Echelle grating

An echelle grating (from French échelle, meaning "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.

The concept of a coarsely-ruled grating used at grazing angles was discovered by Albert Michelson in 1898,[1] where he referred to it as an "echelon".

In echelle gratings, however, this behavior is deliberately used and the blaze is optimized for multiple overlapping higher orders.

Hence the spectrum consists of stripes with different, but slightly overlapping, wavelength ranges that run across the imaging plane in an oblique pattern.

It is exactly this behavior that helps to overcome imaging problems with broadband, high-resolution spectroscopic devices, as in the utilisation of extremely long, linear detection arrays, or strong defocus or other aberrations, and makes the use of readily available 2D-detection arrays feasible, which reduces measurement times and improves efficiency.

Echelle spectrometer: The first standard grating is optimized for a single lower order, while multiple higher orders of the echelle have an optimized output intensity. Both diffractive elements are mounted orthogonally in such a way that the highly illuminated orders of the echelle are transversally separated. Since only parts of the full spectrum of each individual order lie in the illuminated region, only portions of the different orders overlap spectrally (i.e. green line in red portion).