Frequency Stability Of Atomic Clock Research Articles

Reduction of light shifts in a cold-atom CPT clock

Light shifts induced during atom–light interactions significantly affect the medium- and long-term frequency stability of atomic clocks. Here, we employ composite laser pulse sequences to mitigate interrogation-induced light shifts in a cold-atom coherent-population-trapping clock. We obtain the anti-symmetry error signal via modulating the local oscillator phase in the free-evolution time of Ramsey interferometry. Utilizing this signal, we employ two feedback loops to simultaneously eliminate light shifts and stabilize the clock frequency using the auto-balanced Ramsey (ABR) spectroscopy scheme. Our experimental results demonstrate that this approach can reduce the clock frequency’s sensitivity to variations in light shifts by implementing four Ramsey sub-sequences. Furthermore, we show that the ABR spectroscopy scheme enhances the long-term frequency stability of the atomic clock when the averaging time τ > 5000 s.

Evaluation of Stability of Higher-Order Atomic Clocks by Higher-Order Allan Variance

In this paper, we analyze a generalized version of the Allan variance, referred to as a higher-order Allan variance, that can evaluate the frequency stability of multi-dimensional atomic clocks as a function of the measurement interval. It is known that the standard Allan variance, a measure to evaluate the frequency stability of atomic clocks, has a concern that its value for third-order atomic clocks, such as hydrogen maser, is dependent not only on the sampling interval of measurement data, but also on the time of measurement. To resolve this issue, this paper introduces the higher-order Allan variance defined for the higher-order difference of clock reading deviations. The main contribution of this paper is to mathematically prove that the higher-order Allan variance depends only on the sampling interval of clock reading deviations, for which a sufficiently high order of difference is considered. Furthermore, we present numerical simulation studying the numerical stability of the higher-order Allan variance computation for time series data of clock reading deviations.

Control of the VCSEL spectrum by dual microwave frequency modulation.

We propose and investigate a method for controlling the spectrum of the vertical-cavity surface-emitting laser by simultaneous modulation of the injection current at single and doubled frequencies. We experimentally demonstrate the ability to control the power asymmetry of the first-order sidebands and to suppress the carrier by the proposed method. These possibilities are beneficial to improve frequency stability of atomic clocks based on the effect of coherent population trapping.

Comparison and assessment of long-term performance of BDS-2/BDS-3 satellite atomic clocks

Recently, the performance of the new-generation BeiDou Navigation Satellite System (BDS) satellite clocks has attracted considerable attention. There are two types of BDS clocks developed in China—namely, the new rubidium (Rb-II) and passive hydrogen maser (PHM)—and these can affect positioning, navigation and timing (PNT) services. In this paper, to comprehensively evaluate and analyze the performance of BDS satellite atomic clocks, the physical characteristics are exhibited, including phase, frequency, frequency drift and noise. The physical characteristics are applied to detect the switching of satellite clocks. The frequency stability of atomic clocks is assessed by overlapping Allan deviation (ADEV) and overlapping Hadamard deviation (HDEV). The periodic characteristics of satellite clock bias data are studied through spectrum analysis. Based on 550 day precision satellite clock bias data provided by Wuhan University, China, experiments were performed, focusing on the performance of the Rb-II and PHM clocks installed on BDS-3. Some valuable conclusions are obtained: (a) the BDS-3 satellite clocks perform better than BDS-2. The root mean square value of the frequency drift of BDS-3 satellite clocks is enhanced by 41% compared to BDS-2. The noise of BDS-3 is 0.216 ns, which is 55% better than BDS-2. (b) The frequency stability of BDS-2 satellite clocks is worse than that of BDS-3. The mean daily stability of BDS-2 and BDS-3 satellite clocks by ADEV is 5.29 × 10−14 and 5.36 × 10−14, respectively, while that with HDEV is 2.35 × 10−14 and 0.70 × 10−14. For PHM clocks, the stability at 10 000 s with ADEV and HDEV is 2.29 × 10−14 and 2.25 × 10−14, respectively, while the daily stability is 0.77 × 10 −14 and 0.67 × 10−14, respectively. (c) BDS geostationary orbit (GEO) and inclined GEO satellite atomic clocks have multiple significant periodic terms while the medium earth orbit satellite clocks only own one or two, which are related to the corresponding satellite orbital periods.

Microfabricated cells for chip-scale atomic clock based on coherent population trapping: Fabrication and investigation

A universal method for fabrication of miniature cells for frequency standards and quantum magnetometers containing 87Rb atoms in the atmosphere of inert gas neon based on integrated technologies is considered. The results of experimental studies of coherent population trapping signals observed for a series of cells which provided recovery of vapors of an alkali metal from the rubidium dichromate salt with the help of laser radiation are presented. The coherent population trapping signals with a typical linewidth of 2–3 kHz and a signal-to-noise ratio of 1500 in the 1-Hz bandwidth were observed, which allows one to provide a relative frequency stability of atomic clock of 10−11 at 100 s.

Frequency Stability of Atomic Clocks Based on CoherentPopulation Trapping Resonance in 85Rb

An atomic clock system based on coherent population trapping (CPT)resonance in 85Rb is reported, while most past works about theCPT clock are in 87Rb. A new modulation method(full-hyperfine-frequency-splitting modulation) is presented to reducethe effect of light shift to improve the frequency stability of the CPTclock in 85Rb. The experimental results show that the short-termfrequency stability of the CPT clock in 85Rb is in the order of10−10/s and the long-term frequency stability can achieve1.5×10−11/80000 s, which performs as well as 87Rb in CPTresonance. This very good frequency stability performance associatedwith the low-cost and low-power properties of 85Rb indicatesthat an atomic clock based on CPT in 85Rb should be a promisingcandidate for making the chip scale atomic clock.

Continuous light-shift correction in modulated coherent population trapping clocks

The authors demonstrate a simple technique to significantly improve the long-term frequency stability of atomic clocks based on coherent population trapping (CPT). In this technique, the CPT fields are created by a modulated diode laser and a slow servo is used to actively tune the laser modulation index to a value where the light shift vanishes. The observed clock frequency at this modulation index is given by the rubidium hyperfine frequency when no light fields are present, and this makes the clock frequency largely insensitive to variations in laser properties. In addition to reducing stringent requirements on the long-term stabilities of laser temperature, laser frequency, and rf modulation power, this technique may also significantly reduce frequency drifts related to laser aging. In the experiment, they demonstrate improvement by over one order of magnitude in the stability of a clock that is limited by light-shift-induced frequency drifts.