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Optics & Laser Technology

A tunable dual-frequency rubidium Faraday laser

Xiaomin Qin, Zijie Liu, Zhiyang Wang, Hangbo Shi, Xiaolei Guan, Suyang Wei, Jia Zhang, Yu Wang, Tiantian Shi, Jingbiao Chen

Published Jun 26, 2025• 4 citations
Publisher

Abstract

Dual-frequency (DF) lasers with specific frequency intervals have crucial applications in geometric measurement, optical microwave generation, quantum precision measurement, etc. Here, we report a 780 nm DF rubidium Faraday laser, with two modes’ frequencies corresponding to the 85 Rb and 87 Rb D 2 transitions. The microwave signal generated from the two laser modes’ beat has a linewidth of 671 Hz and is tunable from 4.7–4.95 GHz to 5.94–6.7 GHz by adjusting the diode current. Moreover, we use modulation transfer spectroscopy to lock one of the laser modes to the 87 Rb hyperfine transition, and the frequency stability improves nearly an order of magnitude. The noise characteristics are also measured, where the phase noise is suppressed over 25 dB at 100 Hz offset compared to the microwave signal generated by two single-mode lasers’ heterodyne beat. This 780 nm DF rubidium Faraday laser has two coherent modes with frequencies related to Rb transitions, which provides a promising and convenient method for compact atomic physics applications, such as coherent population trapping clocks, atom gravimeters, and so on.

