Abstract

Frequency combs based on mode-locked femtosecond lasers allow measurements of optical transition frequencies with a relative accuracy below 1 part in 10¹⁵. Such measurements are ultimately limited by the frequency instability and uncertainty of the microwave reference realizing the Si-second, such as a caesium fountain clock. As optical frequency standards (OFS) outperform their microwave counterparts with respect to short- term stability and uncertainty, future measurements are likely to be direct comparisons between different optical frequency standards located in different laboratories, and often different countries.

© 2007 IEEE