Relative-phase ambiguities in measurements of ultrashort pulses with well-separated multiple frequency components

Ultrashort-pulse characterization techniques, such as the numerous variants of frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric-field reconstruction, fail to fully determine the relative phases of well-separated frequency components. If well-separated frequency components are also well separated in time, the cross-correlation variants (e.g., XFROG) succeed, but only if short, wellcharacterized gate pulses are used