A bilayer transmission diffraction grating having antireflection lines having rectangular cross-section over grating lines having trapezoidal cross-section is described. The process-dependent grating line profile is accounted for by characterizing the grating line profile and performing electromagnetic wave diffraction simulations, whereby a grating duty cycle is selected that results in an improvement of overall diffraction efficiency and/or reducing polarization dependent loss of a diffraction grating having the characterized grating line profile. Grooves in the substrate between the grating lines further improve diffraction efficiency and reduce polarization dependent loss. The entire grating line profile, including the antireflection line, the grating line, and the groove in the substrate between the grating lines, can be defined using a single etch mask, which reduces process and equipment related manufacturing costs.