No abstract

No abstract

We have fabricated several germanium immersion gratings by single crystal, single point diamond flycutting on an ultraprecision lathe. Use of a dead sharp tool produces groove corners less than 0.1 micron in radius and consequently high diffraction efficiency. We measured first order efficiencies in immersion of over 80% at 10.6 micron wavelength. Wavefront error was low averaging 0.06 wave rms (at 633 nm) across the full aperture. The grating spectral response was free of ghosts down to our detection limit of 1 part in 10 4 . Scatter should be low based upon the surface roughness. Measurement of the spectral line profile of a CO 2 laser sets an upper bound on total integrated scatter of 0.5%.

Recent advances in silicon micromachining techniques (e.g. anisotropic etching) allow the fabrication of very coarse infrared echelle gratings. When used in immersion mode the dispersion is increased proportionally to the refractive index. This permits a very significant reduction in the overall size of a spectrometer while maintaining the same resolution. We have fabricated a right triangular prism (30x60x67 mm with a rectangular entrance face 30x38 mm) from silicon with a grating etched into the face of the hypotenuse. The grating covers an area of 32 mm by 64 mm and has a 97.5 jim periodicity with a blaze angle of 63.4g. The groove surfaces are very smooth with a roughness of a few nanometers. Random defects in the silicon are the dominant source of grating scatter ( 12% at 3.39 jim). We measure a grating ghost intensity of 1.2%. The diffraction peak is quite narrow, slightly larger than the Airy disc diameter at F/12. However due to wavefront aberrations, perhaps 15-20% of the diffracted power is in the peak with the rest distributed in a diameter roughly five times the Airy disc.

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