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Spectroscopy
Published in C. R. Kitchin, Astrophysical Techniques, 2020
Echelle gratings are used for many recently built spectroscopes. The rectangular format (Figure 4.24) of the group of spectral segments after the cross-disperser matches well to the shape of large CCD arrays, so that the latter may be used efficiently. The UVES for ESO’s VLT, for example, operates in the blue, red and NIR regions with two 0.2 × 0.8-m echelle gratings. The gratings have 41 and 31 lines mm−1 and spectral resolutions of 80,000 and 115,000, respectively (Figure 4.24). Also, for the VLT, the X-Shooter instrument has three independent spectrographs each of which uses an echelle grating with a cross disperser. The three components cover the spectral regions 300–559.5 nm, 559.5 nm–1.024 μm and 1.024–2.489 μm and receive ‘their’ portion of the incoming radiation via dichroic mirrors. The maximum spectral resolution is 18,200. Similarly, the Magellan Inamori Kyocera Echelle (MIKE) instrument for the Magellan telescope, which started science operations in 2003 and is still in use at the time of writing, uses two echelle gratings with prism cross dispersers and a dichroic mirror to cover the spectral regions 335–500 nm and 490–950 nm at resolutions up to 83,000.
Diffraction Gratings
Published in Roshan L. Aggarwal, Kambiz Alavi, Introduction to Optical Components, 2018
Roshan L. Aggarwal, Kambiz Alavi
Echelle gratings are used for high-resolution spectroscopy. They are coarsely ruled (large d) blazed gratings with a large blaze angle. The resolving power of an echelle grating used in the Littrow mode at the blaze angle is proportional to the tangent of the large blaze angle. Echelle gratings are generally used with a second grating, or prism, to separate the overlapping diffraction orders. Albert Michelson, who referred to them as echelons, discovered echelle gratings in 1898. The resolving power of an echelle grating relative to that of a conventional blazed grating in the Littrow mode is given by () (λ/Δλ)E(λ/Δλ)C=tanθEtanθC
Simple to Complex Structures Using Wet Bulk Micromachining
Published in Prem Pal, Kazuo Sato, Silicon Wet Bulk Micromachining for MEMS, 2017
Wet anisotropic etching is a simple and effective method to fabricate high-quality diffraction gratings which have a wide range of applications in the infrared (IR) region [4,5,6,7–8]. They are used for high-resolution IR spectrographs, monochromators, and laser tuning. A diffraction grating comprises blazed grooves and therefore is also called an echelle grating. To fabricate a diffraction grating in a silicon wafer, a mask layer on a polished surface is patterned with parallel equidistant stripes using the photolithography process, as presented in Fig. 8.5a. Thereafter anisotropic etching is carried out to form blazed grooves. The groove profile (or blazed angle) of the etched diffraction grating depends on the wafer orientation, the mask edge direction on the wafer surface, and the etchant type (pure or surfactant added). Figure 8.5b shows linear gratings with different blazed angles. For example, the alignment of mask edges along the <110> direction on a {100} wafer provides an echelle pattern with a blaze angle of 54.7° (i.e., angle between the {100} and {111} planes). The apex angle of the grooves is fixed by the angle between opposite pairs of {111} planes. Different blaze angles can be achieved by use of a wafer cut at a nonzero angle relative to the {100} planes. After a groove pattern is completed, the mask layer is removed from the wafer. The blazed grooves are separated by narrow unblazed stripes (i.e., flat-topped grooves). The width of these flat tops can be decreased by using narrower mask stripes and/or by overetching after the formation of grooves. A combination of two-step etching and LOCOS, which is discussed later, is used to fabricate a silicon grating with a triangular profile.
Compact cross-dispersion device based on a prism and a plane transmission grating
Published in Journal of Modern Optics, 2018
An early echelle spectrometer uses two planar reflection gratings operating in two perpendicular planes so that the incident light is first diffracted by an echelle grating into a high diffraction order in one plane, then diffracted by a second reflection grating in a perpendicular plane (3). The resulting dispersion spreads the spectrum out on the detector plane in two dimensions to produce a two-dimensional spectrogram including several lines side by side that correspond to different diffraction orders, namely, several diffraction-order lines side by side with the wavelength varying with its position along each line. However, if large spectral range or high resolving power or both are required, the diffraction-order lines would become crowded near one end of the spectrogram, which tend to make the spectral lines within the order lines indistinguishable.