Features

• Low Period Gratings for Use in Higher Orders
• Ideal for High Resolution Spectroscopy
• Typical resolutions are 80% to 90% of Theoretical
• Bare Aluminum Reflective Coating
• Substrate: Float Glass or Pyrex

High Resolution Echelle Gratings are special low period gratings designed for use in the high orders. They are generally used with a second grating or prism to separate overlapping diffracted orders. Supplied on precision glass substrates, Echelles have a resolution of 80-90% of theoretical. Thorlabs offers a line of high resolution Echelle gratings with high blaze angles making them an ideal solution for high resolution spectroscopy.

Warning:

The surface of a diffraction grating can be easily damaged by fingerprints, aerosols, moisture or the slightest contact with any abrasive material. Gratings should only be handled when necessary and always held by the sides. Latex gloves or a similar protective covering should be worn to prevent oil from fingers from reaching the grating surface. No attempt should be made to clean a grating other than blowing off dust with clean, dry air or nitrogen. Solvents will likely damage the grating's surface.

Thorlabs uses a clean room facility for assembly of gratings into mechanical setups. If your application requires integrating the grating into a sub-assembly or a setup please contact us to learn more about our assembly capabilities.

Special Considerations with Echelle Gratings

The Grating Equation: The general grating equation may be written as: nλ = d(sin θ + sin θ') where n is the order of diffraction, λ is the diffracted wavelength, d is the grating constant (the distance between grooves), θ is the angle of incidence measured from the normal and θ' is the angle of diffraction measured from the normal.

Free Spectral Range: Free spectral range is the maximum spectral bandwidth that can be obtained in a specified order without spectral interference (overlap) from adjacent orders. As grating spacing decreases, the free spectral range increases. It decreases with higher orders. If λ1 and λ2 are lower and upper limits, respectively, of the band of interest, then:

Free Spectral Range = λ2 - λ1 = λ1/n

Using an Echelle Grating: The extremely high blaze angle of the Echelle grating concentrates the energy in the higher orders. In the simplest case where light is incident on the grating at an angle of 0° the grating equation simplifies to nλ = d sin θ' and if solved for sin θ' it becomes:

sin θ' = nλ / d

From this it follows that in higher orders the angular separation between two wavelengths becomes greater. Imagine two lines, one at 600 nm and the other at 605 nm, incident on a grating with 31.6 lines/mm. From the equation above, at n=1 the angular separation is 0.009° but at n=40 the angular separation 0.6°. The disadvantage is the reduced free spectral range, which decreased from 630 nm (630 nm/1) to 15.8 nm (630 nm/40). Often a dispersing prism is used in combination with an Eschelle grating for order sorting.

Thorlabs offers 3 types of Reflection Gratings:

Ruled

Ruled gratings can achieve higher efficiencies than holographic gratings due to their blaze angles. They are ideal for applications centered at the blaze angle. Thorlabs offers replicated ruled diffraction gratings in a variety of sizes and blaze angles.

Holographic

Holographic gratings have a low occurance of periodic errors which results in limited ghosting, unlike ruled gratings. The low stray light of these gratings make them ideal for applications where the signal-to-noise ratio is critical, such as Raman Spectroscopy.

Echelle

Echelle gratings are low period gratings designed for use in the high orders. They are generally used with a second grating or prism to separate overlapping diffracted orders. The are ideal for applications such as high-resolution spectroscopy.

Thorlabs offers 3 types of Transmission Gratings:

UV Transmission

As with all of our transmission gratings, Thorlabs' UV transmission gratings disperse incident light on the opposite side of the grating at a fixed angle. They are ruled and blazed for optimum efficiency in the UV range, are relatively polarization insensitive, and have an efficiency comparable to that of a reflection grating optimized for the UV spectrum. They are ideal for applications that require fixed gratings such as spectrographs.

