An ultrafast monochromator is a monochromator that preserves the duration of an ultrashort pulse (in the femtosecond, or lower, time-scale). Monochromators are devices that select for a particular wavelength, typically using a diffraction grating to disperse the light and a slit to select the desired wavelength; however, a diffraction grating introduces path delays that measurably lengthen the duration of an ultrashort pulse. An ultrafast monochromator uses a second diffraction grating to compensate time delays introduced to the pulse by the first grating and other dispersive optical elements.