What is "echelle grating"

n. (context physics English) A form of diffraction grating optimized for high orders of diffraction

An echelle grating (from French, échelle, meaning stairs or ladder) is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at high incidence angles and therefore in high diffraction orders. Higher diffraction orders allow for increased dispersion (spacing) of spectral features at the detector, enabling increased differentiation of these features. Echelle gratings are, like other types of diffraction gratings, used in spectrometers and similar instruments. They are most useful in cross-dispersed high resolution spectrographs, such as HARPS, PARAS (PRL Advanced Radial Velocity Abu Sky Search) and numerous other astronomical instruments.