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

The optical system of the fountain clock

Xinliang Wang, Jun Ruan, Hui Zhang, Rui Lin, Dandan Liu, Chen Jiang, Yong Guan, Fengxiang Yu, Junru Shi, Shougang Zhang

Published May 1, 2014• 2 citations
Publisher

Abstract

This paper present an optical system for Cs atomic fountain clock that is located on a 1.2 m×1.2 m optical table. Taking advantage of this system, we trap about 108cesium atoms in the magneto-optic trap (MOT) and get the Ramsey fringe of the fountain by scanning the microwave frequency of the interrogation cavity. The full-width at half-maximum of the central fringe (FWHM) is 1.4 Hz.

DOI
10.1109/fcs.2014.6859885

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This paper present an optical system for Cs atomic fountain clock that is located on a 1.2 m×1.2 m optical table. Taking advantage of this system, we trap about 108cesium atoms in the magneto-optic trap (MOT) and get the Ramsey fringe of the fountain by scanning the microwave frequency of the interrogation cavity. The full-width at half-maximum of the central fringe (FWHM) is 1.4 Hz.
We propose and realize a new optical state selection method on a cesium atomic fountain clock by applying a two-laser 3–3′ optical pumping configuration to spin polarize atoms. The atoms are prepared in |F = 3, mF = 0〉 clock state with optical pumping directly after being launched up, followed by a pushing beam to push away the atoms remaining in the |F = 4〉 state. With a state selection efficiency exceeding 92%, this optical method can substitute the traditional microwave state selection, and helps to develop a more compact physical package. A Ramsey fringe has been achieved with this optical state selection method, and a contrast of 90% is obtained with a full width half maximum of 0.92 Hz. The short-term frequency stability of 6.8 × 10−14 (τ/s)−1/2 is acquired. In addition, the number of detected atoms is increased by a factor of 1.7 with the optical state selection.
Our recent work showed the atomic microwave clock configuration beyond fountain is possible by trapped Rb or Cs atoms in optical lattice formed by trapping laser operating at magic wavelength for microwave clock transition where light shifts on both clock levels are matching exactly, and an accuracy of 2 times 10-17at this clock configuration is expected. In this report, we analyze the possibility of 20 s coherent interrogation time of stable Rb or Cs clock transition, which corresponds to a 2 km height atomic fountain. We propose a simple experimental scheme to measure the needed magic wavelengths for atomic microwave clocks in conventional fountain configuration. Moreover, we discuss a new clock scheme combining fountain and lattice configurations, thus provide an easy way to improve fountain clock performance by adding a trapping laser beam in conventional fountain system without any further complicated modifications
We present a coherent population trapping clock system based on laser-cooled 87Rb atoms. The clock consists of a frequency-stabilized CPT interrogation laser and a cooling laser as well as a compact magneto-optical trap, a high-performance microwave synthesizer, and a signal detection system. The resonance signal in the continuous wave regime exhibits an absorption contrast of ∼ 50%. In the Ramsey interrogation method, the linewidth of the central fringe is 31.25 Hz. The system achieves fractional frequency stability of 2.4 × 10 − 11 / τ , which goes down to 1.8 × 10−13 at 20000 s. The results validate that cold atom interrogation can improve the long-term frequency stability of coherent population trapping clocks and holds the potential for developing compact/miniature cold atoms clocks.
A portable cold87Rb atomic clock is under development at NIM. The clock uses87Rb atoms which are trapped and cooled in a small three-dimensional magneto-optical trap (3D-MOT) chamber. Within the 3D-MOT chamber, the free-falling atoms are prepared in∣F=2,mF=0⟩\vert F=2, m_{F}=0\rangle∣F=2,mF​=0⟩clock state with optical pumping (OP) directly after post cooling by2−2′2-2^{\prime}2−2′OP light and repumping. Following optical state selection, the free-falling atoms enter a microwave cavity where they undergo microwave interrogation. Eventually, the number of atoms in two clock states are detected individually by the fluorescence detection method in the detection chamber. The preliminary results show that, the selected atom number has been improved about 3 times with the optical state selection compared to the microwave state selection method. And a Ramsey fringe has been obtained experimentally, and the contrast of the central Ramsey fringe exceeds 88% with a full width half maximum (FWHM) of less than 9 Hz.