High diffraction efficiency varied-line-space concave gratings for the Lyman ultraviolet explorer

Abstract

In this paper we present the design and fabrication of the reflection varied-line-space concave grating (VLSCG) for the project of CAFE (the Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer), which aims to detect and map the warm-hot circumgalactic medium via OVI emission at 103.2 nm and 103.8 nm, using two off-Rowland-circle spectrograph channels. High diffraction efficiency at LUV is supposed for the VLSCG, and an aperture ratio as small as is desired for a compact design. The gratings are fabricated by holographic lithography and ion beam etching techniques. We introduce an additional lens into the normal holographic exposing system to generate the varied-line-space grating patterns. Grooves with triangle profiles are obtained to increase the diffraction efficiency by oblique ion beam bombardment during the etching process. Finally, several VLSCGs with a central line density of 3300 lines/mm have been fabricated successfully. The measured results show that the groove efficiency reaches 51% at 106.4 nm and 31% at 127.4 nm. We imitated the working optical path of the spectrometer and used the order of the VLSCG to measure the image near the exit slit. The results showed that the image of the point source has a vertical extent of 0.68 mm, and the aberrations have been corrected.