We achieved a low-cost and small-sized Rb optical frequency standard based on 85Rb 5S1/2 → 6P3/2 transition with 10−15 stability, which is comparable with that of the best 532 nm I2 optical frequency standards. In this system, we directly lock the 420 nm diode laser on the 5S1/2 F = 3 → 6P3/2 F′ = 4 hyperfine transition line without an additional Pound-Drever-Hall pre-locking system. The signal-to-noise-ratio reaches as high as 350 000 when the averaging time is at 1 s. Eventually by the fluctuation of the residual error signal after locking, the preliminary stability of the optical frequency standard reaches 1.2×10−14/τ, decreasing to 2.1 × 10−15 at 80 s. It shows potential in stability performance, experimental cost, and system volume compared with the 532 nm I2 optical frequency standard as a wavelength standard. It also opens a door for the achievement of wavelength standards by using higher excited states of alkalies.
We achieved a low-cost and small-sized Rb optical frequency standard based on 85Rb 5S1/2 → 6P3/2 transition with 10−15 stability, which is comparable with that of the best 532 nm I2 optical frequency standards. In this system, we directly lock the 420 nm diode laser on the 5S1/2 F = 3 → 6P3/2 F′ = 4 hyperfine transition line without an additional Pound-Drever-Hall pre-locking system. The signal-to-noise-ratio reaches as high as 350 000 when the averaging time is at 1 s. Eventually by the fluctuation of the residual error signal after locking, the preliminary stability of the optical frequency standard reaches 1.2×10−14/τ, decreasing to 2.1 × 10−15 at 80 s. It shows potential in stability performance, experimental cost, and system volume compared with the 532 nm I2 optical frequency standard as a wavelength standard. It also opens a door for the achievement of wavelength standards by using higher excited states of alkalies.
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.
Abstract We achieve a compact ultra-stable 420 nm blue diode laser system by immediately stabilizing the laser on the hyperfine transition line of Rb atom. The Allan deviation of the residual error signal reaches 1 Hz-level Allan deviation within 1 s averaging time, and the fractional frequency Allan deviation is 1.4×10−15/τ 1.4 × 10 - 15 / τ , which shows the best result of frequency-stabilized lasers based on the atomic spectroscopy without Pound–Drever–Hall (PDH) system. The signal-to-noise ratio of the atomic spectroscopy is evaluated to be 3,000,000 from the Allan deviation formula, which is the highest record, to the best of our knowledge. The frequency noise suppression characterization is demonstrated and the maximal noise suppression can be near 40 dB at 6 Hz. As a good candidate of pumping source, the ultra-stable 420 nm diode laser is successfully used in our Rb four-level active optical frequency standard system. The method can be easily extended to other wavelengths ultra-stable lasers with a Allan deviation of 10−15 10 - 15 level retaining an atomic reference with low cost and low complexity while in the absence of an expensive and complicated PDH system.
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 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.
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.
We describe a high-performance, compact optical frequency standard based on a microfabricated Rb vapor cell and a low-noise, external cavity diode laser operating on the Rb two-photon transition at 778 nm. The optical standard achieves an instability of 1.8 × 10 − 13 τ − 1 / 2 for times less than 100 s and a flicker noise floor of 1 × 10 − 14 out to 6000 s. At long integration times, the instability is limited by variations in optical probe power and the ac Stark shift. The retrace was measured to 5.7 × 10 − 13 after 30 h of dormancy. Such a simple, yet high-performance optical standard could be suitable as an accurate realization of the meter or, if coupled with an optical frequency comb, as a compact atomic clock comparable to a hydrogen maser.