Novel structure of the optical elements (i.e. filter) to be operated in
the long, mid, and near infrared wavelengths of lights is provided. The
filter can offer very narrow linewidth, and high reflectivity (or
transmissivity) at the peak wavelength. The optical element consists of
the substrate, first diffraction grating and single uniform surface, and
the second grating. Alternatively, the optical element again consists of
the substrate, single uniform surface and the diffraction grating on the
top of it. Alternatively, filter may also consist of number of sequence
of layers, wherein each sequence comprises the single uniform layer
sandwiched by the two diffraction grating layers. Filter again
alternatively consists of the number of sequences wherein each sequence
comprises the single uniform layer and the single diffraction grating.
Diffraction grating may be two-step grating or multilevel grating with
synchronously or nonsynchronously samples diffraction gratings.