Ikeda, Yuji Kobayashi, Naoto; Kuzmenko, Paul J.;
Little, Steve L.; Yasui, Chikako; Kondo, Sohei; Mito, Hiroyuki;
Nakanishi, Kenshi; Sarugaku, Yuki
2010, SPIE
■ アブストラクト
ZnSe immersion gratings provide the possibility of high resolution
spectroscopy in the wide infrared wavelength region from the NIR (Near
Infrared) to the MIR (Mid Infrared), because ZnSe has a high
refractive index (n ~ 2.45) and a low internal extinction in these
wavelength regions. We are developing ZnSe immersion grating for a
ground-based NIR high-resolution spectrograph and a space MIR
high-resolution spectrograph. We already have produced fine grooves on
the ZnSe flat substrate with a small pitch (~ 30 μm) using nano
precision flycutting technique at the Lawrence Livermore National
Laboratory,1 which satisfies our requirements even for the short NIR
application.2 Our next step is to fabricate a large prism-shaped ZnSe
immersion grating with this technology. The triangle prism has the
entrance surface 50mm × 23mm and the apex angle of 70 deg. Untile now,
we tried three R&D cutting runs. We examined the optical
performances of the immersion grating sample from the second cutting
run, which showed the best performances. Although a lot of chipping
are seen at the edge of the blaze by the microscopic observation, we
found that the groove shape is quite good with the surface
irregularity of 0.74λ (pv) and the random pitch error of 5.2 nm (rms),
which closely meet with our requirements. In the HeNe laser spectra
taken under both grism and immersion configurations, strong ghosts
were observed at the intermedium of the diffracted orders. These
interorder ghosts may originate from the differences of the pitch
and/or shape between odd and even grooves due to the cutting
procedures. In addition, we also investigated a suitable
reflectivecoating for the diffraction surface. As a result, we
concluded that aluminum or cupper by suppering process is the best
materials in the wavelength region of WINERED. Finally, we discuss the
pssible improvement points and prospect for the next trial in this
summer.