It is shown that a pair of perpendicular optical diffraction gratings
leads to a time delay which grows strictly linearly as a function of f
requency. Such a device associated with a simple nonuniform diffractio
n grating allows the realization of lasers with very powerful short pu
lses following the principle of amplification of long optical pulses l
inearly frequency modulated. The compression and expansion factors wit
hin reach are of the order of 10(4) to 10(5).