Steve L. Little

High Efficiency Germanium Immersion Gratings

We have fabricated several germanium immersion gratings by single crystal, single point diamond flycutting on an ultra-precision 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 104. Scatter should be low based upon the surface roughness. Measurement of the spectral line profile of a CO2 laser sets an upper bound on total integrated scatter of 0.5%.

Fabrication and current optical performance of a large diamond-machined ZnSe immersion grating

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.

Fabrication and testing of diamond-machined gratings in ZnSe, GaP, and bismuth germanate for the near infrared and visible

High quality immersion gratings for infrared applications have been demonstrated in silicon and germanium. To extend this technology to shorter wavelengths other materials must be investigated. We selected three materials, zinc selenide, gallium phosphide and bismuth germanate (Bi4Ge3O12), based on high refractive index, good visible transmission and commercial availability in useful sizes. Crystal samples were diamond turned on an ultra-precision lathe to identify preferred cutting directions. Using this information we diamond-flycut test gratings over a range of feed rates to determine the optimal cutting conditions. For both ZnSe and GaP good surface quality was achieved at feed rates up to 1.0 cm/minute using a special compound angle diamond tool with negative rake angles on both cutting surfaces. The surface roughness of the groove facets was about 4 nm. A Zygo interferometer measured grating wavefront errors in reflection. For the ZnSe the RMS error was < λ/20 @633nm. More extensive testing was performed with a HeNe laser source and a cooled CCD camera. These measurements demonstrated high relative diffraction efficiency (> 80%), low random groove error (2.0 nm rms), and Rowland ghost intensities at < 0.1%. Preliminary tests on bismuth germanate show high tool wear.

Fabrication and testing of germanium grisms for LMIRcam

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 2.9°. 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.

Diamond-machined ZnSe immersion grating for NIR high-resolution spectroscopy

ZnSe immersion gratings (n ~ 2.45) provide the possibility of high-resolution spectroscopy for the near-infrared (NIR) region. Since ZnSe has a lower internal attenuation than other NIR materials, it is most suitable for immersion grating, particularly in short NIR region (0.8 - 1.4 μm). We are developing an extremely high-resolution spectrograph with λ/Δλ = 100, 000, WINERED, customized for the short NIR region, using ZnSe (or ZnS) immersion grating. However, it had been very difficult to make fine grooves on ZnSe substrate with a small pitch of less than 50 μm because ZnSe is a soft/brittle material. We have overcome this problem and successfully machined sharp grooves with fine pitch on ZnSe substrates by nano precision fly-cutting technique at LLNL. The optical testing of the sample grating with HeNe laser shows an excellent performance: the relative efficiency more than 87.4 % at 0.633 μm for a classical grating configuration. The diffraction efficiency when used as an immersion grating is estimated to be more than 65 % at 1μm. Following this progress, we are about to start machining a grating on a large ZnSe prism with an entrance aperture of 23mm × 50mm and the blaze angle of 70°.

Materials and Fabrication Issues for Large Machined Germanium Immersion Gratings

LLNL has successfully fabricated small (1.5 cm2 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 63.4° 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.

ZnSe immersion grating in the short NIR region

ZnSe has a high refractive index (n~ 2.45) and low optical loss (< 0.1/cm) from 0.8 to 12 um. Therefore ZnSe immersion gratings can enable high-resolution spectroscopy over a wide wavelength range. We are developing ZnSe immersion gratings for a ground-based NIR high-resolution spectrograph WINERED. We previously produced a large prism-shaped ZnSe immersion grating with a grooved area 50 mm x 58 mm (Ikeda et al. 2010). However, we find two problems as NIR immersion grating: (i) serious chipping of the grooves, and (ii) inter-order ghosts in the diffraction pattern. We believed the chipping to be due to micro cracks just beneath surface present prior to diamond machining. Therefore we removed this damaged region, a few tens of microns thick, by etching the ZnSe grating blank with a mixture of HCl and HNO3. Ghosts appearing halfway between main diffraction orders originate from small differences in spacing between odd and even grooves. Apparently the blank shifts repeatably by about 120 nm in the direction orthogonal to the grooves depending on whether the translation stage holding the blank is moving right to left or left to right. Therefore we remachined the grating only cutting grooves with the stage moving from right to left. After re-cutting, we also deposit the Cu coating with an enhanced interface layer of SiO2 on the groove, which is developed in our previous study. We evaluated the optical performances of this immersion grating. It shows light scattering of 3.8 % at 1μm, no prominent ghosts, and a spectral resolution of 91,200 at 1 μm. However we measured an absolute diffraction efficiency of only 27.3% for TE and 25.9 % for TM waves at 1.55 μm. A non-immersed measurement of the diffraction efficiency of the facet blazed near 20º exceeded 60%, much closer to theoretical predictions. We plan to carry out more tests to resolve this discrepancy.

