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Types of Reflection gratings

Initially, reflection diffraction gratings used in spectrometers were ruled by engines, and since the 1960s are produced by holographic techniques. The latest generation is that of the echelle holographic grating, whose mirroring angle is obtained by ionic machining.

[Types of gratings]

The ruled gratings are cut line by line on special machines called ruling engines. Despite the very high precision of these machines, the gratings incorporated ruling errors which caused phantom spectra (ghosts, caused by periodic defaults) and stray light (caused by random defaults).

[holographic gratings]For holographic gratings, a polished support covered with photosensitive resin is exposed to the interference of two laser beams and processed chemically. This process allows strictly parallel and equally-spaced lines to be obtained. The grating is free of phantom spectra and stray light. However, the brightness achieved by the conventional holographic gratings is not as high as that of ruled gratings, as the groove profile is sinusoidal.

To improve brightness, this sinusoidal profile is then machined by ion bombardment, to achieve an echelle profile according to the selected mirroring angle (blaze angle). Such blazed holographic gratings are much brighter than conventional holographic gratings, as light emission is stronger into the direction of the blaze angle.

Authors: Jean Charles Lefebvre, Jobin-Yvon Emission and Th. Nelis

First published on the web: 15 December 1999.