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IEEE Ultrasonics, Ferroelectrics, and Frequency Control Joint Symposium (UFFC-JS)

Compact 780 nm Rb Optical Frequency Standard on “Optical Cubes”

Qiaohui Yang, Tianyu Liu, Jiqing Lian, Chuanwen Zhu, Jie Miao, Zhendong Chen, Jingmin Chen, Duo Pan, Jingbiao Chen

Published Sep 22, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.1109/uffc-js60046.2024.10794103

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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 mm140 mm100 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.
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.
We demonstrated a compact 780 nm rubidium optical clock, which includes an optical frequency standard and an optical frequency comb, with an optical volume of 11.6 liters. Unlike the 778 nm rubidium atomic clocks based on two-photon transition, here, the laser frequency is stabilized to the Rb D2 transition, using modulation transfer spectroscopy. This approach effectively eliminates Doppler background and provides a high signal to noise ratio and high sensitivity. A nearly 300 MHz microwave signal, whose phase exactly tracks that of the optical frequency standard, is generated via the optical frequency comb, yielding a frequency instability of 1.91 E-13 @1 s and 5.29 E-14 @1000 s in the electronic domain. To the best of our knowledge, this is the most precise frequency stabilization result for the first-excited-state transition of alkali metal atoms to date and represents the first optical clock based on this transition. These results offer a promising approach for the development of portable optical clocks.
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.
Optical clocks with high precision are practical and necessary in atomic physics, geodesy, gravitational wave measurement, etc. Transportability and environmental adaptability are two key indicators to measure the performance of frequency standards. Here, we utilized a highly robust atomic-filter-based Faraday laser as the local oscillator, and achieved two compact Faraday optical frequency standards based on 133Cs6S1/2|F=3⟩→6P3/2|F′=2⟩ and 6S1/2|F=4⟩→6P3/2|F′=5⟩ transitions at 852 nm, leveraging modulation transfer spectroscopy for laser frequency stabilization. The Faraday laser automatically aligns atomic transition spectral lines, due to the use of atomic filters as frequency selective elements. Therefore, the laser wavelength has strong robustness against environmental changes. This setup not only simplifies the operational complexity but also ensures frequency stability. By heterodyne beating between two such compact optical frequency standards, the beatnote achieved a short-term frequency stability of 1.68 × 10−13 at 3 s, remained at the 10−13 level for up to 400 s, and stayed below 2.7 × 10−12 for up to 1600 s, predominantly exhibiting white frequency noise. This underscores the potential of atomic-filter-based Faraday laser in realizing compact and stable frequency standards, mitigating coherent communication error rates, and enhancing the precision of quantum interferometric measurements. Moreover, this approach eliminates the continuous need for external monitoring and adjustments, thereby, offering a practical and robust solution for out of laboratory applications demanding high frequency stability.
Abstract High-precision time–frequency standards play vital roles in fundamental physics, geodesy, navigation, and communication. This paper reports experimental progress on an optical frequency standard (OFS) based on the 5S 1/2 → 5D 5/2 two-photon transition in 87 Rb. A 1556 nm communication-band laser is frequency-doubled to generate a 778.1 nm clock laser, which excites Rb atoms via counter-propagating beams. The 420 nm fluorescence is collected to obtain a high signal-to-noise-ratio transition spectrum. The effects of laser power and vapor cell temperature on the fluorescence intensity were systematically investigated. The impact of laser beam waist and atomic collisions on frequency shifts was analyzed. Results show that replacing a tightly focused beam with a large collimated one reduced the laser power-induced frequency shift by two orders of magnitude. Furthermore, increasing the vapor temperature enhances collision-induced frequency shifts, adversely affecting long-term stability. Under optimized conditions, the system achieved a short-term fractional frequency stability of 2.6 × 10 −13 at 1 s, reaching 5.3 × 10 −15 at 10 000 s. This work lays a critical technical foundation for the development of compact and high-performance OFSs.