For solving the problem which puzzled us for a decade, we construct a high-vaccuum chamber to investigate the influences of the collision between cesium and helium atoms. By virtue of the well-established 822 nm two-photon-transition spectroscopy system, we concluded the relationship of the pressure shift and pressure broadening, and proved that atmospheric helium plays an important role in the inconsistency of the accuracy between secondary clocks.

Due to the inevitable aging and the inconsistent quantum efficiency of the powersensitive elements, laser power detectors can not serve as absolute stardards. Here we propose that power measurement can be referenced to atomic spectrum. By precise measuring AC-Stark effect in cesium two-photon absorption system, laser intensity change can be treat as a digitized value via transition frequency shifts. In the mean time, we contruct a high-finesse cavity which can define the beamshape well inside. Thus, we can calculate the atom-photon interaction range to obtain the exact power.