A diffraction grating comprise a substrate and a plurality of connected diffraction structures formed on the substrate. Each diffraction structure is in the shape of a column and arranged along a concave cylindrical surface, and an axis of each diffraction structure extends along a generatrix of the concave cylindrical surface. A section contour is obtained by a cross section of the diffraction structures. The cross section is perpendicular to each axis of the diffraction structure. The section contour shows the connecting line of apexes of the diffraction structures as a reference curve having a plurality of first inflection points, wherein the diffraction structures are configured for separating the optical signal into a plurality of spectral components and focusing the spectral components onto a focal surface.