Absolute frequency measurements with a set of transportable methane optical frequency standards

For the first time absolute frequency measurements (AFM) referenced to Cs primary standards with a set of 3 double-mode He-Ne/CH/sub 4/ transportable optical frequency standards (TOFS, /spl lambda/=3.39 /spl mu/m, /spl nu/=88 THz) were carried out. The radio-optical frequency chain (ROFC) and the 29-THz CO/sub 2//OsO/sub 4/ secondary frequency standard of BNM-LPTF (France) and the ROFC of PTB were involved in the experiment. The level of accuracy achieved for these cross-measurements is better than 5*10/sup -13/.