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Symposium on Photonics and Optoelectronics

Analysis and Design of High-Performance Coherent Population Trapping Passive Atomic Clock System Based on Vertical-Cavity Surface-Emitting Laser

Chao Guan, Xiaosong Zhu, Qun Zhou

Published May 1, 2011• 1 citation
Publisher

Abstract

Based on the model of the Λ-type three-level atomic system, the principle, the evolutionary equations and the resonance line characteristics of the coherent population trapping (CPT) are analyzed by means of ensemble-averaged density matrix approach. The paper presents a schematic illustration of practical realization and a block scheme of comprehensive analysis of the CPT system. Using vertical-cavity surface-emitting laser (VCSEL) as optical source, we propose an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system. The whole system is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.

DOI
10.1109/sopo.2011.5780574

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Based on the model of the Λ-type three-level atomic system, the principle, the evolutionary equations and the resonance line characteristics of the coherent population trapping (CPT) are analyzed by means of ensemble-averaged density matrix approach. The paper presents a schematic illustration of practical realization and a block scheme of comprehensive analysis of the CPT system. Using vertical-cavity surface-emitting laser (VCSEL) as optical source, we propose an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system. The whole system is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.
Based on the operating principle and the comprehensive analysis of the coherent population trapping (CPT) atomic clock system, the paper presents an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system which uses vertical-cavity surface-emitting laser (VCSEL) as optical source, selects85Rb as operating atoms, and is in resonance with the D1transitions. The system can be used for experimental study of correlation of each parameter and study of the Ramsey-CPT atomic clock. The whole system, which adequately combines the advantages of the ordinary desktop CPT experimental system and the chip-scale atomic clock (CSAC), is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.
Chip-scale atomic clocks are critical for applications requiring compact, energy-efficient, and highly stable timing, such as GPS receivers and communication systems. Coherent population trapping (CPT)-based atomic clocks, which commonly use vertical-cavity surface-emitting lasers (VCSELs), have enabled significant miniaturization. However, their precision is limited by the broad linewidth of VCSELs. Alternative laser technologies, such as distributed Bragg reflector lasers, distributed feedback lasers, and external cavity diode lasers (ECDLs), offer potential improvements but face challenges in achieving compact size and adequate modulation bandwidth. This study presents a 795 nm chip-ECDL with a direct modulation bandwidth up to 4 GHz, specifically designed for CPT chip-scale atomic clocks. Based on the chip-ECDL, a CPT atomic clock with a compact volume of 19.4 ml and power consumption of 1.5 W has been developed. Under identical operating conditions, the CPT signal produced by the chip-ECDL exhibits a 34% reduction in linewidth and a 1.14-fold increase in contrast compared to a VCSEL-based clock. Demonstrating short-term stability better than 5×10−11τ−1/2(1 s<τ<2000 s) and reaching a best stability of 6.6×10−13 at 2000 s, this work represents the application of an ECDL in a CPT chip-scale clock, advancing the development of next-generation, high-performance chip-scale atomic clocks.
The spectroscopic technique of coherent population trapping (CPT) enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards. All-optical interrogation enables the reduction of the size and power consumption of an atomic clock by two orders of magnitude, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources due to their low power consumption and circular output beam. Several research teams are currently using VCSELs for DARPA's chip-scale atomic clock (CSAC) program with the goal of producing an atomic clock having a volume < 1 cm^3, a power consumption < 30 mW, and an instability (Allan deviation) < 1x10^-11 during a 1-hour averaging interval. This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth will be discussed in particular, since atomic clock performance is especially sensitive to this parameter.
We report on the performance of the first table-top prototype atomic clock based on coherent population trapping (CPT) resonances with parallel linearly polarized optical fields (lin‖lin configuration). Our apparatus uses a vertical-cavity surface-emitting laser (VCSEL) tuned to the D1 line of 87Rb with the current modulation at the 87Rb hyperfine frequency. We demonstrate cancellation of the first-order light shift by the proper choice of rf modulation power and further improve our prototype clock stability by optimizing the parameters of the microwave lock loop. Operating in these optimal conditions, we measured a short-term fractional frequency stability (Allan deviation) 2×10−11τ−1/2 for observation times 1 s ≤ τ ≤ 20 s. This value is limited by large VCSEL phase noise and environmental temperature fluctuation. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated lin‖lin CPT resonance clock with environmental impacts minimized.