An echelle diffraction grating differs from a conventional grating (called an echelette) in many ways. An echelle is coarse with fewer grooves per millimeter and is 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. An important limitation of echelles is that the orders overlap unless separated optically, for instance by a cross-dispersing element. A prism or echelette grating is often used for this purpose. This combination leads to an output format well matched to CCD arrays.
Coarse design with fewer grooves per millimeter
High-blaze-angle gratings used in high diffraction orders
High diffraction efficiency in both polarization states
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custom-echelle-gratings - Drawings
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Specifications
Master Grating Options
Echelle gratings are listed in order of groove frequency, with the lowest blaze angle listed first. The maximum ruled area is groove length x ruled width. Click on a Master Grating Code (last 4 digits of a grating's part number) below to view master grating efficiency curves. Use the request a quote to get a quote based on your requirements. For additional diffraction grating options and custom capabilities, please see the Features section.
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°.
Geometry of an echelle grating (Littrow).
Echelles Diffraction Gratings Applications
Echelles are most often used in applications where their high dispersion and resolution are important. This covers atomic spectrometry, 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. Echelles are found on several space spectrographs, including the Hubble Space Telescope.
Echelles Efficiency Behavior
The high efficiency of echelles is maintained near the Littrow angle. This means moving progressively through a series of diffraction orders to cover the entire spectral range. 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.
Catalog Part Number System
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)
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.
BBB
Substrate Dimensions (mm)
Ruled Area (mm)
114
50 x 100 x 16
46 x 92
019
60 x 150 x 25
56 x 142
015
110 x 110 x 16
102 x 102
025
110 x 220 x 30
102 x 206
032
135 x 265 x 45
128 x 254
044
165 x 320 x 50
154 x 306
036
220 x 265 x 45
204 x 254
043
220 x 420 x 74
204 x 410
045
320 x 420 x 74
306 x 410
Grating Substrate Material Options
The standard substrate material for small and medium-sized gratings is specially annealed borosilicate crown glass. Low expansion material can be supplied on request. Float glass may be used for small, diced gratings. In addition, replicas may be furnished on metal substrates, such as copper or aluminum, for applications with extreme thermal conditions. The substrate material codes CC are given below:
Substrate Material Code CC
Substrate Material
AL
Aluminum
BF
Borosilicate float or equivalent
BK
BK-7 glass or equivalent
CU
Copper
FL
Float glass
FS
Fused silica or equivalent
LE
Low-expansion glass
SP
Special glass (unspecified)
TB
BK-7, transmission grade
TF
Fused silica, transmission grade
UL
Corning ULE® glass
ZD
Schott Zerodur®
Diffraction Grating Coating Options
All reflection gratings include a standard aluminum (Al) reflectance coating (Coating Code "01"). Gratings can also be replicated in gold (Au), or overcoated with magnesium fluoride (MgF2) or silver (Ag), to enhance reflectivity in certain spectral regions. The coating material codes DD are shown below:
Coating Code DD
Coating Material
Application
01
Aluminum (Al)
General purpose applications.
02
Gold (Au)
Offers higher reflectivity in the infrared.
03
Magnesium Fluoride (MgF2)
Used to prevent oxidation of aluminum (Al) coatings, which helps maintain high reflectivity in the ultraviolet over time.
06
Protected Silver (Ag)
Offers higher reflectivity in the visible and near infrared. Silver is protected from tarnishing by a dielectric coating, which helps maintain reflection over time.
Testing of Echelles Gratings
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.
Custom Diffraction Gratings
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.
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