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%.
ZnSe immersion gratings provide the possibility of high resolution spectroscopy in the wide infrared wavelength region from the NIR (Near Infrared) to the MIR (Mid Infrared), because ZnSe has a high refractive index (n ∼ 2.45) and a low internal extinction in these wavelength regions. We are developing ZnSe immersion grating for a ground-based NIR high-resolution spectrograph and a space MIR high-resolution spectrograph. We already have produced fine grooves on the ZnSe flat substrate with a small pitch (∼ 30 µm) using nano precision flycutting technique at the Lawrence Livermore National Laboratory, 1 which satisfies our requirements even for the short NIR application.2 Our next step is to fabricate a large prism-shaped ZnSe immersion grating with this technology. The triangle prism has the entrance surface 50mm × 23mm and the apex angle of 70 deg. Untile now, we tried three R&D cutting runs. We examined the optical performances of the immersion grating sample from the second cutting run, which showed the best performances. Although a lot of chipping are seen at the edge of the blaze by the microscopic observation, we found that the groove shape is quite good with the surface irregularity of 0.74λ (pv) and the random pitch error of 5.2 nm (rms), which closely meet with our requirements. In the HeNe laser spectra taken under both grism and immersion configurations, strong ghosts were observed at the intermedium of the diffracted orders. These interorder ghosts may originate from the differences of the pitch and/or shape between odd and even grooves due to the cutting procedures. In addition, we also investigated a suitable reflectivecoating for the diffraction surface. As a result, we concluded that aluminum or cupper by suppering process is the best materials in the wavelength region of WINERED. Finally, we discuss the pssible improvement points and prospect for the next trial in this summer.
LLNL has successfully fabricated small (1.5 cm 2 area) germanium immersion gratings. We studied the feasibility of producing a large germanium immersion grating by means of single point diamond flycutting. Our baseline design is a 63.4º blaze echelle with a 6 cm beam diameter. Birefringence and refractive index inhomogeneity due to stresses produced by the crystal growth process are of concern. Careful selection of the grating blank and possibly additional annealing to relieve stress will be required. The Large Optics Diamond Turning Machine (LODTM) at LLNL is a good choice for the fabrication. It can handle parts up to 1.5 meter in diameter and 0.5 meter in length and is capable of a surface figure accuracy of better than 28 nm rms. We will describe the machine modifications and the machining process for a large grating. A next generation machine, the Precision Optical Grinder and Lathe (POGAL), currently under development has tighter specifications and could produce large gratings with higher precision.
We diamond fly cut 2 sets of germanium grisms for the LMIRcam 3-5 micron Fizeau imager for the combined focus of the Large Binocular Telescope (LBT). The grisms mount in a filter wheel near a pupil to enable moderate resolution (R~300) spectroscopy. Both sets have a measured blaze angle of 2.9º. The first set has a groove period of 40 lines/mm and will be used in first order with peak efficiency at 3.6 µm. The second set has 32 lines/mm. It can operate in first order with an efficiency peak near 4.4 µm and in second order with a peak near 2.3 µm. First results from testing the grisms in the instrument on the sky with the LBT are presented.
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