The group delay of multilayer laser mirrors was measured by determinin
g the spectral position of transmission maxima of a Fabry-Perot interf
erometer formed by the mirrors to be characterized. By optimizing the
spacer thickness, we obtained an accuracy of the group-delay measureme
nt of better than 0.23 fs. To our knowledge, this is the most precise
direct measuring method reported. (C) 1995 Optical Society of America