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European Frequency and Time Forum

Coherent Population Trapped Spectrum Based on Chip-Scale External Cavity Diode Laser

Tian Zhao, Jiqing Lian, S.K. Wu, Jie Miao, Zhendong Chen, Ruoao Yang, Duo Pan, Jingbiao Chen

Published Jun 25, 2024• 1 citation
Publisher

Abstract

This article reports a scheme for obtaining coherent population trapping (CPT) spectrum based on a chip-scale external cavity diode laser (CECDL). The CPT spectrum linewidth is 680Hz. Compared to the scheme using vertical cavity surface emitting laser (VCSEL), CECDL has a comparable small volume and higher laser power of 14mW and narrower linewidth of 270kHz. This is beneficial for improving the frequency stability performance of CPT chip-scale atomic clocks.

DOI
10.1109/eftf61992.2024.10722294

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This article reports a scheme for obtaining coherent population trapping (CPT) spectrum based on a chip-scale external cavity diode laser (CECDL). The CPT spectrum linewidth is 680Hz. Compared to the scheme using vertical cavity surface emitting laser (VCSEL), CECDL has a comparable small volume and higher laser power of 14mW and narrower linewidth of 270kHz. This is beneficial for improving the frequency stability performance of CPT chip-scale atomic clocks.
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.
We investigated, theoretically and experimentally, the influence of the laser spectrum (in particular the linewidth) on the coherent population trapping (CPT) resonance in87Rb buffer gas cell. We investigated the case of linearly polarized laser fields. The experiments have been performed by using either a current-modulated vertical cavity surface emitting laser (VCSEL) or a phase-locked-laser (PLL) system. A model based on the density matrix approach has been used for the data interpretation. We found that, under experimental conditions suitable for compact clock applications, the fact that VCSELs have a broader spectrum than PLLs does not influence significantly the CPT signal amplitude. We demonstrate that a not negligible role is played by the destructive interference between different CPT excitation channels and propose further investigations to evaluate this contribution with respect to the other causes. Finally, we discuss the application of this scheme in compact atomic clocks.
We propose a pulse excitation method of coherent population trapping (CPT) with a liquid crystal optical modulator. Since liquid crystals enable reductions in size, weight, cost, and power consumption compared with acousto-optical modulators (AOMs), our method is suitable for chip-scale atomic clocks (CSACs). Experiments showed that pulse excitation with a liquid crystal modulator can narrow the CPT resonance linewidth and reduce the light shift effect using a 87Rb gas cell and the D1-line vertical-cavity surface-emitting laser (VCSEL). The CPT resonance linewidth and light shift sensitivity were less than one-eighth and one-third those for continuous excitation, respectively. They also showed that our method is comparable to that based on an AOM.
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.
A lin⊥lin quasi-bichromatic light can be employed to realize a double-Λ coherent population trapping (CPT) resonance configuration, thereby enabling a CPT atomic clock with improved performance. A method for generating such light is proposed, in which two lasers with different linewidths are used to achieve a microwave frequency separation of several gigahertz. The effects of the optical injection ratio from the master extended-cavity diode laser into the slave vertical-cavity surface-emitting laser and the microwave power modulating the slave laser on the locking range were systematically investigated. The influence of both the injection ratio and microwave power on the contrast, linewidth, and the ratio of contrast to linewidth of the CPT signal is also studied. To enhance the trapping efficiency, single-pass and multi-pass interactions between the lin⊥lin quasi-bichromatic light and 87Rb atoms are investigated, with near-purely coherent states achieved under multi-pass interactions. Our scheme offers a promising solution for high-performance atomic clock applications.