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IEEE Space, Aerospace and Defence Conference (SPACE)

A Homodyne technique to measure the frequency chirp of Raman beams in gravimetry

Hriday Dath, Ahmed Rabeeh I C, V N Radhika, Umesh R. Kadhane, Rajesh George P, Joji J. Chaman

Published Jul 21, 2025• 0 citations
Publisher

Abstract

The frequency chirp for Doppler correction applied to Raman beams during Mach Zehnder interferometry is a good indicator of the precise measurement of acceleration due to gravity experienced by the ultra-cold atomic ensemble. In general, the frequency chirp rate is scanned for the interferometry central fringe for Gravimetry applications. Precise measurement of chirp rate is of paramount importance to measure the phase difference between the interference arms and in turn measure the gravity by accuracy of 10−6m/s2. We present a homodyne interferometry technique to precisely measure the chirp rates applied to the Raman beams. This scheme uses conventional sensors/detectors and performs well in their limited dynamic range or bandwidth. Our analysis allows us to measure and characterize the chirp rate, which we find in perfect agreement with expected values.

DOI
10.1109/space65882.2025.11171444

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The frequency chirp for Doppler correction applied to Raman beams during Mach Zehnder interferometry is a good indicator of the precise measurement of acceleration due to gravity experienced by the ultra-cold atomic ensemble. In general, the frequency chirp rate is scanned for the interferometry central fringe for Gravimetry applications. Precise measurement of chirp rate is of paramount importance to measure the phase difference between the interference arms and in turn measure the gravity by accuracy of 10−6m/s2. We present a homodyne interferometry technique to precisely measure the chirp rates applied to the Raman beams. This scheme uses conventional sensors/detectors and performs well in their limited dynamic range or bandwidth. Our analysis allows us to measure and characterize the chirp rate, which we find in perfect agreement with expected values.
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 a precision measurement of local gravity acceleration g in Wuhan by a compact cold atom interferometer. The atom interferometer is in vertical Mach—Zehnder configuration realized using a π/2 - π - π/2 Raman pulse sequence. Cold atoms were prepared in a magneto-optical trap, launched upward to form an atom fountain, and then coherently manipulated to interfere by stimulated Raman transition. Population signal vs Raman laser phase was recorded as interference fringes, and the local gravity was deduced from the interference signal. We have obtained a resolution of 7 × 10 −9 g after an integration time of 236s under the best vibrational environment conditions. The absolute g value was derived from the chirp rate with a difference of 1.5 × 10 −7 g compared to the gravity reference value. The tidal phenomenon was observed by continuously monitoring the local gravity over 123 h.
Atom interferometry using stimulated Raman transitions in a retroreflected configuration is the first choice in high-precision measurements because it provides low phase noise, a high-quality Raman wave front, and a simple experimental setup. However, it cannot be used for atoms at zero velocity because two pairs of Raman lasers are simultaneously resonant. Here we report a method which allows this degeneracy to be lifted by using a frequency chirp on the Raman lasers. Using this technique, we realize a Mach-Zehnder atom interferometer hybridized with a force balanced accelerometer which provides horizontal acceleration measurements with a short-term sensitivity of $3.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}\phantom{\rule{0.16em}{0ex}}\mathrm{s}}^{\ensuremath{-}2}/\sqrt{\text{Hz}}$. This technique could be used for multiaxis inertial sensors, tiltmeters, or atom interferometry in a microgravity environment.
We present here an analysis of the influence of the frequency dependence of the Raman laser light shifts on the phase of a Raman-type atom gravimeter. Frequency chirps are applied to the Raman lasers in order to compensate gravity and ensure the resonance of the Raman pulses during the interferometer. We show that the change in the Raman light shift when this chirp is applied only to one of the two Raman lasers is enough to bias the gravity measurement by a fraction of $\ensuremath{\mu}\mathrm{Gal}$ ($1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{Gal}\phantom{\rule{0.28em}{0ex}}=\phantom{\rule{0.28em}{0ex}}{10}^{\ensuremath{-}8}\phantom{\rule{0.28em}{0ex}}\mathrm{m}/{\mathrm{s}}^{2}$). We also show that this effect is not compensated when averaging over the two directions of the Raman wave vector kkk. This thus constitutes a limit to the rejection efficiency of the kkk-reversal technique. Our analysis allows us to separate this effect from the effect of the finite speed of light, which we find in perfect agreement with expected values. This study highlights the benefit of chirping symmetrically the two Raman lasers.
Atom interferometry gravimeter with stimulated Raman transitions is realized by atom matter wave, it is now widely used in gravity measurements. We are now building a feasible system of cold atom gravimeter, it is based on the atom interferometry technology by coherently manipulating the cold atoms in a fountain with specific Raman lasers, the cold atom wave packet is splitted, combined, and then re-splitted in the process. Then the atomic wave packet will acquire different phase because of the different evolution path. The precise acceleration can be deduced through the precision measurement of atomic interference fringes phase, and this will be a high precision standard of acceleration. At present, the preparation of Raman laser and the precise control of the laser Frequency have been finished, and they have been proved to meet the requirements of the experiment.
Cold atom interferometer based on two-photon stimulated Raman transition has been proved to be a promising way to measure g with higher precision. In this work, we first introduce the principle of atomic interference gravimetry, including the interaction between the atoms and light, the interferometry with the Raman beams and the measure of the g. Then our gravity acceleration measurement system will be introduced, including atomic fountain, Raman system and detection system. With this device, we can get the value of the g, with the uncertainty in the order of ten minus seven.