Paul J. Kuzmenko

On-sky operations and performance of LMIRcam at the Large Binocular Telescope

The L/M-band (3−5 μm) InfraRed Camera (LMIRcam) sits at the combined focal plane of the Large Binocular Telescope Interferometer (LBTI), ultimately imaging the coherently combined focus of the LBT’s two 8.4-meter mirrors. LMIRcam achieved first light at the LBT in May 2011 using a single AO-enabled 8.4-meter aperture. With the delivery of LBT’s final adaptive secondary mirror in Fall of 2011, dual-aperture AO-corrected interferometric fringes were realized in April 2012. We report on the performance of these configurations and characterize the noise performance of LMIRcam’s HAWAII-2RG 5.3-μm cutoff array paired with Cornell FORCAST readout electronics. In addition, we describe recent science highlights and discuss future improvements to the LMIRcam hardware.

Experimental Performance of a Miniature Fabry-Perot Fiber Optic Hydrophone

The advantages of fiber optic sensors in hydrophone applications are well known. They offer excellent sensitivity, resistance to electromagnetic interference, and all active electronics are kept out of the water. One aspect where they are not clearly superior to conventional piezoelectric transducers is their size. For some applications size is critical and existing designs are too large.

Zinc sulfide and zinc selenide immersion gratings for astronomical high-resolution spectroscopy: evaluation of internal attenuation of bulk materials in the short near-infrared region

We measure the internal attenuation of bulk crystals of chemical vapor deposition zinc selenide (CVD-ZnS), chemical vapor deposition zinc sulfide (CVD-ZnSe), Si, and GaAs in the short near-infrared (sNIR) region to evaluate the possibility of astronomical immersion gratings with those high refractive index materials. We confirm that multispectral grade CVD-ZnS and CVD-ZnSe are best suited for the immersion gratings, with the smallest internal attenuation of att=0.01 to 0.03 cm−1 among the major candidates. The measured attenuation is roughly in proportion to −2, suggesting it is dominated by bulk scattering due to the polycrystalline grains rather than by absorption. The total transmittance in the immersion grating is estimated to be at least >80%, even for the spectral resolution of R=300,000. Two potential problems, the scattered light by the bulk material and the degradation of the spectral resolution due to the gradient illumination in the diffracted beam, are investigated and found to be negligible for usual astronomical applications. Since the remaining problem, the difficulty of cutting grooves on CVD-ZnS and CVD-ZnSe, has recently been overcome by the nanoprecision fly-cutting technique, ZnS and ZnSe immersion gratings for astronomy can be technically realized.

Fabrication and testing of a silicon immersion grating for infrared spectroscopy

recent advances in silicon micromachining techniques 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 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 micrometers periodicity with a blaze angle of 63.4 degree(s). The groove surfaces are very smooth with a roughness of a few nm. Random defects in the silicon are the dominant source of grating scatter. 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.

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.

Improving the optical performance of etched silicon gratings

Silicon diffraction gratings fabricated by photolithography and anisotropic chemical etching offer unique advantages over conventional ruled gratings. Previous work showed a high amount of scatter from etched gratings and several waves of wavefront distortion due to an overetch problem. We recently began to study ways to improve the optical performance of etched silicon gratings. Defect etching was used to verify that our starting material was of high quality and that it could be polished to the required optical flatness without producing subsurface damage. We compared the effect of different etch mask materials and etching variables on the scatter and wavefront aberration of test gratings. Gratings masked with silicon nitride achieved sufficiently low wavefront distortion to be useful in high resolution spectroscopy. The scatter was improved by a factor of tow over the grating fabricated several years earlier, but is still a factor of 3 worse thana ruled echelle. A better lithography mask should reduce the scatter. The major issues in fabricating large etched gratings for astronomy are achieving good uniformity across the surface and the compatibility of thick silicon disks with standard semiconductor processing hardware.

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.

Modeling, fabrication, and testing of a diamond-machined germanium immersion grating

ABSTRACT We investigated the fabrication of an immersion grating by diamond machining grooves on the hypotenuse of a right angle germanium prism. A numerical modeling code was used to optimize the blaze angle and to model grating efficiency over the wavelength range of 7.5 to 13.5 tim. The sensitivity in polarization is predicted to be small. We alsoquantified the effects of non-zero radius of curvature of the diamond tool. An atomic force microscope measured an rmsroughness of 1 . 1 nm on the grating facets. The diffracted wavefront error in reflection at 633 nm was 0.01 waves. Therefore optical aberration and scatter over the operating wavelengths will be negligible.Keywords: Immersion grating, germanium, diamond machining 1. INTRODUCTION Spectrometers are valuable tools for determining the composition and properties of distant, inaccessible objects both inastronomy and in environmental remote sensing. Diffraction gratings, which spectrally disperse electromagnetic wavesusing a periodic structure, are a key component in most modern spectrometers. The classic reflection grating consists of

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.