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Chinese Physics B

Review of chip-scale atomic clocks based on coherent population trapping

Zhong Wang

Published Mar 1, 2014• 36 citations
Publisher

Abstract

Research on chip-scale atomic clocks (CSACs) based on coherent population trapping (CPT) is reviewed. The background and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the reliability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance's collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.

DOI
10.1088/1674-1056/23/3/030601

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Research on chip-scale atomic clocks (CSACs) based on coherent population trapping (CPT) is reviewed. The background and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the reliability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance's collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.
To Obtain an appropriate coherent population trapping (CPT) signal through a miniature atomic vapor cell is one of the key steps to implement a chip-scale atomic clock (CSAC). In the present experiment, the high S/N differential CPT spectral line has been achieved with a miniature atomic vapor cell through modulating the laser and extracting 87Rb atom laser interacting signal using the phase sensitive demodulation technique. With the spectral line, the dependence of CPT signal on the working parameters and the dependence of frequency stability of CSAC on the quality of the CPT signal have been studied; the obtained experimental results agree well with the theoretical prediction, which can be used as the recommended working parameters for CSAC. The methods exploited in the experiment can be implemented in the resource of a CSAC, therefore this experiment explores for CSAC the practical approaches of performance optimization.
Ever since the idea of the chip-scale atomic clock (CSAC) based on coherent population trapping (CPT) transition theory was proposed, the CSAC has been used widely in many various fields because of its characteristics of low power consumption, small size and good performance. It can be used in micro-positioning navigation and timing system, geophysical prospecting of the ocean bottom, and 5G communication system. Researchers work on reducing its volume and power consumption to make it more widely used. In the meantime, we all made great effort to improve its long-term stability. In this paper, we report our progress toward chip-scale atomic clock based on the87Rb CPT transition during 2019. We reduced the volume of the CSAC from 20 cm3to 12 cm3. For the physical part, we optimized the structure of the physical package to make sure it is possible for mass production. For the electrical part, we redesigned and rearranged the circuit, optimized the temperature control algorithm, employed a disciplining method. Finally, the frequency stability of CSAC was obtained to be 3.66×10-11, 6.39×10-12and 3.42 ×10-13at the interrogation time of 1 s, 103s and 104s respectively.
We have undertaken the development of a chip-scale atomic clock (CSAC) whose design goals include short-term stability, sigma(sub y)(tau=1 hour), of 1x10(exp 11) with a total power consumption of 30 mW and an overall device volume of 1 cu cm. The stringent power requirement dominates the physics package architecture, dictating a small (<5 mm(exp 3) gaseous atomic ensemble interrogated by a low-power semiconductor laser. At PTTI 2002, we reported on initial experimental investigations leading to the decision to employ the coherent population trapping (CPT) interrogation technique. This paper describes our further progress on the CSAC effort, including the development of custom vertical cavity surface emitting laser (VCSEL) sources and techniques for microfabricating miniature cesium vapor cells comprised of anodically bonded silicon and glass. Measurements of the signal contrast and linewidth of both the cesium D1 and D2 resonance transitions are compared, and frequency stability measurements of the CSAC testbed are presented.
More and more, precise timekeeping is becoming a crucial element of space-system operations. Not only are atomic clocks required at mission control ground stations, but they are expected to fly on spacecraft ranging in size from the “large” to CubeSats. This, of course, requires that the size, weight, and power (SWaP) of the clock cannot be sacrificed for frequency stability, and that the frequency stability of the device cannot be sacrificed for SWaP. Traditional space clocks (e.g. Rb clocks, Cs clocks, and hydrogen masers) require microwave cavities, rf-discharge lamps, alkali ovens, hydrogen dissociators, filter vapor-cells, or state-selecting magnets. In contrast, Coherent-Population-Trapping (CPT) clocks can have very low SWaP and have led to the development of the chip-scale atomic-clock (CSAC) – perfect for CubeSat missions. In the standard CPT clock, a narrow atomic resonance is formed by a bichromatic optical field. Unfortunately, in the standard CPT clock design the atomic signal suffers a low signal-to-noise ratio (SNR), due to atoms becoming systemically stuck in “trap” states inaccessible to the laser light. At The Aerospace Corporation’s Physical Sciences Laboratories, we have constructed a CPT atomic clock testbed, with the goal of accelerating CSAC development for next-generation space missions. Specifically, our plan is to investigate atomic physics issues that influence CSAC performance and SWaP, and prioritizing issues not being addressed in academic or industrial laboratories. In this presentation, we will discuss our CPT testbed, and recent work investigating CPT-enhancement strategies for improved clock performance that recover atoms from trap states and improve the SNR. Additionally, we will discuss our studies of the physical processes taking place in these CPT-enhancement strategies, and how they can affect advanced-CSAC functioning and design.
An ultra-low power consumed chip-scale atomic clock (CSAC) based on rubidium 87 coherent population trapping (CPT) resonance was reported in this paper. During the previous year, we focused on the power consumption deduction and dedicated to improve the frequency stability simultaneously. The power consumption of physical package was dramatically dropped to 20 mW, and thus the total power consumed by the CSAC is below 160 mW. Long term frequency stability with 1.65×10-11at 100s., and 8.91×10-12at 1000s is obtained.