Echelle grating

Echelle gratings are special that hold large diffraction efficiencies in high Beugungsordungen. Usually we use blazed grating with high blaze angles of about 60-75 °. Unlike conventional diffraction gratings, echelle have relatively little furrows visible light approximately 20-100 per millimeter.

Term origin

The word comes from French échelle = ( rung ) ladder, stairs. There are two explanations for the name: The grid itself resembles a staircase, and the individual lines of an échelle spectrogram are parallel like the rungs of a ladder.

The échelle spectrograph

The drawing shows the basic structure of échelle spectrograph. The light from below on a slit with a downstream optical echelle grating G1. The dispersion spectrum falls on a conventional grid G2. The blue and red arrow indicate the position of the diffraction spectrum. The echelle grating is operated in high diffraction orders that fall largely on each other. The figure shows an example of two orders O1 and O2. In fact, the colors mix. If one were to insert at this point a screen, like watching a faded gray, not the spectral colors.

The grid G2 is oriented perpendicular to the echelle grating G1 and separates the orders. The spatial splitting by G1 continues in G2. Instead a continuous spectrum that spreads from above (red line ) to bottom ( blue line) over the entire screen S, a spectral band bounded by the width of G2 is displayed. The left blue arrow marked S on the short-wave edge of the order O1, the left red arrow the long-wave. The other two arrows point to the band that forms the grating G2 for the second order O2.

The second diffraction grating separates the superposed diffracted orders, which generates the echelle grating. A Échellespektrum consists of nearly parallel diffraction bands. With increasing order, the diffraction patterns are compressed, which results in a tilting of the strips on the screen. The drawing exaggerated the effect. Is chosen for the separation instead of a diffraction grating G2, a prism with a non-constant dispersion, the tapes extend on the screen is not linear but curved.

Areas of application

Echelle gratings are often used in astronomy for the absorption of stellar spectra with high resolution, because a high-resolution spectrum can be detected with a very wide range of wavelengths at once. In contrast, conventional high-resolution diffraction grating for a relatively narrow range are optimized; also very long CCD sensors or more juxtaposed CCDs would be necessary to capture the complete spectrum first ( or second ) order. When échelle spectrograph allows the spectrum with a relatively small square CCD record.

Echelle achieve good spectral resolution in a compact design. Uses an Gitter/Gitter-, but more often a grating / prism combination. As a so-called echelle diffraction grating with high efficiency is used in high diffraction orders. With sequential monochromators a prism for pre-selection of the wavelength range used. In polychromators the various diffraction orders are projected in two dimensions with a second grating or prism in the area. As a receiver commercial photographic plates, secondary electron multiplier are used behind gap masks and semiconductor surface detectors.

Echelle spectrometer are widely used in analytical chemistry for the quantitative and qualitative determination of elements, in particular in the optical ICP emission spectrometry (ICP -OES) and gas chromatography ( see Echelle plasma emission detector ). The good spectral resolution in the UV region for the separation of the line- rich spectra ICP is particularly effective in this combination.