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 \vert F=4, m_F=0\rangle 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 \vert F=4\rangle \to \vert F'=4\rangle transition and the other is resonant with \vert F=3\rangle \to \vert F'=4\rangle transition. With optical pumping, the population accumulated in the \vert m_F=0\rangle 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.