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Improving the start-up characteristics of the rubidium atomic clock

Yonggang Guo, Lili Zhu, Shiwei Wang, Jun Zhang, Fucheng Shi, Jinhai Zhang, Jianxiang Wang, Wenxin Li, Zhidong Liu

Published Apr 1, 2022• 4 citations
Publisher

Abstract

Considering the importance of start-up characteristics of the rubidium atomic clock in engineering applications, the objective of this paper is to optimize the start-up characteristics of the rubidium atomic clock by studying the theory of the rubidium atomic frequency standards, especially the light pumping process, and the effect of light intensity on frequency accuracy. Our analysis demonstrated that frequency accuracy is proportional to the light intensity, and hence, we propose a method for actively optimizing the start-up characteristics of the rubidium atomic clock by utilizing the fluctuations in light intensity. Additionally, some related experiments using the proposed method indicate that the light intensity–frequency coefficient of the rubidium atomic clock is improved from 1.84 × 10−9 to 4.21 × 10−10 V−1 within 30 min after the rubidium atomic clock is locked, and also, the lockout time is less than 5 min with a wide working temperature range (0–50 °C), indicating a significant improvement in the start-up characteristics of the rubidium atomic clock.

DOI
10.1063/5.0086523

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Considering the importance of start-up characteristics of the rubidium atomic clock in engineering applications, the objective of this paper is to optimize the start-up characteristics of the rubidium atomic clock by studying the theory of the rubidium atomic frequency standards, especially the light pumping process, and the effect of light intensity on frequency accuracy. Our analysis demonstrated that frequency accuracy is proportional to the light intensity, and hence, we propose a method for actively optimizing the start-up characteristics of the rubidium atomic clock by utilizing the fluctuations in light intensity. Additionally, some related experiments using the proposed method indicate that the light intensity–frequency coefficient of the rubidium atomic clock is improved from 1.84 × 10−9 to 4.21 × 10−10 V−1 within 30 min after the rubidium atomic clock is locked, and also, the lockout time is less than 5 min with a wide working temperature range (0–50 °C), indicating a significant improvement in the start-up characteristics of the rubidium atomic clock.
We demonstrate the implementation of a Rb87 chip-scale atomic clock (CSAC) that has improved long-term stability. A simple method of reducing the frequency drift in the CSAC is proposed. As well as the well-known effect of light intensity on the clock frequency, our analysis shows that the frequency drift that is due to laser frequency detuning (LFD) variation originates from asymmetry in the coherent laser fields, and thus, we propose to actively compensate the clock frequency by using variations in the light intensity and LFD. We performed experiments to obtain precise clock frequency sensitivities to the light intensity and LFD, and the frequency drift was reduced from 7.1×10−11/day to 6.9×10−13/day with the frequency compensation method. Additionally, the CSAC with an optimized configuration of light intensity and microwave power showed evident long-term frequency stability improvement, from 8.9×10−11 to 8.7×10−12 at 105 s. Therefore, our method is useful in reducing the frequency drift of CSACs and could be potentially used in applications that require moderate long-term stability of sub-1×10−11.
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.
Since the 1960s, alkali-based RF discharge lamps have been used as frequency-stable light sources due to their reliable performance [1]. However, it is often reported that rubidium (Rb) lamp-based standards suffer from lamp-related frequency ageing phenomena such as light-shift effects [2], [3]. To understand this behaviour, we are characterising a commercial Rb-based RF discharge lamp over extended periods and will present an analysis of data collected so far. Electrical and optical measurements on a Rb lamp are presented here after about 40 days of monitoring. The output of the lamp was coupled into an optical fibre to observe its emitted light characteristics with an optical spectrum analyser. Optical spectra results indicated that the lamp had substantial buffer gas content. The lamp spectra exhibited various buffer gas excitation lines as well as strong Rb excitation lines of 780.0 nm (D2 line) and 794.8 nm (D1 line). Subsequently, the lamp was monitored for output optical power, temperature, and electric current dynamics in a laboratory environment. During monitoring, sudden changes in light output were observed along with longer term trends. From previous studies, we can expect that temporal changes in the light intensity of the lamps may lead to observable frequency fluctuations in an atomic clock [4].
We present our recently experimental results on a DFB laser diode pumped Rubidium atomic frequency standard. Our experiment was based on a homemade lamp-pumped Rubidium frequency standard with the discharge lamp assembly removed. In the experiment, we first locked a DFB laser diode to the87Rb cycling transition (5S1/2, F=2→5P3/2, F=3) by means of saturation absorption locking technique. After that, we optimized the parameters carefully related to the clock performance, such as temperature of the physical package, amplitude of the modulation signal for the laser locking. The preliminary short-term stability of our clock is measured 5.25×10−12τ−1/2(1s<τ<100s).
We measure the light shift of the rubidium atomic clock under interaction of a 780 nm laser, which has potential application in the laser power stabilization. At the same time, in order to improve the performance of rubidium clock, we demonstrate a Faraday laser of which the cavity mode is locked to the peak of the transition spectrum, thus suppressing the otherwise existed power and frequency noise caused by the cavity mode fluctuations. This Faraday laser is suitable for the optical pumped rubidium atomic clock with the frequency being stably on resonance with the rubidium 780nm transition, and has potential applications on the cesium beam clocks and fountain clocks.
We have realized and studied a rubidium atomic frequency standard based on a paraffin-coated cell, exhibiting a short-term frequency stability <3 × 10−12 τ−1/2 between τ = 1 and 100 s. Characterization of the wall-coating is performed by measuring the T1 and T2 relaxation times. Perturbations of the medium- to long-term clock stability, due to variations in the laser-intensity, laser frequency, the microwave power shift, and the shifts due to temperature variations are measured and analyzed. A method for reducing the intensity light-shift by detuning the laser frequency and the resulting improvement in clock stability is demonstrated. This work is of relevance for further improvements on Rb cell standards using anti-relaxation wall-coating technology.