Diffraction gratings and geometrical optics

We show that diffraction gratings can be naturally placed in the framework of geometrical optics. In this context, the relationship between path length and group delay, τ(ω) = l(ω)/c, is a straightforward result of Fermat’s principle. This result eases some of the conceptual difficulties involved in the analysis of grating-pair pulse compressors. Our method is equally applicable to both phase and amplitude gratings.