Echelle gratings differ from conventional ruled gratings in many ways. Echelle gratings are coarse (i.e., they have few grooves per millimeter) and are used at high angles in high diffraction orders. The virtue of an echelle lies in its high efficiency and low polarization effects over large spectral intervals. Together with high dispersion, this leads to compact, high-resolution instruments.
Echelles are most often used in applications where high dispersion and resolution are important, such as atomic absorption spectroscopy, laser tuning, and astronomy. Since they operate in many diffraction orders, echelles are capable of wide wavelength coverage, being used from 100 nm into the infrared. Richardson Gratings have been used in several space spectrographs, including the Hubble Space Telescope.
The two design parameters that define an echelle grating are its groove frequency and its blaze angle. Available groove frequencies presently range from 23 to 316 g/mm. Blaze angles include, but are not limited to, 32°, 44°, 63.4°, 71.5°, and 79°; the last four are chosen because their tangents are 2, 3, 4, and 5. Echelles are often specified by the "R number", which equals this tangent. For example, an R4 echelle is one with blaze angle arctan(4) = 76°. Applications requiring angles above 63.4° have been in use since the mid 1980s.
Overlapping of diffraction orders is an important limitation of echelle gratings. These orders can be separated optically by using a cross-dispersing element, such as a prism or echelette grating. The combination of grating and dispersing element leads to an output format well matched to CCD or CID detectors. Echelles are often used in or near Littrow configuration, in which the angle of incidence, alpha, equals the angle of diffraction, beta.
Commonly used sizes vary from 50 by 100 mm to 306 by 408 mm, where the shorter number specifies the groove length and the longer number the ruled width. Smaller and intermediate sizes are also available. Because dispersion is high, it is important to maintain constant groove spacing, which is why echelles are often replicated onto materials with low thermal expansion.
In order to satisfy the needs of large astronomical spectrographs our engineering team has developed a process to make echelles larger than the standard 408 mm ruled width limit. This is accomplished by the precise double replication of a single ruling onto a larger substrate; the resulting grating is called a mosaic.
The majority of echelles are supplied with aluminum surfaces. For applications below 200 nm, it is necessary to overcoat echelles with a thin layer of magnesium fluoride (MgF2) to prevent the oxidation of the aluminum, which could result in the loss of ultraviolet efficiency.
The high efficiency of echelles is maintained near the Littrow angle. In order to cover the entire spectral range, echelle based system must move progressively through a series of diffraction orders. Within each order, the efficiency will be maximum at the middle (typically reaching 50 to 75%), but dropping to about one-half these values at the crossover points. Interorder efficiency behavior closely follows scalar theory; however, when the diffraction order of use is low, or when the diffraction angles are high, the detailed efficiency properties are governed by electromagnetic (vector) theory. Accurate theoretical formulation of this case is a recent achievement.
Echelle gratings are subjected 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. |