Features

• 45% to 65% Efficiencies at Peak Wavelength
• Versions Optimized for the UV and Visible Spectrum
• Relatively Independent of Angle of Incidence
• Substrate: Borofloat
• Three Sizes: 12.7 x 12.7 x 6.0 mm, 25 x 25 x 6.0 mm, and 50 x 50 x 9.5 mm

Thorlabs offers a selection of holographic diffraction gratings optimized for high end applications that require a high signal to noise ratio. Holographic gratings feature low stray light, which nearly eliminates all ghosting that ruled gratings suffer from and makes them an ideal solution for applications such as Raman spectroscopy that require high signal to noise ratios.  Our holographic gratings are optimized for the UV or Visible spectral ranges and are available in three sizes with groove densities from 600 to 3600 lines/mm. Three sizes are available, 12.7 x 12.7 x 6.0 mm, 25 x 25 x 6.0 mm, and 50 x 50 x 9.5 mm, with dimensional tolerances of ± 0.5 mm.

Mounts and Adapters

Thorlabs offers a variety of mounts and adapters for precise and stable mounting and aligning square optics. All of Thorlabs' gratings can be mounted directly into the KM100C Right-Handed or KM100CL Left-Handed Kinematic Rectangular Optic Mount. Gratings can also be mounted in one of three Kinematic Grating Mount Adapters which can be used with any of Thorlabs' Ø1" Mirror Mounts, including the POLARIS-K1 ultra-stable kinematic mirror mount. See the Mounting tab for further info.

Warning:

Optical gratings can be easily damaged by moisture, fingerprints, aerosols, 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.

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.

Diffraction Gratings Tutorial

Diffraction gratings, either transmissive or reflective, can separate different wavelengths of light using a repetitive structure embedded within the grating. The structure affects the amplitude and/or phase of the incident wave, causing interference in the output wave. In the transmissive case, the repetitive structure can be thought of as many tightly spaced, thin slits. Solving for the irradiance as a function wavelength and position of this multi-slit situation, we get a general expression that can be applied to all diffractive gratings,

(1)

Figure 1. Transmission Grating

known as the grating equation. The equation states that a grating with spacing , of ^th order, will diffract light at a wavelength of at an angle of . The diffracted angle, , is the output angle as measured from the surface normal of the grating. It is easily observed from Eq. 1 that for a given order , different wavelengths of light will exit the grating at different angles. For white light sources, this corresponds to a continuous, angle-dependent spectrum.

Transmission Grating

One popular style of grating is the transmission grating. This type of grating is created by scratching or etching a transparent substrate with a repetitive, parallel structure. This structure creates areas where light can scatter. A sample transmission grating is shown in Figure 1.

The transmission grating, shown in Figure 1, is comprised of a repetitive series of grooves of narrow width and separation . The incident light impinges on the grating at an angle , as measured from the surface normal. The light of order exiting the grating leaves at an angle of , relative to the surface normal. Utilizing some geometric conversions and the general grating expression (Eq. 1) an expression for the transmissive diffraction grating can be found:

(2)

Figure 2. Reflective Grating

Reflective Grating

Another very common diffractive optic is the reflective grating. A reflective grating is made by depositing a metallic coating on an optic and ruling parallel grooves in the surface. Reflective gratings can also be made of epoxy and/or plastic imprints from a master copy. In all cases, light is reflected off of the ruled surface at different angles corresponding to different orders and wavelengths. An example of a reflective grating is shown in Figure 2. Using a similar geometric setup as above, the general expression for the reflective grating is identical to the tranmission grating equation (see Eq. 2).

Both the reflective and transmission gratings suffer from the fact that the zeroth order mode contains no diffraction pattern and appears as a surface reflection or transmission, respectively. Solving Eq. 2 for this condition, = , we find the only solution to be =0, independent of wavelength or gratings spacing. At this condition, no wavelength-dependent information can be obtained, and all the light is lost to surface reflection or transmission.

This issue can be resolved by creating a repeating surface pattern, which produces a different surface reflection geometry. Gratings of this type are commonly referred to as blazed (or ruled) gratings. An example of this repeating surface structure is shown in Figure 3.

Blazed (Ruled) Grating

Figure 4. Blazed Grating, 0^th Order Reflection

Figure 3. Blazed Grating Geometry

The blazed grating shown in Figure 3 is characterized by two main variables: , the groove or facet spacing, and , the blaze angle. The blaze angle is the angle between the surface structure and the surface parallel. It is also the angle between the surface normal and the facet normal.

The blazed grating features the same geometries as the transmission and reflection gratings discussed thus far; the incident and exit angles are determined from the surface normal of the grating. However, the significant difference is the surface reflection geometries are determined based on the blaze angle, , and NOT the surface normal. This results in the ability to change the diffraction geometries by only changing the blaze angle of the grating.

The 0^th order reflection from a blazed grating is shown in Figure 4. The incident light at angle is reflected at for = 0. From Eq. 2, the only solution is = . This is analogous to specular reflection from a flat surface.

Figure 5. Blazed Grating, Specular Reflection

The specular reflection from the blazed grating is different from the flat surface due to the surface structure, as shown in Figure 5. The specular reflection, , from a blazed grating occurs at the blaze angle geometry. Performing some simple geometric conversions, one finds that

                                                                                                (3)

Utilizing Eqs. 2 and 3, we can find the grating equation for a blazed grating at twice the blaze angle:

                                                                                       (4)

There is one final case that plays an important role in monochromators and spectrometers, the Littrow configuration. In this configuration, the angle of incidence of the incoming and diffracted light are the same, = , and > 0 so

                                                                                      (5)

The Littrow configuration angle, , is dependent on the most intense order ( = 1), the design wavelength, , and the grating spacing . It is easily shown that the Littrow configuration angle, , is equal to the blaze angle, , at the design wavelength. The Littrow / Blaze angles for all Thorlabs' Blazed Gratings can be found in the grating specs tables.

It is easily observed that the wavelength dependent angular separation increases as the diffracted order increases for light of normal incidence (for =0, increases as increases). There are two main drawbacks for using a higher order diffraction pattern over a low order one: (1) a decrease in efficiency at higher orders and (2) a decrease in the free spectral range, , defined as:

                                                                                             (6)

where is the central wavelength, and is the order.

The first issue with using higher order diffraction patterns is solved by using an Echelle grating, which is a special type of ruled diffraction grating with an extremely high blaze angle. The high blaze angle is well suited for concentrating the energy in the higher order diffraction modes. The second issue is solved by using another optical element: grating, dispersive prism, or other dispersive optic, to sort the wavelengths/orders after the Echelle grating.

Figure 6. Holographic Grating

Holographic Gratings

While blazed gratings offer extremely high efficiencies at the design wavelength, they suffer from periodic errors, such as ghosting, and relatively high amounts of scattered light, which could negatively affect sensitive measurements. Holographic gratings are designed specially to reduce or eliminate these errors. The drawback of holographic gratings compared to blazed gratings is reduced efficiency.

Holographic gratings are made from master gratings by similar processes to the ruled grating. The master holographic gratings are typically made by exposing photosensitive material to two interfering laser beams. The interference pattern is exposed in a periodic pattern on the surface, which can then be physically or chemically treated to expose a sinusoidal surface pattern. An example of a holographic grating is shown in Figure 6.

 

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.