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Applied Physics Letters

A 780 nm optical frequency standard based on diffuse laser cooled 87Rb atoms

Xiaolei Guan, Jia Zhang, Xun Gao, Yu Wang, Tiantian Shi, Jingbiao Chen

Published Jan 20, 2025• 12 citations
Publisher

Abstract

We demonstrate an optical frequency standard by locking the output frequency of an external cavity diode laser to the 780 nm D2 transition of diffuse laser cooled 87Rb atoms using frequency modulation spectroscopy (FMS). A 50-cm-long cold atomic cloud is obtained within a ϕ 2 cm ×l 50 cm vacuum glass tube, featuring greater simplicity and reliability. By analyzing the impact of power and detuning of cooling and repumping lasers, and power of frequency-modulated laser on FMS based on cold atoms, we choose the optimal parameter and lock the laser frequency to the 87Rb 52S1/2 (F = 2) → 52P3/2 (F′ = 3) cycling transition. The cold-atom-based optical frequency standard achieves an in-loop frequency stability of 3.3 × 10−15 at 1 s, with peak-to-peak frequency fluctuations being less than 50 Hz within 6000 s. This represents the integration of cold atoms into an optical frequency standard utilizing FMS for frequency stabilization. In contrast to the common cold-atom optical frequency standards, our approach requires fewer lasers and eliminates the need for a complex and expensive Pound–Drever–Hall (PDH) system. Its low complexity makes it a promising candidate for development into a space cold-atom clock, promoting advancements in space science and navigational positioning. Additionally, it possesses great potential to serve as an absolute wavelength standard.

DOI
10.1063/5.0250471

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We demonstrate an optical frequency standard by locking the output frequency of an external cavity diode laser to the 780 nm D2 transition of diffuse laser cooled 87Rb atoms using frequency modulation spectroscopy (FMS). A 50-cm-long cold atomic cloud is obtained within a ϕ 2 cm ×l 50 cm vacuum glass tube, featuring greater simplicity and reliability. By analyzing the impact of power and detuning of cooling and repumping lasers, and power of frequency-modulated laser on FMS based on cold atoms, we choose the optimal parameter and lock the laser frequency to the 87Rb 52S1/2 (F = 2) → 52P3/2 (F′ = 3) cycling transition. The cold-atom-based optical frequency standard achieves an in-loop frequency stability of 3.3 × 10−15 at 1 s, with peak-to-peak frequency fluctuations being less than 50 Hz within 6000 s. This represents the integration of cold atoms into an optical frequency standard utilizing FMS for frequency stabilization. In contrast to the common cold-atom optical frequency standards, our approach requires fewer lasers and eliminates the need for a complex and expensive Pound–Drever–Hall (PDH) system. Its low complexity makes it a promising candidate for development into a space cold-atom clock, promoting advancements in space science and navigational positioning. Additionally, it possesses great potential to serve as an absolute wavelength standard.
We proposed a scheme for realizing active Faraday optical frequency standards using diffuse laser cooling of alkali-metal atoms. A 1-m-long cold87Rb atomic cloud was prepared within a ϕ 2 cm ×$l$1 m vacuum glass tube, featuring a maximum optical density of 4. By utilizing this setup to construct the Faraday anomalous dispersion optical filter, we are able to achieve an ultra-narrowband laser gain close to the natural linewidth of the atomic transition and corresponding to the hyperfine transition frequency. This ultra-narrowband laser gain can directly determine the output frequency of the active Faraday optical frequency standard operating in the “bad cavity” regime, significantly suppressing the cavity-pulling effect. Furthermore, due to the suppression of atomic collision and Doppler frequency shifts by the cold atoms, it is expected to further narrow the laser linewidth of the cold-atom-based active Faraday optical frequency standard to Hz level.
We demonstrate a straightforward frequency-locking technique that employs external modulation saturation absorption spectroscopy to achieve a high-performance 780 nm laser system. By externally modulating the laser frequency through a fiber electro-optic modulator, this scheme can stabilize multiple transitions of the 87Rb D2 line with excellent performance. Experimental results establish fractional frequency instability of 2.21 × 10-13 at 1 s with a linewidth of 1.46 kHz for the cycling transition 5S1/2(Fg = 2) → 5P3/2(Fe = 3). Notably, we have locked the system to the repumping transition 5S1/2(Fg = 1) → 5P3/2(Fe = 2), achieving a record-low instability of 1.98 × 10-12 at 1 s averaging time. This simplified laser system, featuring exceptional short-term frequency stability and multi-frequency locking capability, serves as a critical subsystem for the compact cold-atom optical clock, enabling advanced precision metrology applications. Furthermore, its metrological performance enables immediate applications in other quantum sensors, such as Rydberg electrometry, atomic magnetometry, and matter-wave interferometry.
We make a detailed comparison of frequency modulation spectroscopy (FMS) and modulation transfer spectroscopy (MTS) and of their applications in stabilizing laser frequency. In both FMS and MTS, intensity modulations of probe beam are observed in saturated absorption spectroscopy of 87Rb atoms. However, the two modulation profiles appear different due to their distinct origins. Demodulation gives rise to frequency discrimination spectra, which can serve as error signals for frequency locking. The optimal error signal of FMS has a larger linear slope but also a slanted baseline, in contrast to that of MTS. We apply FMS and MTS to stabilize two 780 nm external cavity diode lasers to D2 lines of 87Rb atoms. We measure laser linewidth and stability using beat notes of two independently locked lasers. A laser linewidth as narrow as 12.9 kHz is achieved in FMS locking, compared to 70.8 kHz in MTS locking. A laser stability at the level of 10−12 is reached in both schemes. Nevertheless, FMS locking offers a better short-term stability, whereas MTS locking yields a smaller long-term drift. Our investigation of FMS and MTS would be valuable for making a choice of these spectroscopy techniques for laser stabilization as well as other relevant applications.
The free-fall atomic clock has shown considerable potential for long-term stability and mobility, yet reports on its design and implementation remain limited. Herein, we introduce a simple integrated optical system for free-fall cold Rb atomic clocks. A distributed feedback (DFB), frequency-doubled fiber laser, and a 780 nm DFB laser diode are utilized as the cooling and repumping light source, respectively. Both lasers feature a broad mode-hop-free tuning range and reduced sensitivity to acoustic noise. Benefiting from a high signal-to-noise ratio of modulation transfer spectroscopy (MTS), the linewidth of the fiber laser is measured to be 42 kHz. The power and frequency management of the cooling beams is implemented by two 110-MHz acousto-optic modulators (AOMs), enabling a frequency detuning as large as −92 MHz. The power-frequency management module is integrated into a single side bench of 316 mm × 270 mm × 52 mm. The power fluctuation of the probe beam is measured to be 0.8% (RMS) over 24 hours measurement. Using an ultra-high vacuum system, the functionality of the optical system is preliminarily verified by the time-of-flight (TOF) signal of the cold atom cloud.
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.