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IEEE Ultrasonics, Ferroelectrics, and Frequency Control Joint Symposium (UFFC-JS)

Estimation of a Light Shift Using Various Atomic Resonances in Ramsey-Coherent Population Trapping Sequence

Masahiro Fukuoka, Sigeyoshi Goka

Published Sep 22, 2024• 0 citations
Publisher

Abstract

This study aimed to detect the light shift by utilizing various atomic resonances observed under pulsed excitation with drive current modulation (DCM). Light shift degrades the performance of atomic clocks, and several studies have attempted to mitigate or correct it. Pulsed excitation with DCM facilitates the observation of various Ramsey-coherent population trapping (CPT) resonances owing to wavelength variation. Further, in the Ramsey-CPT sequence, a semi-continuous wave CPT resonance (SCW-CPT resonance), which is a continuous wave CPT resonance from pulsed lasers, is also observed. As these atomic resonances exhibit different frequency sensitivities to laser intensity, the frequency shifts differ based on the changes in laser intensity. This facilitates the detection of the light shift. By simultaneously measuring the resonance frequencies of two Ramsey-CPT resonances and the SCW-CPT resonance while varying the laser intensity to induce light shifts, we measured the resulting frequency changes. Each resonance exhibited a different frequency shift in response to the light shift. Therefore, this study conformed the feasibility of detecting and estimating the light shift in an operational atomic clock.

DOI
10.1109/uffc-js60046.2024.10793892

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This study aimed to detect the light shift by utilizing various atomic resonances observed under pulsed excitation with drive current modulation (DCM). Light shift degrades the performance of atomic clocks, and several studies have attempted to mitigate or correct it. Pulsed excitation with DCM facilitates the observation of various Ramsey-coherent population trapping (CPT) resonances owing to wavelength variation. Further, in the Ramsey-CPT sequence, a semi-continuous wave CPT resonance (SCW-CPT resonance), which is a continuous wave CPT resonance from pulsed lasers, is also observed. As these atomic resonances exhibit different frequency sensitivities to laser intensity, the frequency shifts differ based on the changes in laser intensity. This facilitates the detection of the light shift. By simultaneously measuring the resonance frequencies of two Ramsey-CPT resonances and the SCW-CPT resonance while varying the laser intensity to induce light shifts, we measured the resulting frequency changes. Each resonance exhibited a different frequency shift in response to the light shift. Therefore, this study conformed the feasibility of detecting and estimating the light shift in an operational atomic clock.
Abstract In this study, we proposed a method to observe the semi-cw excited coherent population trapping (SCW-CPT) and the Ramsey-CPT resonance during a Ramsey pulse sequence. By simultaneously observing the Ramsey-CPT and SCW-CPT resonances, which have distinct light shift coefficients, it is easy to detect light shift from the resonance frequency ratio of these two atomic resonances. Experimental results demonstrated the capability of the proposed method to simultaneously observe both the Ramsey-CPT and SCW-CPT resonances. The frequency difference attributable to the light shift was 308.9 Hz, indicating the successful detection of the light shift. Long-term measurements revealed that the frequency drift rates of the two atomic resonances differ by a factor of 3.5, and the variation in the frequency ratio enabled the detection of fluctuations in these two resonance frequencies.
Abstract We investigated the light shift of Ramsey coherent population trapping (CPT) resonance by drive current modulation (DCM), which can be realized with low power consumption in a small system, to improve the performance of a CPT atomic clock. The light shift makes the greatest contribution to the degradation of long-term frequency stability of a CPT atomic clock. We also devised digitizer-based DCM to reduce the uncertainty in the frequency of light shift due to wavelength variation at the time of observation. The results show that light shift with DCM is reduced compared with cw excitation and is consistent with theoretical calculations. The estimated frequency uncertainty of the light shift due to wavelength variation using the proposed method is 2.1 × 10−13, which is approximately 165 times better than that of the conventional method without a digitizer, thus demonstrating the effectiveness of the proposed method for future small atomic clocks.
SummaryWe investigated the Ramsey–coherent population trapping (CPT) characteristics of a new observation method that employs different laser sideband combinations as the two interrogation pulses during Ramsey–CPT resonance observation. In an experimental setup using a gas cell containing Cs atoms, Ramsey–CPT resonance was observed using the 1st-1stsideband combination for the common preparation pulse and the 1st-1stand C-2nd sideband combinations for the detection pulse. The Ramsey–CPT spectral shapes were inverted to each other. The light shift characteristic was 2.7 times different to its light sensitivity. This difference in the light shift characteristics is expected to allow the estimation of the light frequency shift in the Ramsey–CPT atomic clock during operation.
Light shifts are known to be an important limitation to the mid- and long-term fractional frequency stability of different types of atomic clocks. In this article, we demonstrate the experimental implementation of an anti-light-shift interrogation protocol onto a continuous-wave (cw) microcell atomic clock based on coherent population trapping (CPT). The method, inspired by the autobalanced Ramsey spectroscopy technique demonstrated in pulsed atomic clocks, consists in the extraction of atomic based information from two successive light-shifted clock frequencies obtained at two different laser-power values. Two error signals, computed from the linear combination of signals acquired along a symmetric sequence, are managed in a dual-loop configuration to generate a clock frequency free from light shift. Using this method, the sensitivity of the clock frequency to both laser-power and microwave-power variations can be reduced by more than an order of magnitude compared to normal operation. In the present experiment, the consideration of the nonlinear light-shift dependence allows enhancement of the light-shift mitigation. The implemented technique allows an improvement of the clock Allan deviation for time scales higher than 1000 s. This method can be applied in various kinds of atomic clocks such as CPT-based atomic clocks, double-resonance Rb\mathrm{Rb}Rb clocks, or cell-stabilized lasers.
In the atomic frequency standard, the light shift inevitably affects the long-term stability of the atomic clock. In this work, we accurately calculated the light shifts of electromagnetically induced transparency (EIT) and coherent population trapping (CPT) clocks operating under continuum and pulse sequence regimes by numerically solving the Liouville density matrix equation, including all the relaxation terms of a three-level system. The results show that the light shifts under pulse excitation are smaller than those under continuous excitation and the light shifts of the CPT process are much smaller than those of the EIT process under both excitations. It is also found that the light shifts in the continuous excitation increase and those in the pulse excitation decrease with an increase in the Rabi frequency. The light shifts will increase with an increase of the relaxations of the ground states for both the processes under both regimes. Compared to all process atomic clocks, the light shift of pulse-excited atomic clocks is the smallest, which will be suitable for many applications.
This paper presents a theoretical and experimental investigation on the light shift (AC Stark shift) of the 62S1/2(F = 3 ↔ 4) transition in133Cs, as observed through coherent population trapping (CPT) under a pulsed excitation. The calculated results are good agreement in the experimental results on various parameters and show that the light shift under pulsed excitation is 20∼30 times less than that under continuous excitation. In addition, this paper also presents a two-step pulse method for suppressing the light shift.