International Comparisons of Transportable Femtosecond Laser Frequency Combs

Two types of comparisons using transportable femtosecond laser combs have been performed in France and the USA, respectively. The agreement among the combs, when referenced to a hydrogen maser, is on the sub-hertz level at 563 THz. When the combs are referenced to an optical standard, the agreement among four combs was much improved and found to be < 1 mHz at 456 THz