Progress in the fabrication of a prototype ZnSe immersion grating for the WINERED spectrograph

The WINERED spectrograph requires a large (9 cm × 9 cm aperture), 70° blaze ZnSe immersion grating to achieve a resolution of ~100,000. It will be implemented as a mosaic of 3 gratings of aperture 3 cm × 9 cm. LLNL is diamond machining a subscale prototype (2.3 cm × 5.0 cm aperture) to demonstrate feasibility. This is a 10× increase in the size of gratings machined at LLNL. The first cutting of the grating had large wave front errors due to non-working thermal control. After repairs we recut a grating with an rms wavefront error of 0.11 wave @633 nm, which meets WINEREDs full aperture requirement. The first cutting had good quality grooves, but the recut showed bad chipping. A second recut after polishing away the grating had pitted groove faces. It is known that diamond machining and mechanical polishing create subsurface damage. This damage can cause brittle fracture during subsequent machining. We plan to chemically etch ZnSe substrates to remove damaged layers prior to machining our next gratings.

Machining of ZnSe grisms for the Rapid Infrared Imager Spectrograph (RIMAS): effect of diamond crystal orientation

The Rapid Infrared Imager/Spectrograph (RIMAS) is an instrument designed to observe gamma ray burst afterglows. Dispersion in the moderate resolution mode (R~4000) is provided by ZnSe grisms: one covering the Y and J bands and the other covering the H and K. Each has a clear aperture of 44 mm. For the HK grism the blaze is 49.9° with a 20 line/mm period. The grooves cover an area of 69 mm x 45 mm. The HK grism was diamond machined on the Precision Engineering Research Lathe (PERL) at LLNL. Chipping of the grooves increased from moderate to severe as the cutting progressed resulting in excess scattered light and reduced diffraction efficiency. High magnification optical microscopy and SEM of the cutting edges indicated damage to the tool caused by wear. A comparison of the outcomes of ZnSe gratings and grisms machined at LLNL indicated that chipping was minimal in low blaze angle cuts but moderate to severe with the blaze angle near 45° as in the HK grism. Vendor records showed that the (100) crystal planes of the diamond were aligned parallel to the tool shank. Therefore the (100) planes are closely aligned with the cutting edge in low blaze angle tools but 45° off in the HK tool. We believe that this misalignment of the cutting edge with the (100) crystal plane in the HK tool produced excessive tool wear resulting in the chipped grooves observed.

Technique for diamond machining large ZnSe grisms for the Rapid Infrared/Imager Spectrograph (RIMAS)

The Rapid Infrared Imager/Spectrograph (RIMAS) is an instrument designed to observe gamma ray burst afterglows following initial detection by the SWIFT satellite. Operating in the near infrared between 0.9 and 2.4 μm, it has capabilities for both low resolution (R~25) and moderate resolution (R~4000) spectroscopy. Two zinc selenide (ZnSe) grisms provide dispersion in the moderate resolution mode: one covers the Y and J bands and the other covers the H and K. Each has a clear aperture of 44 mm. The YJ grism has a blaze angle of 49.9° with a 40 μm groove spacing. The HK grism is blazed at 43.1° with a 50 μm grooves spacing. Previous fabrication of ZnSe grisms on the Precision Engineering Research Lathe (PERL II) at LLNL has demonstrated the importance of surface preparation, tool and fixture design, tight thermal control, and backup power sources for the machine. The biggest challenges in machining the RIMAS grisms are the large grooved area, which indicates long machining time, and the relatively steep blaze angle, which means that the grism wavefront error is much more sensitive to lathe metrology errors. Mitigating techniques are described.