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Applied Optics and Photonics China (AOPC)

Characteristics of wavelength standard system based on two-photon transition in 87Rb atoms

Peixuan Song, Yali Liu, Wei Li

Published Oct 28, 2025• 0 citations
Publisher

Abstract

Optical wavelength standards, based on the optical transitions of atoms or molecules, provide ultra-high precision frequency references and are the core technology of next-generation optical clocks. Their stability far exceeds that of the microwave frequency range, laying the foundation for precision metrology and cutting-edge scientific research. The wavelength standard device based on the 5S1/2-5D3/2 hyperfine level two-photon transition of 87Rb atoms exhibits significant advantages such as no Doppler broadening, high signal-to-noise ratio (SNR), and potential for miniaturization and chip integration, showing important application potential in precision measurement fields like rubidium atomic clocks and optical communication. In this paper, an experimental setup for two-photon transition was constructed using an external cavity diode laser (ECDL), achieving stable locking of the 778 nm wavelength by modulating the laser current. The effects of the temperature and length of the atomic vapor cell on the blue fluorescence linewidth, SNR, and frequency stability were systematically studied, further optimizing the system. By using an atomic vapor cell designed at Brewster's angle, optical reflection loss was effectively reduced, compensating for the impact of shortened optical path on frequency stability in miniaturized atomic vapor cells. This provides new research directions and performance enhancement solutions for the miniaturization and chip integration of two-photon transition systems. In the improved system, a Brewster-angle glass vapor cell with a length of 30 mm was used and heated to 110°C, achieving a linewidth of 1.69 MHz full width at half maximum (FWHM) and a SNR of 20 dB. The system demonstrated a instability of 9.86×10-11 over a 1-second averaging time and a minimum instability of 5.79×10-11 over a 5.7-second averaging time. This achievement provides key technological support for the practical application of the next generation of miniaturized optical atomic clocks, with the potential for widespread use in portable metrology equipment and space science through integrated design and noise suppression optimization in the future.

DOI
10.1117/12.3083733

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Optical wavelength standards, based on the optical transitions of atoms or molecules, provide ultra-high precision frequency references and are the core technology of next-generation optical clocks. Their stability far exceeds that of the microwave frequency range, laying the foundation for precision metrology and cutting-edge scientific research. The wavelength standard device based on the 5S1/2-5D3/2 hyperfine level two-photon transition of 87Rb atoms exhibits significant advantages such as no Doppler broadening, high signal-to-noise ratio (SNR), and potential for miniaturization and chip integration, showing important application potential in precision measurement fields like rubidium atomic clocks and optical communication. In this paper, an experimental setup for two-photon transition was constructed using an external cavity diode laser (ECDL), achieving stable locking of the 778 nm wavelength by modulating the laser current. The effects of the temperature and length of the atomic vapor cell on the blue fluorescence linewidth, SNR, and frequency stability were systematically studied, further optimizing the system. By using an atomic vapor cell designed at Brewster's angle, optical reflection loss was effectively reduced, compensating for the impact of shortened optical path on frequency stability in miniaturized atomic vapor cells. This provides new research directions and performance enhancement solutions for the miniaturization and chip integration of two-photon transition systems. In the improved system, a Brewster-angle glass vapor cell with a length of 30 mm was used and heated to 110°C, achieving a linewidth of 1.69 MHz full width at half maximum (FWHM) and a SNR of 20 dB. The system demonstrated a instability of 9.86×10-11 over a 1-second averaging time and a minimum instability of 5.79×10-11 over a 5.7-second averaging time. This achievement provides key technological support for the practical application of the next generation of miniaturized optical atomic clocks, with the potential for widespread use in portable metrology equipment and space science through integrated design and noise suppression optimization in the future.
Optical frequency standards, or lasers stabilized to atomic or molecular transitions, are widely used in length metrology and laser ranging, provide a backbone for optical communications and lie at the heart of next-generation optical atomic clocks. Here we demonstrate a compact, low-power optical frequency reference based on the Doppler-free, two-photon transition in rubidium-87 at 778 nm implemented on a micro-optics breadboard. Our optical reference achieves a fractional frequency instability of 2.9×10−12/ τ for averaging times τ less than 103 s, has a volume of ≈35 cm3 and operates on ≈450 mW of electrical power. The advanced optical integration presented here demonstrates a key step towards the development of compact optical clocks and the broad dissemination of SI-traceable wavelength references.
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.
High precision portable and deployable frequency standards are required for modern navigation and communication technologies. Optical frequency standards are attractive for their improved stability over their microwave counterparts; however, increased complexities have anchored them in the laboratory. Sacrificing sensitivity of the most stable optical clocks has led to the recent development of deployable and portable optical frequency standards, leveraging hot atomic or molecular vapor. The short term limit for a majority of previous reports on two-photon rubidium standards is either the shot-noise or intermodulation limit hindering the one second fractional frequency stability to around 1×10−13/τ1\times10^{-13}/\sqrtτ1×10−13/τ​. The answer for the shot-noise limit is to increase optical power and collected fluorescence, while the intermodulation limit solution requires improvements in laser linewidth, stimulated Brillouin scattering (SBS) lasers are known to reduce frequency noise, suppressing noise of the pump laser at high offset frequencies. We investigate an optical frequency standard based on the two-photon transition in 87^{87}87Rb probed with a narrow linewidth photonic integrated circuit SBS laser with a quality factor over 130 million and instantaneous linewidth <<< 10 Hz. The use of a narrow linewidth clock laser coupled with operating at higher optical intensities yields clock instabilities of 2×10−142\times10^{-14}2×10−14 at one second, currently the best reported short-term stability for a two-photon rubidium optical frequency standard.
High-precision, portable, and deployable frequency standards are required for modern navigation and communication technologies. Optical frequency standards are attractive for their improved stability over their microwave counterparts; however, increased complexities have anchored them in the laboratory. Sacrificing the sensitivity of the most stable optical clocks has led to the recent development of deployable and portable optical frequency standards, leveraging hot atomic or molecular vapor. The short-term limit for a majority of previous reports on two-photon rubidium standards is either the shot-noise or intermodulation limit hindering the 1 s fractional frequency stability to around 1 × 10 − 13 . The answer for the shot-noise limit is to increase optical power and collected fluorescence, while the intermodulation limit solution requires improvements in laser linewidth, stimulated Brillouin scattering (SBS) lasers are known to reduce frequency noise, suppressing noise of the pump laser. We investigate an optical frequency standard based on the two-photon transition in Rb 87 probed with a narrow photonic integrated circuit SBS laser with a quality factor over 130 million and instantaneous linewidth < 10 Hz . The use of a narrow clock laser coupled with operating at higher optical intensities yielded clock instabilities of 2 × 10 − 14 at 1 s, currently the best reported short-term stability for a two-photon rubidium optical frequency standard.
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.
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.