DOI
10.1016/j.optlastec.2025.113431

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Dual-frequency (DF) lasers with specific frequency intervals have crucial applications in geometric measurement, optical microwave generation, quantum precision measurement, etc. Here, we report a 780 nm DF rubidium Faraday laser, with two modes’ frequencies corresponding to the 85 Rb and 87 Rb D 2 transitions. The microwave signal generated from the two laser modes’ beat has a linewidth of 671 Hz and is tunable from 4.7–4.95 GHz to 5.94–6.7 GHz by adjusting the diode current. Moreover, we use modulation transfer spectroscopy to lock one of the laser modes to the 87 Rb hyperfine transition, and the frequency stability improves nearly an order of magnitude. The noise characteristics are also measured, where the phase noise is suppressed over 25 dB at 100 Hz offset compared to the microwave signal generated by two single-mode lasers’ heterodyne beat. This 780 nm DF rubidium Faraday laser has two coherent modes with frequencies related to Rb transitions, which provides a promising and convenient method for compact atomic physics applications, such as coherent population trapping clocks, atom gravimeters, and so on.
We demonstrate a dual-frequency (DF) Faraday laser operating on 780 nm using an87Rb Faraday anomalous dispersion optical filter (FADOF) as the frequency selection element. The frequencies of the two modes correspond to the transition of87Rb 52S1/2(F=1) to 52P3/2and 52S1/2(F=2) to 52P3/2, respectively. The output laser wavelength has good robustness to the laser diode’s current and temperature fluctuations. The frequency difference between the two modes is tunable with the temperature of87Rb vapor cell and cavity length. Besides, the two modes share good coherence with the most probable linewidth of the beat-note spectra being 3 kHz. Such a 780 nm DF Faraday laser can be used in precision measurement, such as, laser frequency stabilization, optical microwave generation, and coherent population trapping (CPT) atomic clocks.
The Faraday laser, with Faraday anomalous dispersion optical filter (FADOF) as frequency-selection element, is a natural narrow-bandwidth light source for laser physic experiments. In this work, a dual-frequency (DF) Faraday laser is demonstrated for the first time on Cs D2 line at 852 nm. The frequencies of the two modes of DF Faraday laser hinge on the peak transition frequencies of 852 nm FADOF transmittance spectrum corresponding to the ground state F=4 and F=3. The frequency difference between the two modes is tunable over a range of 1.4 GHz with the temperature of Cs vapor cell inside the FADOF. The linewidth of each laser mode is less than 33 kHz. Furthermore, the most probable linewidth of the beat-note spectra between the two modes is 902.95 Hz at different vapor-cell temperatures. Such a DF Faraday laser has many potential applications in atomic physics, such as sub-Doppler spectroscopy and coherent population trapping atomic clocks.
A laser with a switchable wavelength between 770 nm and 767 nm, corresponding to the potassium D1 and D2 lines, is a promising tool for manipulating potassium atoms. Here, we develop a potassium Faraday laser using a potassium atom-based Faraday anomalous dispersion optical filter (FADOF) as its frequency selection element. By dynamically adjusting the central frequency of the peak transmission across the entire transmitted spectrum of the potassium FADOF, the laser can operate either in single mode at either 770 nm or 767 nm, or emit both simultaneously. In single-mode operation, the laser wavelength remains aligned with the atomic transitions, exhibiting a narrow linewidth of 14.96 kHz at 770 nm and 13.07 kHz at 767 nm. In dual-mode operation, both modes are sustained continuously, with frequency drifts of less than 0.3 GHz for 200 minutes. Furthermore, we perform modulation transfer spectroscopy (MTS) and stabilize the laser to the 39 K 4S 1/2 F = (1, 2) → 4P 1/2 F ′ = 2 and 4S 1/2 F = (1, 2) → 4P 3/2 F ′ transitions. The corresponding frequency stability, measured by the optical heterodyne beat note between two identical systems, is 1.57 × 10 −12 and 8.77 × 10 −13 at 1 s, respectively. This architecture introduces a novel approach to the generation of wide-range wavelength switching within a single laser. In addition, with its narrow linewidth and impressive frequency stability, it serves as a reliable source for ultracold atom physics and quantum precision measurement.
In the development of atomic physics, laser sources with Frequencies corresponding to atomic transition and high stability are essential. The Faraday laser is a special diode laser using the Faraday anomalous dispersion optical filter (FADOF) to realize frequency selection, so the output laser frequency is automatically limited to the atomic Doppler broadening. However, the frequency of a Faraday laser corresponds to the range around only one atomic hyperfine transition. Here, we realize a switchable Faraday laser with two isotopes laser frequencies corresponding to 85Rb 52S1/2 (F=3)→52P3/2 and 87Rb 52S1/2 (F=2)→52P3/2 transitions based on a single isotope 87Rb-FADOF. The laser has good robustness against the fluctuation of diode current and temperature, with wavelength fluctuating within 0.8 pm from 16 to 30 °C of diode temperature, and has a free-running linewidth of 18 kHz. We also lock the laser frequency to the two cycling transitions of 85Rb 52S1/2 (F=3)→52P3/2 (F′=4) and 87Rb 52S1/2 (F=2)→52P3/2 (F′=3) by the modulation transfer spectroscopy technique. The Allan deviation of the residual error signal is 3×10−14/τ, and the frequency stability of the beat detection reaches 2.8×10−12 at 1 s integration time. This 780 nm switchable Faraday laser expands the application scenarios of Faraday lasers, which can be used in laser cooling atoms, optical frequency standards, and other quantum precision measurement fields.
The dual-frequency Faraday laser, utilizing the two-peak transmission of a Faraday anomalous dispersion optical filter (FADOF), can achieve simultaneous lasing on two modes both determined by the atomic transition. The beating frequency between the two modes is close to the frequency interval between hyperfine energy levels of the atomic ground state. Since the frequency of the beat signal and output laser modes all depend on the cavity length, locking one of them can also improve the stability of the others. Therefore, by locking the beat frequency to a stable microwave frequency reference, the stability of the reference can be transferred to the laser frequencies; Similarly, locking one of the output laser frequencies will also transfer its stability to the beat frequency in the microwave regime. This can serve as a means to bridge the gap between the optical and microwave frequency regimes, and can have wide applications, such as absolute frequency measurement of a laser, or photonic microwave generation.
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.