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Practical ZnSe immersion grating for astronomical applications in the short NIR region

Yuji Ikeda (Photocoding), Naoto Kobayashi (Univ. Tokyo), Paul J. Kuzmenko, Steve L. Little (LLNL), Sayumi Kaji (Kyoto Sangyo Univ.), Yuki Sarugaku (JAXA), Chikako Yasui (Univ. Tokyo), Sohei Kondo (Kyoto Sangyo Univ.), Kei Fukue (Univ. Tokyo)
2014年6月27日 SPIE conference
"Astronomical Telecopes and Instrumentation" @Monréal
■ アブストラクト
ZnSe has a high refractive index (n~2.45) and low optical attenuation (< 0.1cm-1) from 0.8 to 12 um. Therefore ZnSe immersion gratings can enable high-resolution spectroscopy over a wide wavelength range. We are devolping ZnSe immersion gratins for a gruond-based NIR high-resolution spectrograph WINERED (Kondo et al. 2010), and a space MIR high-resolution spectrograph HRS mounted on the infrared space telescope, SPICA (Sarugaku et al. 2012). LLNL previously machined a 30 micron pitch grating on a 25 mm diameter ZnSe substrate (Kuzmenko et al. 2008) with good surface irregularity (< lambda/8), low surface roughness (< 5 nm rms), and small random pitch error (< 3 nm), which satisfied requirements for short NIR applications (Ikeda et al. 2008). Extending this technology, we produced a large prism-shaped ZnSe immersion grating with a grooved area 50 mm x 58 mm (Ikeda et al. 2010). However, we observed (i) serious chipping of the grooves, and (ii) inter-order ghosts in the diffraction pattern. We believed the chipping to be due to micro cracks just beneath surface present prior to diamond machining. So we removed this damaged region, a few tens of microns thick, by etching the ZnSe grating blank with a mixture of HCl and HNO3. Ghosts appearing halfway between main diffraction orders could originate from small differences in spacing between odd and even grooves. Hence, we re-machined the grating only cutting grooves with the stage moving from right to left. As a result of these efforts, we have a practical ZnSe immersion grating with a relative diffraction efficiency of about 73% at 1.0 um. There is little chipping and no ghosts exceeding 0.3% of the peak diffraction. In this paper, we discuss these results and also report on the deposition of an enhanced Cu reflective coating (Kuzmenko et al. 2012) directly on the grooved surface.