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Acta Physica Sinica

Influence of Raman laser sidebands effect on the measurement accuracy of cold atom gravimeter

Bin Wu, Bing Cheng, Zhijie Fu, Dong Zhu, Li-Ming Wu, Kainan Wang, Helin Wang, Zhaoying Wang, Xiao-Long Wang, Qiang Lin

Published Jan 1, 2019• 2 citations

Abstract

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.

DOI: 10.7498/aps.68.20190581

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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 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.

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.

Cold-atom interferometer based on stimulated Raman transition is interesting for both fundamental researches and precision measurements. Measuring g, the free fall acceleration, plays an important role in these applications. We describe the theory of atom interferometry, including the evolution of the atomic wavepacket in gravity field, the laser-atom interaction, and the interference of the atomic waves. In this article, we introduce the procedure of measuring g using atom interferometer, the experimental setup, and the tendency of the future development. The works of the domestic and international groups for g measurement are concluded. Furthermore, the noise limits and dominant systematic effects in atom gravimeters are analyzed. The suppressions of the background vibration, the Raman phase noise, and the evaluation of the main systematic effects, are introduced. Finally, the other applications using the atom interferometers for precision measurement are presented, such as in measuring the Newton’s gravitational constant, in testing the weak equivalence principle and the other fundamental applications in physics studies.