Development of a Fine Grating on ZnS for a Wideband Spectral Disperser in Characterizing Exoplanets using Space-borne Telescopes

摘要

We present the fabrication and optical testing of a fine grating on a ZnSsubstrate to be used as a wideband infrared spectral disperser and for whichthe primary application is measurement of the composition of the atmospheres oftransiting exoplanets using space-borne infrared astronomical telescopes. Agrating with a blaze angle of 2.1 deg. and pitch of 166.667 midron wasconstructed on a roughly flat 10 mm x 10 mm substrate with a maximum thicknessof 1 mm. To obtain high accuracy, the sample was fabricated on a ZnSmonocrystal using a high performance processing machine at Canon Inc. Thesurface roughness measured with a microscope interferometer was 2.6 nm rms. Theshape of the fabricated grating edges was examined with a scanning electronmicroscope. The diffraction efficiency was evaluated by optical experiments at{\lambda} = 633 nm, 980 nm, and 1550 nm, and compared with the efficienciescalculated using a Fourier Modal Method. The results showed that thedifferences between the diffraction efficiencies obtained from experiment andby calculation were between just 0.9 % and 2.4 %. We concluded that the qualityof the fabricated ZnS grating was sufficiently high to provide excellentdiffraction efficiency for use in the infrared wavelength region. We alsopresent the design of a spectral disperser in CdTe for future more advancedperformance.