VIS Transmission

As with all of our transmission gratings, Thorlabs' VIS transmission gratings disperse incident light on the opposite side of the grating at a fixed angle. They are ruled and blazed for optimum efficiency in the VIS range, are relatively polarization insensitive, and have an efficiency comparable to that of a reflection grating optimized for the VIS spectrum. They are ideal for applications that require fixed gratings such as spectrographs.

Near IR Transmission

As with all of our transmission gratings, Thorlabs' NIR transmission gratings disperse incident light on the opposite side of the grating at a fixed angle. They are ruled and blazed for optimum efficiency in the NIR range, are relatively polarization insensitive, and have an efficiency comparable to that of a reflection grating optimized for the NIR spectrum. They are ideal for applications that require fixed gratings such as spectrographs.

Selecting a grating requires consideration of a number of factors, some of which are listed below:

Efficiency:
Ruled gratings generally have a higher efficiency than holographic gratings. However, holographic gratings tend to have less efficiency variation accross their surface due to how the grooves are made. The efficiency of ruled gratings may be desireable in applications such as fluorescence excitation and other radiation-induced reactions.

Blaze Wavelength:
Ruled gratings have a sawtooth groove profile created by sequentially etching the surface of the grating substrate. As a result, they have a sharp peak around their blaze wavelength. Holographic gratings are harder to blaze, and tend to have a sinusoidal groove profile resulting in a less intense peak around the design wavelength. Applications centered around a narrow wavelength range could benefit from a ruled grating blazed at that wavelength.

Wavelength Range:
Groove spacing determines the optimum spectral range a grating covers and is the same for ruled and holographic gratings having the same grating constant. As a rule of thumb, the first order efficiency of a grating decreases by 50% at 0.66λB and 1.5λB, where λB is the blaze wavelength. Note: No grating can diffract a wavelength greater than 2 times the groove period.

Stray Light:
Due to a difference in how the grooves are made, holographic gratings have less stray light than ruled gratings. The grooves on a ruled grating are machined one at a time which results in a higher frequency of errors. Holographic grooves are made all at once which results in a grating that is virtually free of errors. Applications such as Raman spectroscopy, where signal-to-noise is critical, can benifit from the limited stray light of the holographic grating.

Resolving Power:
The resolving power of a grating is a measure of its ability to spatially separate two wavelengths. It is determined by applying the Rayleigh criteria to the diffraction maxima; two wavelengths are resolvable when the maxima of one wavelength coincides with the minima of the second wavelength. The chromatic resolving power (R) is defined by R = λ/Δλ = nN, where Δλ is the resolvable wavelength difference, n is the diffraction order, and N is the number of grooves illuminated.

For further information about gratings and selecting the grating right for your application, please visit our Grating Tutorial.

Caution:

The surface of a diffraction grating can be easily damaged by fingerprints, aerosols, moisture or the slightest contact with any abrasive material. Gratings should only be handled when necessary and always held by the sides. Latex gloves or a similar protective covering should be worn to prevent oil from fingers from reaching the grating surface. Solvents will likely damage the grating's surface. No attempt should be made to clean a grating other than blowing off dust with clean, dry air or nitrogen. Scratches or other minor cosmetic imperfections on the surface of a grating do not usually affect performance and are not considered defects.

 

Thorlabs' selection of gratings can be mounted in the KGM Series Grating Mount Adapters, as shown in the photograph to the right. These mounts can accommodate gratings and rectangular mirrors up to 60 mm tall and are compatible with all Ø1", front-loading, unthreaded mirror mounts.

To secure the grating, simply slide the upper and lower clamps on the adapter to match the desired optic height. These clamps slide in machined grooves and are tightened into place with two M3 screws that are located on the back side of the mount (see below).

Back Side of KGM Series Adpater

The 3-point, kinetic mounting mechanism consists of two bottom lines of contact and a top flat-spring contact, as shown in the photograph to the right. A nylon-tipped, locking setscrew, which is located on the top clamp, provides added holding force.