the additional sidebands to lower than -50 dB, and the phase shift caused by ASBs is demonstrated to be effectively suppressed from 358.8 mrad to 2.2 mrad. Furthermore, this system has already applied on atom gravimeter to achieve primary gravity measurement. It shows that FADOF-based Raman laser is a new scheme for a compact atom absolute gravimeter.
Phase-modulator-generated Raman laser is widely used in atom gravimeters to simplify system and improve robustness. However, the induced additional sidebands (ASBs) lead to systematic errors in gravity measurement. This work presents a novel method to generate optical single-sideband (OSSB) Raman laser based on Faraday anomalous dispersion atomic filter (FADOF). The experimental result indicates that it can reduce the additional sidebands to lower than -50 dB, and the phase shift caused by ASBs is demonstrated to be effectively suppressed from 358.8 mrad to 2.2 mrad. Furthermore, this system has already applied on atom gravimeter to achieve primary gravity measurement. It shows that FADOF-based Raman laser is a new scheme for a compact atom absolute gravimeter.
The technology of electro-optic modulation is one of the several methods of generating the Raman beams. The experimental system based on this method is simple and much easier to implement, and the environmental adaptability is strong as well. However, this kind of modulation technology will produce additional laser lines, which may affect the measurement accuracy of cold atom gravimeter. Based on a homemade transportable cold atom gravimeter, the influence of Raman sideband effect on the accuracy of cold atom gravimeter is investigated in this paper. We analyze in detail the relationship between Raman sideband effect and some experimental parameters, such as the height of Raman retro-reflection mirror, the time of free fall of the atoms, the detuning of Raman laser, etc. It is found that those parameters have a dominant influence on the measured gravity resulting from Raman sideband effect. Besides, it is also found that the gravity measurements will be sensitive again to some experimental parameters in the case of Raman sideband effect while these parameters are usually insensitive in case of laser system without sideband effect. Finally, we investigate the relationship between Raman sideband effect and Raman detuning, and presente a method of evaluating the gravity induced by Raman sideband effect. The experimental results in this paper can provide a reference for reducing the influence of Raman sideband effect on the accuracy evaluation of cold atomic gravimeter.