Micro V-groove grinding technique of large Germanium Immersion Grating element for mid-infrared spectrograph

Abstract

The 8.2 m SUBARU large space telescope in Hawaii requires a mid-infrared high dispersion spectrograph with a resolution of 200 000 at 10 μm which employs a Germanium Immersion Grating (GIG) element. For this, the GIG element with a large number of sharp and smooth micro V-grooves must be fabricated. Previous studies were focused on the fabrication of a prototype GIG element. The grinding system developed however was not applicable for the fabrication of practical larger GIG element due to insufficient machining space and fast wear of the small grinding wheel used. The ultimate goal of this project was therefore to develop a new grinding system capable of fabricating larger GIG elements for practical use. This paper discusses the results achieved in the first phase of the research, which focused on the principle of the grinding process and grinding system, as well as describes results of micro-truing and micro-grinding experiments using a #4000 metal bonded diamond grinding wheel. Wear variation of the grinding wheel and its effects on the corner radius of micro V-grooves are also discussed. The minimum wheel tip radius of 8.2 μm was achieved using this #4000 grinding wheel by micro-truing, and V-groove corner radiuses ranging from 15 μm to 25.8 μm were also achieved using the same grinding wheel.