Blazed gratings on convex substrates for high throughput spectrographs

Abstract

In next generation space and ground-based instrumentation for Earth and Universe Observation, new instrument concepts include often non planar gratings. Their realization is complex and costly. We propose a new technology for designing and realizing convex blazed gratings for high throughput spectrographs. For this purpose, the requirements are driven by a Digital-Micromirror-Device-based (DMD) MOS instrument we are developing, called BATMAN. The two-arm instrument is providing in parallel imaging and spectroscopic capabilities. The objects/field selector is a 2048 x 1080 micromirrors DMD, placed at the focal plane of the telescope; it is used as a programmable multi-slit mask at the entrance of the spectrograph. The compact Offner-type spectrograph design contains a low density convex grating to disperse light. For optimization of the spectrograph efficiency, this convex grating must be blazed. A blazed reflective grating has been designed with a period of 3300 nm and a blaze angle of 5.04°, and fabricated into convex substrates with 225 mm radius of curvature and a footprint diameter of 63.5 mm. The blaze is optimized for the center wavelength of 580 nm within the spectral range of 400 – 800 nm. Such grating has been fabricated by using lithography, angular Ar ion etching, transfer of the blazed grating from a flat surface onto a convex substrate with a flexible stamp, etched into the substrate by RIE etching. and finally coated with a silver-based layer. With a final 7° blaze angle over the whole surface, efficiency close to 90% on the 1st diffraction order at 700nm has been obtained, measured on BATMAN spectroscopic arm. An optimized device with the exact required blaze angle would reach the same efficiency and be centered on the mid of 400-800nm wavelength band: its realization is on-going. The wavefront error of the diffracted beam will also be optimized. The grating brings a significant contribution in the total amount of straylight at instrument level. Their straylight level remains a critical issue, and its reduction by specific and controlled implementation of improvements in manufacturing process is a challenge to tackle. Straylight measurement has been done and shows a BRDF cosq values of 10-8 sr-1 on the optical surface and 10-7 sr-1 on the structured features. This new type of non-planar reflective gratings will be the key component for future high throughput spectrographs in space missions.