Optical frequency standards

Alan A. Madej, J. E. Bernard

Duo Pan, Xiaobo Xue, Xiaogang Zhang +1 more

Active optical frequency standard, or active optical clock, is a new concept of optical frequency standard, where a weak feedback with phase coherence information in optical bad-cavity limitation is formed, and the continuous self-sustained coherent stimulated emission between two atomic transition levels with population inversion is realized. Through ten years of both theoretical and experimental exploration, the narrow linewidth and suppression of cavity pulling effect of active optical frequency standard have been initially proved. In this paper, after a simple review, we will mainly present the most recent experimental progresses of active optical frequency standards in Peking University, including 4-level cesium active optical frequency standards and active Faraday optical frequency standards. The future development of active optical frequency standards is also discussed.

Mizuhiko Hosokawa

Dwayne Knight, K. I. Pharaoh, S.L.T. Drysdale +3 more

Benjamin Parker, Stephen A. Webster, S.N. Lea +2 more

Comparison of microwave frequency standards by satellite techniques is currently limited in instability at the 10-15level at one day [1]. However, the best optical frequency standards have already demonstrated an instability of ~ 5 × 10-17at 2000 s and an estimated uncertainty of 9 × 10-18[2]. The predicted improvement in optical frequency standards motivates the need for a new transfer method capable of performing below these predicted levels of instability. The transmission of cw optical frequencies through optical fibre networks is the most promising alternative to satellite methods when it comes to comparing remote optical frequency standards. Intra-lab comparison of optical frequency standards can already be carried out with the use of femtosecond optical frequency combs, which can easily bridge the frequency gap between the different standards under test. However, a particularly more challenging task is to compare different standards at remote European laboratories. The comparison of remote standards, particularly like-for-like systems (such as the Yb+clocks at NPL and PTB), will be important in determining the relative uncertainty and reproducibility of these clocks. For this remote comparison method to operate below the instability level of the optical standards, a highly stable phase-coherent local oscillator is needed so as not to degrade the stability of the atomic reference being compared.

Peiliang Liu, Yao Huang, Bian Wu +4 more

Frequency comparison is one of the most efficient ways to evaluate the performance of a frequency standard. Based on the pre-existing 40Ca+ optical frequency standard, we set up the second 40Ca+ optical frequency standard, which has been improved in the materials and structure of ion traps for better control of the magnetic field. After the compensation, the residual magnetic field at the position of the ion is adjusted to be ∼500 nT with a long time jitter of ∼10nT, which is better than the pre-existing 40Ca+ optical frequency standard. We realize the '4-point-closed-loop locking' on the second 40Ca+ optical frequency standard after a series of preparatory works. Through half an hour of measurement time, the two frequency standards exhibited a stability of 2.1 × 10−13 τ −1/2 and a relative frequency difference of 1.5 (2.9) Hz.