An optical pulse-width modifier structure includes a first diffraction grating and an optically unpowered reversing mirror. An optical path extends between the first diffraction grating and the optically unpowered reversing mirror. A second diffraction grating lies on the optical path between the first diffraction grating and the optically unpowered mirror. A set of optically powered mirrors lies on the optical path between the first diffraction grating and the second diffraction grating. The diffraction gratings and mirrors are positioned such that an input light beam is diffracted from the first diffraction grating, reflected from each of the set of optically powered mirrors, diffracted from the second diffraction grating, reflected from the optically unpowered reversing mirror back to the second diffraction grating, diffracted from the second diffraction grating, reflected from each of the set of optically powered mirrors, and diffracted from the first diffraction grating as an output light beam. The present approach produces a differential path length as a function of wavelength.