Blazed grating

A blazed grating – also called echelette grating (from French "échelle" = ladder) – is a special type of diffraction grating. It is optimized to achieve maximum grating efficiency in a given diffraction order. For this purpose, maximum optical power is concentrated in the desired diffraction order while the residual power in the other orders (particularly the zeroth) is minimized. Since this condition can only exactly be achieved for one wavelength, it is specified for which "blaze wavelength" the grating is optimized (or "blazed"). The direction in which maximum efficiency is achieved is called the "blaze angle" and is the third crucial characteristic of a blazed grating directly depending on blaze wavelength and diffraction order.
Blaze angle.
Like every optical grating, a blazed grating has a constant line spacing formula_1, determining the magnitude of the wavelength splitting caused by the grating. The grating lines possess a triangular, sawtooth-shaped cross section, forming a step structure. The steps are tilted at the so-called blaze angle formula_2 with respect to the grating surface. Accordingly, the angle between step normal and grating normal is formula_2.
The blaze angle is optimized to maximize efficiency for the wavelength of the used light. Descriptively, this means formula_2 is chosen such that the beam diffracted at the grating and the beam reflected at the steps are both deflected into the same direction. Commonly blazed gratings are manufactured in the so-called Littrow configuration.
Littrow configuration.
The Littrow configuration is a special geometry in which the blaze angle is chosen such that diffraction angle and incidence angle are identical. For a reflection grating, this means that the diffracted beam is back-reflected into the direction of the incident beam (blue beam in picture). The beams are perpendicular to the step and therefore parallel to the step normal. Hence it holds in Littrow configuration formula_5.
Diffraction angles at the grating are not influenced by the step structure. They are determined by the line spacing and can be calculated according to the grating equation:
where:
For the Littrow configuration, this becomes formula_15. By solving for formula_2 the blaze angle can be calculated for arbitrary combinations of diffraction order, wavelength and line spacing:
Blazed transmission grating.
Blazed gratings can also be realized as transmission gratings. In this case the blaze angle is chosen such that the angle of the desired diffraction order coincides with the angle of the beam refracted at the grating material.
Echelle grating.
A special form of a blazed grating is the "echelle grating". It is characterized by particularly large blaze angle (>45°). Therefore, the light hits the short legs of the triangular grating lines instead of the long legs. Echelle gratings are mostly manufactured with larger line spacing but are optimized for higher diffraction orders.
Echelle gratings are useful in planet-finding astronomy, and are used on the successful HARPS and PARAS (PRL Advanced Radial-velocity All-sky Search) spectrograph.