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Atomic clock stability under dynamic excitation of coherent population trapping resonance in cells without buffer gas

Sergey M Kobtsev, Daba A. Radnatarov, Sergey Khripunov, Ivan Popkov, Valeriy Andryushkov, Tatiana Steschenko

Published Feb 22, 2018• 4 citations
Publisher

Abstract

The present work for the first time reports on studies of properties of dynamically excited coherent population trapping (CPT) resonances in cells without buffer gas (both with and without anti-relaxation coating of their inner walls). Over the 0−3 kHz range of CPT resonance scanning frequencies, the discriminant curve slope was determined, as well as measured stability of atomic clocks. The work provides details of the experiments and analyses prospects of application of results for improvement of atomic clock stability. The work also discusses the newly discovered different character of CPT resonance shape evolution in cells with and without buffer gas.

DOI
10.1117/12.2288872

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The present work for the first time reports on studies of properties of dynamically excited coherent population trapping (CPT) resonances in cells without buffer gas (both with and without anti-relaxation coating of their inner walls). Over the 0−3 kHz range of CPT resonance scanning frequencies, the discriminant curve slope was determined, as well as measured stability of atomic clocks. The work provides details of the experiments and analyses prospects of application of results for improvement of atomic clock stability. The work also discusses the newly discovered different character of CPT resonance shape evolution in cells with and without buffer gas.
This work presents a study of the short-term stability properties of Rb atomic clocks based on coherent population trapping (CPT) under dynamic excitation of a CPT resonance in cells without buffer gas. It is demonstrated that in an optical cell with anti-relaxation coating of its inner walls, the best stability is achieved at the scanning frequency of the frequency difference of the bichromatic pump field equal to 2 kHz at the resonance width of 450 Hz. In cells without a wall coating, the optimal scanning frequency range is found to be 1.2–2.8 kHz at the resonance width of 29 kHz. Examination of the slope of the stabilization system’s discriminant curve at zero error signal for buffer-gas-free cells uncovered an amount of correlation between the discriminant curve slope and atomic clock stability. It is demonstrated that the highest stability of atomic clocks in dynamic excitation mode is achieved at a ratio of scanning frequency and amplitude around 1.
This work reports for the first time development and experimental study of a laboratory prototype of an atomic clock with stability of 2×10–11τ –1/2 (Allan deviation) over time 1 s < τ < 1000 s (7×10–13 over 1000 s) on the basis of a coherent population trapping (CPT) resonance in 87Rb in a compact spherical cell having 1.3-cm diameter and without any buffer gas. Demonstrated are dependencies of the width of the studied CPT resonance upon the amplitude and modulation frequency of optical frequency difference (3.417 GHz) of a bichromatic circularly polarised pump field. For the first time, a considerable non-linearity was discovered in the dependence upon the reference difference frequency of the spectral position of the CPT resonance when the modulation frequency of the optical frequency difference is varied within a broad range.
In recent years, the coherent population trapping (CPT) phenomenon using a very small cell has attracted considerable attention for its potential application to ultra-compact atomic clock. For the miniaturization of CPT clock, it is important to clarify influence by the collision with atoms and cell walls. We study the effect of cell geometry on the CPT spectrum using a thin Cs vapor cell without buffer gas. When the cell length is below 1 mm and the laser intensity is sufficiently weak, the transit-time broadening is largely suppressed because only slow atoms have enough time to settle down on the dark state and contribute to the CPT signals. Furthermore, we show that introduction of buffer gas may broaden the CPT resonance in extremely thin cells. We also show that sub-Doppler signal which can be used for stabilization of laser frequency is obtained by using the CPT signal.
We propose a novel way of radiation wavelength stabilisation in atomic clocks based on coherent population trapping (CPT). It uses quadrature-phase component of the CPT signal within the CPT resonance feedback loop and automatic wavelength locking of radiation from a VCSEL to the atomic absorption line. We demonstrate advantages and limitations of the new method applied to vapour cells with buffer gas or anti-relaxation coating. Also provided are measurements of short- and long-term stability of a CPT-based atomic clock using the proposed method with various optical cells. Discussed are the prospects of this method in chip-scale atomic clocks.