Comparative characterization of the different methane-stabilized optical frequency standards and absolute frequency measurements

This paper reports on the results of comparative characterization and absolute frequency measurements of the UME and PTB He-Ne/CH 4 methane-stabilized optical frequency standards. The influence of different parameters that have to be kept under control in order to achieve frequency repeatability below 10 -13 was investigated. A method of synchronizing the frequency of a CH 4 laser and H maser was implemented to establish an absolute frequency scale for the He-Ne laser. The first direct frequency comparisons between the UME transportable optical frequency standard and PTB linear optical Ramsey frequency standard were carried out.