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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Frequency stabilization of 423 nm laser for calcium-beam optical frequency standard

Shengnan Zhang, Xiaogang Zhang, Haosen Shang, Jingbiao Chen, Haijun Chen

Published Jul 1, 2017• 1 citation
Publisher

Abstract

The Ca beam optical frequency standard has the potential to be small optical clock like small Cs beam clock in microwave regime. For improving the signal-to-noise ratio of the clock transition spectroscopy, the electron shelving detection, which uses the fast1S0-1P1423nm transition to detect the narrow1S0-3P1657nm clock transition, was applied for Ca beam optical frequency standard. Therefore the frequency stability of 423 nm laser to some extent determines the performance of the optical frequency standard. In this paper, we stabilize the 423 nm laser frequency to Ca beam signal with side-band locking technique and estimate the preliminary stability of 423 nm laser which is 1.4×10-11at 1 s and 2×10-12at 100 s.

DOI
10.1109/fcs.2017.8089026

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The Ca beam optical frequency standard has the potential to be small optical clock like small Cs beam clock in microwave regime. For improving the signal-to-noise ratio of the clock transition spectroscopy, the electron shelving detection, which uses the fast1S0-1P1423nm transition to detect the narrow1S0-3P1657nm clock transition, was applied for Ca beam optical frequency standard. Therefore the frequency stability of 423 nm laser to some extent determines the performance of the optical frequency standard. In this paper, we stabilize the 423 nm laser frequency to Ca beam signal with side-band locking technique and estimate the preliminary stability of 423 nm laser which is 1.4×10-11at 1 s and 2×10-12at 100 s.
The narrow linewidth of1S0-3P1657 nm clock transition makes it a promising quantum frequency reference. In this experiment, the fast1S0-1P1423 nm transition is used to detect the narrow1S0-3P1657 nm clock transition. Therefore, the frequency stability of the 423 nm laser determines the performance of the optical frequency standard. Here we investigate the frequency shift properties of the neutral calcium atoms'1S0-1P1transition at 423 nm. We measure the frequency shift caused by temperature, laser power, and laser beam pointing.
The optical frequency transitions with long-lifetime spin triplet states produced by the two valence electrons in the outermost layer of alkaline earth atoms make calcium atom a promising quantum frequency reference. Electron-shelving detection improves the signal-to-noise ratio of the clock transition spectrum, enabling a calcium atomic beam optical frequency standard. This scheme employs the 1S0-1P1 transition at 423 nm to detect the 1S0-3P1 transition at 657 nm, emphasizing the importance of 423 nm laser stability. While the performance of these clocks is well-studied, research on the atomic beam itself is limited. This study investigates frequency shifts in neutral calcium atoms’ 1S0-1P1 transition at 423 nm and their 1S0-3P1 transition at 657 nm. We measure the frequency shift caused by temperature, laser power, and laser alignment, providing insights to optimize the design of the calcium beam’s physical apparatus.
In order to develop small optical clock for various transportable applications with better stability and accuracy than that of commercial small Cs clock and H-maser clock, our group has proposed a small Ca atomic beam clock scheme with electron-shelving detection. Its potential stability was experimentally demonstrated recently. Here, we first demonstrated the dispersive detection of 657 nm transition via the Faraday rotation effect of 423 nm transition based on a thermal Ca atomic beam, this alternative detection method can effectively detect the clock signal by the ground state atoms after the clock laser interrogation. The Faraday anomalous dispersion optical filter of 423 nm transition can reach a maximum transmission of 10%. With a magnetic field of 84 G, the observed 3.3% of the ground state atoms has been excited to the σ±level of metastable3P1(m=±1) state, which are separated 176.6 MHz to m=0 level.
In this paper a small Calcium beam optical frequency standard utilizing fully-sealed Calcium beam vacuum tube owning smaller size compared with vacuum system with flanges is demonstrated. Adopting the electron shelving detection method, we detect the clock transition 657 nm saturation absorbing spectroscopy whose linewidth is 500 kHz. The instability of this small optical frequency standard is 6.8×10-14at 1 second and 1.8×10-15after 1000 seconds of averaging by comparing with another small Calcium beam optical frequency standard whose reported instability is 3.0×10-14at 1 second. The optical part of the whole system is specially designed for robustness and miniaturization.
Research on the development of the optical frequency standard based on trapped and cold 40 Ca + with the 4s 2 S 1/2 -3d 2 D 5/2 clock transition at 729 nm is reported. A single calcium ion was trapped and laser cooled in the Paul trap and stay in trap for more than 15 days. The linewidth of a 729 nm laser was reduced to less than 10 Hz by locking to a cavity for longer than 50 hours uninterruptedly. The overall systematic uncertainty of the clock transition has been characterized to be better than 6.5×10 −16 . The absolute frequency of the clock transition was measured at 10 −15 level using an optical frequency comb referenced to a Hydrogen maser, which was calibrated to the SI second through the global positioning system (GPS). The frequency value was 411042129776393.0(1.6) Hz after the correction of the systematic shifts.