Diffraction grating element, optical module and optical communications system

A diffraction grating element, comprising:

• a first medium having a refractive index of n1;
• a second medium arranged so as to be in contact with said first medium, said second medium having a refractive index of n2 lower than that of said first medium;
• and
• a diffraction grating provided at the interface between said first medium and said second medium,
• wherein one of said first medium and said second medium is a solid, and the other thereof is a solid or a liquid, and
• wherein, taking the period of said diffraction grating to be Λ, the height of said diffraction grating to be H, the duty ratio of the width of said first medium with respect to the period Λ in said diffraction grating to be f, and the normalized height expressed by (n1/n2−1) H/Λ to be Hnorm, in a two-dimensional plane based on coordinate values (Hnorm, f), the normalized height Hnorm and the duty ratio f lie within a region enclosed by linking in sequence, by means of line segments, the point (0.50, 0.32), the point (0.50, 0.75), the point (2.00, 0.90), the point (4.00, 0.90), the point (2.20, 0.76), the point (0.75, 0.32), and the point (0.50, 0.32), or within a region enclosed by linking in sequence, by means of line segments, the point (2.25, 0.20), the point (2.25, 0.44), the point (2.75, 0.44), the point (2.75, 0.20), and the point (2.25, 0.20).