Optical clocks based on stimulated emission radiation

Atomic clocks operating at microwave region, are di-vided into two categories according to the mode of ope- ration, namely the passive mode, in which the standard frequency of a clock signal is locked on to a very narrow atomic spectral line such as in cesium beam and rubi- dium gas cell clocks, and the active mode, where the atomic ensemble itself can produce a high stable and accurate standard frequency signal such as hydrogen maser. The hydrogen maser has been the most stable atomic clock for nearly half a century, and has been applied in a variety of scientific research fields^1].