Improvement of the short-term stability of atomic fountain clock with state preparation by two-laser optical pumping*

To improve the signal to noise ratio (SNR) and the short-term stability of cesium atomic fountain clocks, the work of two-laser optical pumping is presented theoretically and experimentally. The short-term stability of the NIM6 fountain clock has been improved by preparing more cold atoms in the | F = 4, m(F) = 0 clock state with a shortened cycle time. Two pi-polarized laser beams overlapped in the horizontal plane have been applied after launching, one is resonant with | F = 4 -> | F ' = 4 transition and the other is resonant with | F = 3 -> | F ' = 4 transition. With optical pumping, the population accumulated in the | m(F) = 0 clock state is improved from 11% to 63%, and the detection signal is increased by a factor of 4.2, the SNR of the clock transition probability and the short-term stability are also improved accordingly.