Tautochronism principle and grating dispersive delay lines

Dispersion delay lines (DDLs) are used for dispersion compensation in ultrafast optics. Note that the correct determination of the time delay of DDLs is based on the tautochronism principle: in an optical system consisting of refracting and reflecting elements the time delay between any two wavefronts is the same for all rays. But for diffraction gratings this principle is not valid; the time delay between the wavefronts of incident and diffracted waves for different light rays is different. However, fortunately, a pair of diffraction gratings can be combined so that the input and the output wavefronts of the system still satisfy the tautochronism principle. There are only two such grating systems: Treacy's system and Martinez's system. Both these systems are closely related to each other: in Martinez's system one can see virtual Treacy's system. Note that according to the tautochronism principle, dispersion Martinez's system and virtual Treacy's system are equal in value but opposite in sign.