A diffractive optical element of the blazed-binary grating type is disclosed,
with
which a high diffraction efficiency can be achieved over substantially the entire
used wavelength region. The diffractive optical element has a periodic structure
taking a grating unit including a plurality of grating portions with different
grating width and grating interval as one period. The diffraction element further
includes a plurality of first grating portions made of a first material and a plurality
of second grating portions made of a second material having a different refractive
index than the first material that are arranged between the first grating portion.
