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.
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).
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
Authors: Jean Charles Lefebvre, Jobin-Yvon Emission and Th. Nelis
First published on the web: 15 December 1999.