title: Simple design of slanted grating with simplified modal method: comment.
abstract: In the recent Letter [Opt. Lett.39, 781 (2014)10.1364/OL.39.000781OPLEDP0146-9592] that proposes a method to design slanted parallelogram gratings based on the simplified modal method, the analysis of the modal field in the grating layer is incorrect; therefore, the validity of the equivalence between slanted parallelogram gratings and upright rectangular gratings that the authors thereby have deduced is questionable or at least in need of substantial modifications.
authors.forename: Lifeng
authors.initials: L
authors.lastname: Li