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Journal of Physics B Atomic Molecular and Optical Physics

Beam geometry and temperature effects on the frequency stability of a 8 7 Rb two-photon optical frequency standard

Jiongyang Zhang, Yuhua Xiao, Ji Wang, Wenxi Zhang, Hu Dai, Yanying Cheng, Jiqing Lian, Jinhan Zhang, Wei Yang, Detian Li, Hao Zhai

Published May 5, 2026• 1 citation
Publisher

Abstract

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.

DOI
10.1088/1361-6455/ae6161

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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.
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.
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.
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 3.5×10−13τ−1/2 until 200 s, in good agreement with a phase noise level of +43dBrad2/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. Retaining 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.
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.
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.
Two-photon vapor cell-based optical clocks are strong candidates for next-generation portable atomic standards, offering simplicity, compactness, and high performance. Their narrow clock transitions with counter-propagating beams enable first-order Doppler-free operation. However, systematic perturbations such as the AC Stark shift, temperature-induced shift, and drifts resulting from the laser system pose challenges cause instabilities to medium- to long-term performance. This paper provides a comprehensive overview of Rb two-photon vapor cell optical standards, focusing on the long-term performance-limiting effects and potential mitigation strategies, aiming for clock stabilities better than 1 × 10−15 over the averaging time of a day and beyond.