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Chinese Optics Letters

Compact optical frequency standard based on a miniature cell using modulation transfer spectroscopy

Jiqing Lian, Qiaohui Yang, Tianyu Liu, Duo Pan, Jie Miao, Zhendong Chen, Jingming Chen, Chen Jiang, Lina Bai, Zhidong Liu, Jingbiao Chen

Published Jan 1, 2025• 2 citations
Publisher

Abstract

This study investigates the application of miniature quantum references within small-scale optical atomic frequency standards, utilizing a diminutive 87 Rb glass cell, dimensions of 6 mm × 6 mm × 6 mm, to establish a quantum frequency standard.By employing the transition spectrum from 5 2 S 1/2 F = 2 to 5 2 P 3/2 F 0 =3, this study successfully demonstrates the development of a compact rubidium atomic optical frequency standard via modulation transfer spectroscopy (MTS).Subsequent to frequency stabilization, the 780 nm clock laser exhibits a linewidth of 6.9 kHz, and its out-of-loop short-term stability reaches 4.1 × 10 -13 @ 1 s, as confirmed through beat frequency analysis.This research not only provides a practical blueprint for the development of small optical atomic frequency standards but also lays down essential groundwork for future advancements in chip-level optical frequency standard technologies.

DOI
10.3788/col202523.041201

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This study investigates the application of miniature quantum references within small-scale optical atomic frequency standards, utilizing a diminutive 87 Rb glass cell, dimensions of 6 mm × 6 mm × 6 mm, to establish a quantum frequency standard.By employing the transition spectrum from 5 2 S 1/2 F = 2 to 5 2 P 3/2 F 0 =3, this study successfully demonstrates the development of a compact rubidium atomic optical frequency standard via modulation transfer spectroscopy (MTS).Subsequent to frequency stabilization, the 780 nm clock laser exhibits a linewidth of 6.9 kHz, and its out-of-loop short-term stability reaches 4.1 × 10 -13 @ 1 s, as confirmed through beat frequency analysis.This research not only provides a practical blueprint for the development of small optical atomic frequency standards but also lays down essential groundwork for future advancements in chip-level optical frequency standard technologies.
This work introduces an innovative optical frequency standard that leverages MEMS vapor cells to achieve exceptional stability. To attain these objectives, we have integrated the modulation transfer spectroscopy (MTS) technique, which not only ensures high stability but also contributes to minimal power consumption in a compact form factor. Employing MTS with a high signal-to-noise ratio and utilizing a full bandwidth, high-speed servo feedback, we have successfully stabilized the laser frequency at the hyperfine transitions of rubidium atomic lines within the MEMS vapor cell. Furthermore, after the laser is locked, there is a significant narrowing effect on the laser linewidth. The current specifications of our compact optical frequency standard include a frequency Allan deviation of 4.8E-13@1s and the laser linewidth of only 18 kHz.
Atomic clocks represent the most advanced instruments for providing time-frequency standards, with increasing demand for designs that offer high frequency stability while minimizing size. Central to an atomic clock’s function is the atomic vapor cell, which serves as the quantum reference. Compared to traditional cells, wafer-level micro-electro-mechanical systems (MEMS) vapor cells enable cost-effective, scalable production and facilitate integration with silicon-based chips. In this work, we present a wafer-level MEMS vapor cell featuring an innovative silicon-glass-silicon transverse optical path structure. A single wafer is used to fabricate 24 identical atomic vapor cells, each with precise dimensions of 14 mm×14 mm×4.3 mm, ensuring scalability. We demonstrate an optical frequency standard that combines modulation transfer spectroscopy (MTS) with a MEMS vapor cell, featuring a compact design with excellent performance. This frequency standard achieves stability over averaging times of 1–400 s, with short-term stability of 2.6×10 −13 at 1 s and 5.1×10 −14 at 200 s. The laser linewidth is only 3.9 kHz, marking a substantial improvement over existing thermal standards, and opening potential applications in navigation, radar, and precision measurement. This work provides a crucial step toward the development of chip-scale optical clocks.
Atomic clock is currently the highest precision time frequency standard device, which is widely used in communication, positioning, navigation and other advanced fields. However, optical frequency standards often face problems such as huge volume, complex system and high cost, which greatly limit their application scenarios. Here we report a compact 780 nm Rb optical frequency standard on “optical cubes”, which can greatly reduce the volume and maintain high stability. Profit from the robust architecture of “optical cubes”, the physical volume of the optical frequency standard can be greatly reduced to 140 mm*140 mm*100 mm, which based on modulation transfer spectroscopy. The “optical cubes” are a concept that all optical components can be mounted on universal glass cubes. At present, the compact optical frequency standard’s frequency instability reached short-term stability of 7.2E-13@1s.
We experimentally study the character of saturated absorption spectroscopy (SAS) of the 87 Rb 5 S 1/2 → 6 P 3/2 transition at 420 nm in a miniature cubic glass cell (3 × 3 × 3 mm 3 ). The influences of the laser intensity and cell temperature on the saturated absorption resonance are comprehensively investigated to determine the optimal parameters for a compact stabilized frequency standard. By stabilizing the laser frequency to the optimized SAS signal, a beatnote stability of 2.4 × 10 −13 at 1 s is measured between two identical SAS systems, corresponding to a single-laser stability of 1.7 × 10 −13 at 1 s, assuming equal contribution from each laser. A further analysis of the noise budget reveals that the main contributions of short-term stability arise from the intermodulation effect and the laser frequency noise through the frequency modulation to amplitude modulation (FM-AM) conversion process. Our work provides a promising route toward high-performance miniaturized optical frequency standards based on high-lying excited-state transitions.
A compact rubidium optical clock is presented, incorporating an optical frequency standard, a fiber frequency comb, and associated control electronics. Modulation transfer spectroscopy, devoid of Doppler background and characterized by a high signal-to-noise ratio and sensitivity, is employed to lock the laser frequency to the hyperfine transitions of rubidium atomic lines. The clock exhibits a short-term stability of 4.10 × 10-13at a 1-second gate time, and the stability remains below 7.27 × 10-13up to 1000 seconds. These results offer a promising alternative approach for the development of portable optical clocks.
We report on the development and short-term stability characterization of an optical frequency reference based on the spectroscopy of the rubidium two-photon transition at 778 nm in a microfabricated vapor cell. When compared against a 778 nm reference signal extracted from a frequency-doubled cavity-stabilized telecom laser, the short-term stability of the microcell frequency standard is 3.5 ×\times× 10−13^{-13}−13 τ−1/2τ^{-1/2}τ−1/2 until 200~s, in good agreement with a phase noise level of +++ 43 dBrad2^22/Hz at 1~Hz offset frequency. The two main contributions to the short-term stability of the microcell reference are currently the photon shot noise and the intermodulation effect induced by the laser frequency noise. With still a relevant margin of progress, these results show the interest of this spectroscopic approach for the demonstration of high-stability miniaturized optical vapor cell clocks. Such clocks are poised to be highly beneficial for applications in navigation, communications, and metrology.