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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Ramsey–CPT Resonance Observation Using Different Laser Sideband Combinations for the Two Interrogation Pulses

Masahiro Fukuoka, Shigeyoshi Goka

Published Apr 24, 2022• 2 citations
Publisher

Abstract

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.

DOI
10.1109/eftf/ifcs54560.2022.9850881

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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.
A gas cell atomic frequency standard based on coherent population trapping (CPT) with Ramsey interrogation pulses is developed. The clock resonance is detected in a Cs cell with buffer gas using two phase-locked diode lasers tuned to the D1 line. A preliminary short-term stability of3.5×10−12t−1/23.5 \times 10^{-12}t^{-1/2}3.5×10−12t−1/2is reported. The CPT specificity allows a new interrogation–detection sequence, i.e., the continuous pulse train, which, with phase modulation, could lead to better short-term stability compared to usual continuous CPT clocks. The parameters optimizing the stability include a high Rabi frequency, a very short measurement time, and an interrogation time equal to the lifetime of the hyperfine coherence.
We propose a two-step pulse observation method to enhance the frequency stability of coherent population trapping (CPT) atomic clocks. The proposed method is a Raman–Ramsey scheme with low light intensity at resonance observation and provides a Ramsey-CPT resonance with both reduced frequency sensitivity to the light intensity and a high signal-to-noise ratio by reducing the repumping into a steady dark state. The resonance characteristics were calculated using density matrix analysis of a Λ-type three-level system that was modeled on the 133 Cs D 1 line, and the characteristics were also measured using a vertical-cavity surface-emitting laser and a Cs vapor cell.
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.
Light-induced frequency shifts can be a key limiting contribution to the mid- and long-term frequency instabilities in atomic clocks. In this letter, we demonstrate the experimental implementation of the combined error signal interrogation protocol to a cold-atom clock based on coherent population trapping (CPT) and Ramsey spectroscopy. The method uses a single error signal that results from the normalized combination of two error signals extracted from two Ramsey sequences of different dark periods. The single combined error signal is used to stabilize the atomic clock frequency. Compared to the standard Ramsey-CPT interrogation, this method reduces the clock frequency sensitivity to light-shift variations by more than one order of magnitude. This method can be applied in various kinds of Ramsey-based atomic clocks, sensors, and instruments.
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.
We experimentally investigated the characteristics of two-photon transmission resonances in Rb vapor cells with different amounts of buffer gas under the conditions of steady-state coherent population trapping (CPT) and a pulsed Raman–Ramsey (RR) CPT interrogation scheme. We particularly focused on the influence of the Rb atoms diffusing in and out of the laser beam. We showed that this effect modifies the shape of both CPT and RR resonances as well as their projected performance for CPT clock applications. In particular we found that at moderate buffer gas pressures RR-CPT did not improve the projected atomic clock stability compared to the regular steady-state CPT resonance.