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

An Integrated Pulsed Optically Pumped Rb Clock

Qiang Hao, Shaojie Yang, Peter Yun, Shuai Nie, Jun Ruan, Shougang Zhang

Published May 15, 2023• 0 citations
Publisher

Abstract

Rb atomic clock refers to the microwave transition of87Rb atoms, generally considered as a low-cost and low-performance atomic frequency standard. The maturity of semiconductor laser offers us a new opportunity to improve the Rb atomic clock. However, there are also many challenges in terms of electronic system and optical bench. Despite the performance superiority of the pulsed optically pumped (POP) Rb atomic clock has been demonstrated in the laboratory, there still no integrated prototype emerges to date. Here, we report the design and characteristics of an integrated POP Rb atomic clock. The primary test shows short-term stability of 3.2×10-13τ-1/2(1-1000s) and 4000s frequency stability of 8.5×10-15in air. The size and power consumption are 23 L and 45 W, respectively.

DOI
10.1109/eftf/ifcs57587.2023.10272106

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European Frequency and Time Forum•Apr 1, 2012•1 citation
Rb atomic clock refers to the microwave transition of87Rb atoms, generally considered as a low-cost and low-performance atomic frequency standard. The maturity of semiconductor laser offers us a new opportunity to improve the Rb atomic clock. However, there are also many challenges in terms of electronic system and optical bench. Despite the performance superiority of the pulsed optically pumped (POP) Rb atomic clock has been demonstrated in the laboratory, there still no integrated prototype emerges to date. Here, we report the design and characteristics of an integrated POP Rb atomic clock. The primary test shows short-term stability of 3.2×10-13τ-1/2(1-1000s) and 4000s frequency stability of 8.5×10-15in air. The size and power consumption are 23 L and 45 W, respectively.
Pulsed optically pumped (POP) Rb atomic clock is considered to be a powerful technique for the Rb atomic clock due to the capability of light-shift mitigation and atomic spectroscopy narrowing. Relevant investigations are conducted over two decades, however, no integrated prototype of such a kind of clock has been reported yet. Here, we present an integrated prototype of the POP Rb atomic clock and quantitatively characterize its physical properties. The atomic clock shows a short-term fractional frequency stability of $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}/\sqrt{\ensuremath{\tau}}$ (where $\ensuremath{\tau}$ is the averaging time), and comes to $2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ with 40 000 s averaging (drift removed) under atmospheric conditions. Physical effects contributing to the long-term frequency stability are carefully analyzed and cavity-pulling shift is identified as the leading limit. The techniques presented here enable realizing a low ${10}^{\ensuremath{-}15}$ level vapor-cell atomic clock under atmospheric conditions, which could benefit a wide range of terrestrial applications, such as quantum communication, decimeter-level terrestrial positioning.
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.
In this paper we report a laboratory prototype of pulsed optically pumped (POP) rubidium frequency standard. In POP operation the pumping, interrogation, and detection phases are separated in time, avoiding the coupling between microwave and optical coherence. The POP87Rb vapor-cell frequency standard was studied experimentally based on the detection of free induction microwave signal or optical absorption signal. Ramsey fringes were obtained based on87Rb in buffer gas, with FWHM of 65Hz and 150Hz for microwave and optical central Ramsey fringes respectively. The atomic quality factor was on the order of 108. With microwave detection, the achieved frequency stability was 1.2×10-12τ-1/2(τ=1~100s).
We present theoretical and experimental studies concerning the realization of a rubidium vapor cell frequency standard based on the pulsed optical pumping (POP) technique. This technique is extremely powerful since it avoids in principle the light shift effect and the noise conversions from the laser to the clock signal. The detection of the clock transition in the microwave domain (the so called POP maser) has been extensively investigated at INRIM and a frequency stability of 1.1 × 10-12τ-1/2 has been measured until integration times of 105 s, reaching the level of 5 × 10-15 (drift removed). The optical detection is still under evaluation but this technique may lead in effect to some advantages with respect to the microwave detection, such as a lower or negligible cavity pulling, a lower operation temperature (which implies a lower spin exchange contribution) and a higher signal-to-noise ratio. All these features may lead to a better frequency stability performance than that previously reached. Read More: http://www.worldscientific.com/doi/abs/10.1142/9789812838223_0040
We demonstrate a high-performance pulsed optically pumped (POP) Rb vapor-cell clock based on a magnetron-type microwave cavity of only 44 cm3 external volume. Using optical detection, an unprecedented 35% contrast of the Ramsey signal has been obtained. Both the signal-to-noise ratio (of 30 000) and the estimated shot-noise limit of 1.7 × 10−14 τ−1/2 are at the same level as those found with a bigger cylindrical TE011 cavity (100 cm3 inner volume) and are sufficient for achieving excellent clock stability. Rabi oscillations are measured and indicate a sufficiently uniform microwave magnetic field distribution inside the cavity. The instability sources for the POP clock's performance are analyzed. A short-term stability of 2.1 × 10−13 τ−1/2 is demonstrated which is consistent with the noise budget.