Reevaluation of the Optically Pumped Cesium Frequency Standard NRLM-4 With an H-Bend Ring Cavity

The largest source of uncertainty of the optically pumped cesium (Cs) frequency standard (NRLM-4), i.e., the distributed cavity phase shift, was reduced by replacing the microwave cavity with a new ring-type one. An H-bend-type ring cavity is adopted for the first time. Due to the novel cavity and the improvement of the Cs beam alignment, the distributed cavity phase shift was significantly reduced from 2 x 10^-14 to 1.4 x 10^-15. The frequency stability was measured to be 8 x 10^-13 tau^-1/2 . The total uncertainty was reevaluated to be 6.7 x 10^-15, which is four times better than the previous value.