Visible Transmission Gratings
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Top Image: On one edge of the grating, an arrow parallel to the grating's surface indicates the blaze direction.
Bottom Image: On the opposite edge of the grating, an arrow perpendicular to the grating's surface indicates the transmission direction.
Thorlabs' Visible Transmission Gratings are ideal for fixed grating applications such as spectrographs. The incident light is dispersed on the opposite side of the grating at a fixed angle. Transmission gratings offer low alignment sensitivity, which helps minimize alignment errors. In most cases, the efficiency of these gratings is comparable to that of reflection gratings such as Ruled Gratings or Holographic Gratings when used in the same wavelength range.
Transmission gratings require relatively low groove densities to maintain high efficiency. As the diffraction angles increase with higher groove densities, the refractive properties of the substrate material limits the transmission at the higher wavelengths and performance drops off. The grating dispersion characteristics, however, lend themselves to compact systems utilizing small detector arrays. These gratings are also relatively polarization insensitive. Our visible transmission gratings are offered with four different groove densities in three different sizes. Please see the Selection Guide tab to choose the right grating type for your application.
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:
Thorlabs offers 3 types of Transmission Gratings:
Selecting a grating requires consideration of a number of factors, some of which are listed below:
For further information about gratings and selecting the grating right for your application, please visit our Grating Tutorial.
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 experssion that can be applied to all diffractive gratings,
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.
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:
Figure 2. 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, 0th Order Reflection
Figure 3. Blazed Grating Geometry
The blazed grating, also known as the echelette grating, is a specific form of reflective or transmission diffraction grating designed to produce the maximum grating efficiency in a specific diffraction order. This means that the majority of the optical power will be in the designed diffraction order while minimizing power lost to other orders (particularly the zeroth). Due to this design, a blazed grating operates at a specific wavelength, known as the blaze wavelength.
The blaze wavelength is one of the three main characteristics of the blazed grating. The other two, shown in Figure 3, are , 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 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 0th 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
Utilizing Eqs. 2 and 3, we can find the grating equation for a blazed grating at twice the blaze angle:
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:
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 and relatively low groove density. 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
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.