Silicon Immersion Gratings for Very High Resolution Infrared Spectroscopy

摘要

In our group, development of large format silicon immersion gratings with sizes up to 4 inches in diameter is a routine practice. The first silicon anamorphic immersion grating has an 80x50 mm~2 etched grating area, a 63.5° blazed angle and a 5.4 l/mm groove density (or 185μm period) on a 30mm thick silicon substrate. The groove density is about 4 times coarser than any existing commercial echelle grating, allowing a complete coverage of a cross-dispersed echelle spectrum on a 1k x 1k IR array at R = 220,000 in the K band. The optical measurements show the grating has a high quality wavefront and surface. The rms wavefront error is 0.125 waves and the integrated scattered light is ～1% at 0.6328nm. A silicon immersion grating with an 85x50 mm etched area, a 54.7° blazed angle and 16.1 l/mm groove density on a 40 mm thickness allows for complete wavelength coverage of 1.2-2.4 μm on a 2kx2k IR array. We are in the middle of processing a silicon disk with a 6 inch diameter and 2.5 inch thickness to make a large format silicon immersion grating for the Gemini next generation Advanced Cryogenic Echelle Spectrograph (ACES) and space applications.