Absolute frequency measurements of 633 nm iodine-stabilized helium neon lasers

The frequency of a helium–neon laser stabilized to the hyperfine component f (alternatively denoted a 16 ) of the 127 I 2 11-5 R(127) line at 633 nm has been measured with respect to the SI second using a femtosecond optical frequency comb generator based on a mode-locked Ti : sapphire laser and microstructure fibre. The standard uncertainty of this measurement is 0.6 kHz. The same laser was taken to BIPM mid-way through the measurements and its frequency compared to that of the BIPM's continuously maintained iodine-stabilized helium–neon laser BIPM-4. From this comparison, the frequency of BIPM-4 when stabilized to the same iodine hyperfine component f is 473 612 353 608.1 (0.7) kHz.