For a conventional (echelette) grating, the longer groove facet (of length t) would face toward the incident and diffracted light; for an echelle, the shorter, steeper groove facet (of length s) is facing toward the light. Echelles are often used in or near the Littrow configuration (shown), in which the angle of incidence α equals the angle of diffraction β, though they have been used with as much as α – β = 40° between the beams.

The two design parameters that define an echelle grating are its groove frequency G (= 1/d) and its blaze angle θ. Echelles presently range in groove frequency from 23 to 316 mm^–1. Blaze angles include, but are not limited to, 32°, 44°, 63.4°, 71.5°, 76° and 79°; the last four are chosen because their tangents are 2, 3, 4 and 5. Often echelles are specified by their "R number", which equals this tangent; for example, an R4 echelle in one with blaze angle arctan(4) = 76°.

All standard Richardson gratings have a part number according to the following format:

AA BBB CC DD - EEE x

• AA indicates the type of grating (e.g., diced, plano, grism).
• BBB indicates the size of the grating substrate.
• CC indicates the substrate material
• DD indicates the type of coating
• EEE indicates the master grating groove parameters
• x indicates the master grating type (e.g., ruled, holographic, echelle)

Please see Diffraction Grating Part Number System for additional information.

Commonly used sizes vary from 50 by 100 mm to 308 by 408 mm, where the shorter number specifies the groove length and the longer number the ruled width. Because dispersion is high, it is important to maintain constant groove spacing, which is why echelles are often replicated onto low thermal expansion materials. A recent development has been to make echelles larger than the standard 408 mm ruled width limit, in order to satisfy the needs of large astronomical spectrographs. This can be accomplished by the precise double replication of a single ruling onto a larger substrate; the resulting grating is called a mosaic.

Echelle gratings are subject to careful testing. Resolution close to the theoretical limit can be verified by interferometric and Foucault wavefront tests, and also by observation of the hyperfine spectra of mercury. Efficiency is determined with mercury and laser light sources, to ensure narrow spectral line widths. Exact blaze angles are derived by one of several methods.

Newport is pleased to discuss special and unusual applications that are not addressed by our build to order catalog diffraction gratings.  In some instances, none of the hundreds of master gratings we have in stock meet specifications, so a new master may be required. Please see Custom Diffraction Gratings for additional information on our capabilities.