Red laser beam split by a diffraction grating. Diffraction grating is an optical element whose properties are periodically modulated in one dimension. Transmissive diffraction gratings consist of many thin lines of either absorptive material or thin grooves (indentations) on an otherwise transparent substrate. Light transmission through a diffraction grating occurs along discrete directions, called diffraction orders. Here a diode laser beam (635 nm) is split into three diffraction orders (+1, 0, -1). This grating groove density is 500 lines/mm.