At the end of the 1990s the mode-locked femtosecond laser and its associated
optical frequency comb made an unanticipated entry into the field of precision
optical frequency metrology (Udem et al., 1999a, 1999b; Diddams et al., 2000;
Jones et al., 2000). It is difficult to overstate the impact of this new technology
on frequency metrology research (Hall, 2006; Hänsch, 2006). In simple
terms, the femtosecond laser frequency comb transformed optical frequency standards
into optical clocks by providing a straightforward means to count optical
cycles (Diddams et al., 2001; Ye et al., 2001). Just as important, a stabilized
frequency comb permits the easy comparison of optical standards separated
by hundreds of terahertz (Diddams et al., 2001; Stenger et al., 2002),
offers new tools for frequency transfer (Holman et al., 2004; Foreman et al.,
2007), introduces new approaches to length metrology (Ye, 2004; Minoshima
and Matsumoto, 2000), and provides the carrier–envelope control required for
phase-sensitive nonlinear optics and attosecond physics (Paulus et al., 2001;
Baltuska et al., 2003 ˘ ).