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European Frequency and Time Forum

Optimisation of Double Resonance Optically Pumped Rb Atomic Clocks Using a Reconfigurable Physics Package Testbed

Nitika Gupta, Hugh A. Klein, Guilong Huang, Rabia Ince, Nyra Ashraf, Ulas Sabahattin Gokay, Laurence Nicholls, Mohsin Haji

Published Jun 25, 2024• 0 citations
Publisher

Abstract

The optimisation of a double resonance optically pumped (DROP) rubidium (Rb) atomic clock using a reconfigurable physics package testbed is reported. The testbed can incorporate numerous sizes and variations of the following components: optical sources, Rb vapour cells and microwave cavities. DROP measurements were carried out for a range of different optical powers and temperatures. For one of the Rb vapour cells studied, near optimum physics package conditions were achieved at 75 °C with a DROP signal contrast of 8.7%, a linewidth of 2.3 kHz, and an optical power of 25 In addition, the extent of helium permeation through the walls of a commercial Rb integrated cell was determined by immersing a Rb atomic clock in pure helium at atmospheric pressure and monitoring the drift of the 10 MHz output. A fractional drift rate in normal air (with 5 ppm helium) was determined to be nearly 10-11/month.

DOI
10.1109/eftf61992.2024.10722418

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The optimisation of a double resonance optically pumped (DROP) rubidium (Rb) atomic clock using a reconfigurable physics package testbed is reported. The testbed can incorporate numerous sizes and variations of the following components: optical sources, Rb vapour cells and microwave cavities. DROP measurements were carried out for a range of different optical powers and temperatures. For one of the Rb vapour cells studied, near optimum physics package conditions were achieved at 75 °C with a DROP signal contrast of 8.7%, a linewidth of 2.3 kHz, and an optical power of 25 In addition, the extent of helium permeation through the walls of a commercial Rb integrated cell was determined by immersing a Rb atomic clock in pure helium at atmospheric pressure and monitoring the drift of the 10 MHz output. A fractional drift rate in normal air (with 5 ppm helium) was determined to be nearly 10-11/month.
We report our studies on using microwave-optical double-resonance (DR) spectroscopy for a high-performance Rb vapour-cell atomic clock in view of future industrial applications.The clock physics package is very compact with a total volume of only 0.8 dm 3 .It contains a recently in-house developed magnetron-type cavity and a Rb vapour cell.A homedmade frequency-stabilized laser system with an integrated acousto-optical-modulator (AOM)for switching and controlling the light output power-is used as an optical source in a laser head (LH).The LH has the overall volume of 2.5 dm 3 including the laser diode, optical elements, AOM and electronics.In our Rb atomic clock two schemes of continuous-wave DR and Ramsey-DR schemes are used, where the latter one strongly reduces the light-shift effect by separation of the interaction of light and microwave.Applications of the DR clock approach to more radically miniaturized atomic clocks are discussed.
We report our investigations on a compact high-performance rubidium (Rb) vapourcell clock based on microwave-optical double-resonance (DR).These studies are done in both DR continuous-wave (CW) and Ramsey schemes using the same Physics Package (PP), with the same Rb vapour cell and a magnetron-type cavity with only 45 cm 3 external volume.In the CW-DR scheme, we demonstrate a DR signal with a contrast of 26% and a linewidth of 334 Hz; in Ramsey-DR mode Ramsey signals with higher contrast up to 35% and a linewidth of 160 Hz have been demonstrated.Short-term stabilities of 1.4×10 -13 τ -1/2 and 2.4×10 -13 τ -1/2 are measured for CW-DR and Ramsey-DR schemes, respectively.In the Ramsey-DR operation, thanks to the separation of light and microwave interactions in time, the light-shift effect has been suppressed which allows improving the long-term clock stability as compared to CW-DR operation.Implementations in miniature atomic clocks are considered. IntroductionSince the first invention of lamp-pumped rubidium (Rb) clocks in the 1960's [1,2], the demand for a stable, reliable, and compact Rb frequency standard with low power consumption has been growing, for various applications fields such as industry, telecommunication, navigation, space applications (e.g.GPS [3], GALILEO [4] and COMPASS [5]).Recently, thanks to the semiconductor laser technology, laser-pumped Rb clocks have been considered in several studies, for their better optical pumping efficiency results in short-term frequency stabilities at least one order of magnitude better than their lamp-pumped counterparts [6][7][8][9].This opened up also several new schemes for vapour-cell atomic clocks, such as the continuous-wave (CW) Rb clock [9], coherent population trapping (CPT) Cs clock [10] and pulsed optical pumping (POP) Rb clock [11].In this paper we report our studies on high-performance Rb frequency standards based on the laser-microwave double-resonance (DR) scheme, using both CW and Ramsey interrogation.Both CW-DR and Ramsey-DR schemes require two resonant electromagnetic fields of light and microwave.In the case of CW-DR, the laser optically pumps the vapour of the Rb atoms by creating a polarization of the atomic ground-state; simultaneously the microwave field, generated from a quartz oscillator and is near-resonant with the 87 Rb hyperfine clock transition (energy levels 5 2 S 1/2 │F g =1, m F =0˃ ↔│F g =2, m F =0˃), is applied.The opacity of the Rb vapour to the transmitted light signal as a function of microwave frequency is a measure of the atomic ground state polarization known as CW-DR signal [9].In CW-DR, the steps of optical pumping, microwave interrogation and optical detection are applied simultaneously (figure 1(b)).In the case of Ramsey-DR, these three sequences are separated in time, see figure 1(c): first, during optical pumping a strong laser pulse creates a significant atom population imbalance between
We present first results on a laser-pumped, double resonance (DR) Rubidium atomic clock, based on micro-fabricated Rb vapour cells. The study aims towards a miniature atomic clock based on DR, using a low-power laser as pump light source and a miniaturized microwave coupling structure (that will replace the bulky microwave cavity employed in today's much bigger Rb clocks). Good signal amplitude, low background light level, and no need for GHz laser modulation are among the advantages of this approach.
Experiments were carried out to evaluate the performances of a semiconductor laser pumped rubidium (87Rb) atomic clock. Two kinds of Rb gas cells were used and their performances were compared [gas cell A (natural rubidium (87Rb/85Rb =\frac{3}{7}) and buffer gases) and gas cell B (87Rb and buffer gases)]. The highest microwave frequency stabilities were estimated as3.4 \times 10^{-12} \tau^{-1/2}and2.7 \times 10^{-12} \tau^{-1/2}at the optimal gas cell temperatures of 60°C and 48°C for the gas cellsAandB, respectively (τ: integration time). The light shift, i.e., microwave frequency shift induced by laser light, was measured as -0.50 Hz/MHz and -0.11 Hz/MHz for the gas cellsAandBat their optimal operating conditions given above. As an improved experiment by utilizing high temporal coherence of the laser, a novel double resonance spectral line shape with a drastically narrower linewidth was demonstrated. A technique, similar to FM laser spectroscopy, was employed for this purpose by utilizing laser FM sidebands which are induced by microwave frequency modulation and nonlinear susceptibility of three-level87Rb atoms. The minimum linewidth obtained was 20 Hz, which can be used as a sensitive frequency discriminator for an improved87Rb atomic clock.
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.