  We present the fabrication and optical testing of a fine grating on a ZnS
substrate to be used as a wideband infrared spectral disperser and for which
the primary application is measurement of the composition of the atmospheres of
transiting exoplanets using space-borne infrared astronomical telescopes. A
grating with a blaze angle of 2.1 deg. and pitch of 166.667 midron was
constructed on a roughly flat 10 mm x 10 mm substrate with a maximum thickness
of 1 mm. To obtain high accuracy, the sample was fabricated on a ZnS
monocrystal using a high performance processing machine at Canon Inc. The
surface roughness measured with a microscope interferometer was 2.6 nm rms. The
shape of the fabricated grating edges was examined with a scanning electron
microscope. The diffraction efficiency was evaluated by optical experiments at
{\lambda} = 633 nm, 980 nm, and 1550 nm, and compared with the efficiencies
calculated using a Fourier Modal Method. The results showed that the
differences between the diffraction efficiencies obtained from experiment and
by calculation were between just 0.9 % and 2.4 %. We concluded that the quality
of the fabricated ZnS grating was sufficiently high to provide excellent
diffraction efficiency for use in the infrared wavelength region. We also
present the design of a spectral disperser in CdTe for future more advanced
performance.
