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Applied Optics

Frequency stability of a pulsed optically pumped atomic clock with narrow Ramsey linewidth

Haixiao Lin, Jianliao Deng, Jinda Lin, Song Zhang, Yuzhu Wang

Published Apr 12, 2018• 14 citations
Publisher

Abstract

In general, the linewidth of the Ramsey central fringe (RCF) is equal to 1/(2T), where T is the Ramsey free-evolution time. We demonstrate that the RCF linewidth of a pulsed optically pumped (POP) atomic clock with orthogonal polarization detection based on the magneto-optical rotation effect can be narrowed down to 1/(4T). The Allan deviation of the POP atomic clock decreases from 2.4×10−13τ−1/2 to 1.4×10−13τ−1/2. This corresponds to an improvement in the frequency stability by about 60%. We also estimate the main noise sources that limit the short-term frequency stability of the POP atomic clock.

DOI
10.1364/ao.57.003056

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In general, the linewidth of the Ramsey central fringe (RCF) is equal to 1/(2T), where T is the Ramsey free-evolution time. We demonstrate that the RCF linewidth of a pulsed optically pumped (POP) atomic clock with orthogonal polarization detection based on the magneto-optical rotation effect can be narrowed down to 1/(4T). The Allan deviation of the POP atomic clock decreases from 2.4×10−13τ−1/2 to 1.4×10−13τ−1/2. This corresponds to an improvement in the frequency stability by about 60%. We also estimate the main noise sources that limit the short-term frequency stability of the POP atomic clock.
In this paper we report the results we obtained with a prototype of pulsed optically pumped (POP) Rb clock. In terms of frequency stability, an Allan deviation σy(τ)=1.7 ×10-13τ-1/2has been measured, where τ is the integration time. In addition, the long term frequency drift results below 10-14/day. In the paper we also describe a technique to stabilize the amplitude of the microwave signal using the atoms themselves as discriminator. Moreover, an anomalously large temperature sensitivity of the clock frequency and a technique to keep it under control are also reported. Finally, we describe some non linear effects that modify the shape of the clock signal represented by the Ramsey central fringe.
We demonstrate a high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1/(2TR), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3×10−13 τ−1/2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained in the conventional continuous wave regime, with the use of an additional laser power stabilization setup. The pulsed interaction allows to reduce significantly the clock frequency sensitivity to laser power variations, especially for high values of TR. This pulsed CPT clock, ranking among the best microwave vapor cell atomic frequency standards, could find applications in telecommunication, instrumentation, defense or satellite-based navigation systems.
We present a prospective experimental study on double-resonance Ramsey spectroscopy in a micro fabricated Rb vapor cell, in view of future miniature atomic clocks with improved stability performance. Ramsey signals with a narrow central fringe width of 1.5 kHz combined with a competitive contrast of > 5 % are observed, resulting in a clock short-term stability of ≈ 1×10-11τ−1/2. The employed pulsed optical pumping (POP) scheme bears promise for reduced light-shift effects and thus for improved clock stability on medium- to long-term timescales.
Herein, we report the scheme of an optically pumped atomic clock based on a cold cesium atomic beam source. We propose the laser system and physical mechanism of this atomic clock, wherein the atomic beam travels in an upper parabolic trajectory, thereby eliminating the light shift effect. In the experiments, when the length of the free evolution region was 167 mm, the line width of the Ramsey fringe was 37 Hz. When the expected signal-to-noise ratio of the Ramsey fringe that can be achieved is 36,000, the expected short-term frequency stability is about 3.6 × 10–14/√τ, which is significantly higher than that of a conventional optically pumped cesium clock of similar volume.
In this paper, we report a dispersive detection technique with ultrahigh resonance contrast based on magneto-optical effect. To demonstrate specifically the advantages of the dispersive detection, we applied it in the pulsed optically pumped (POP) clock to obtain the clock signal. The contrast of Ramsey central fringe versus the parameters of pumping light, detection light and microwave power are studied experimentally. An ultrahigh resonance contrast greater than 90% is obtained. The frequency stability of POP clock in terms of Allan deviation for dispersive detection is improved by an order of magnitude over the absorptive detection under equivalent condition. We also present the preliminary experimental result in case of off resonance detection where a laser with large frequency detuning is used as detection light. The signal intensity detected by off resonance light is improved about 70% than the resonant light.
We report a high-performance pulsed optically pumped (POP) Rb clock based on a novel magnetron-type microwave cavity. The cavity has a volume of 30 mL, enabling a highly homogenous microwave field distribution. With the laser frequency tuning to the ground-state hyperfine level F = 2 of the D2line, we observe a Ramsey fringe contrast of 52% in terms of optical absorption detection. The resultant shot noise limit is estimated to be 1.6 × 10-14τ-1/2. The clock frequency stability is measured as 2.3 × 10-13τ-1/2(1-100 s), which is limited by the relative intensity noise of the light.