Diffraction Gratings type and choice
Diffraction gratings are optical components critical for a wide variety of applications including spectrometers, other analytical instruments, telecommunications, and laser systems. Gratings contain a microscopic and periodic groove structure - which splits incident light into multiple beam paths through diffraction, causing light of different wavelengths to propagate in different directions. This makes the function of diffraction gratings similar to that of dispersion prisms, although the prism separates wavelengths through wavelength-dependent refraction instead of diffraction (Figure 1).
Figure 1: While dispersion prisms separate wavelengths through refraction (top), diffraction gratings instead separate wavelengths through diffraction because of their surface structure (bottom).
A grating’s groove pattern, or the spacing between grooves (d), determines the angles at which different orders are diffracted. In some situations, the groove spacing may be designed to vary across the grating for different levels of diffraction across the part. The grating’s groove profile, on the other hand, describes their shape and determines how much light is diffracted and how much simply reflects off of, or transmits through, the grating. Efficiency charts are used to characterize the percentage of light that will be diffracted at each wavelength. Efficiency will be unique for different polarization states, so efficiency charts usually show different curves for both s- and p-polarization. Metallic or dielectric coatings are often added to gratings to make them reflective and/or increase efficiency.
What Should You Look for When Choosing a Grating?
When choosing a grating it is important to specify the wavelength range, blaze wavelength (which is the wavelength in the diffracted spectrum with the highest efficiency), and blaze angle. The blaze angle describes the first order diffracted angle of the blaze wavelength. At this angle, α and β are equal in Equation 1 and incident light is diffracted back in the exact same direction it came from. This situation is also called the Littrow configuration. Coming close to this angle in a system results in maximum efficiency.
Groove density, or frequency, is typically specified, and this is the inverse of the groove spacing (d). A key property of the optical system is its level of dispersion, but this depends on both the properties of the grating and how it is used. A grating itself cannot be given a specification detailing how a certain amount of rotation corresponds with a certain separation of wavelengths without knowing other system details. A grating’s resolving power may also be specified, which is related to the system’s spectral resolution. However, this resolution depends on both the grating and the system’s entrance and exit slits. The grating’s resolving power (R) is dependent on the spectral order (m) and the number of grooves under illumination (N):
There are often so many grooves under illumination that the entrance and exit slits are the limiting factors for system resolution, not the grating. Efficiency curves may also be useful for verifying the level of diffraction across all wavelengths that will be used in the application.
Gratings should be at least as large as the incident light cone or beam, or else light from the edges will be lost. Therefore, a grating should always be underfilled to prevent stray light from bouncing around the system and creating false signals.
Types of Gratings
Reflection vs. Transmission Gratings
The two broadest categories of diffraction gratings are reflection and transmission gratings. Figures 1 and 2 show reflection gratings, which are essentially mirrors with microscopic grooves. All diffracted orders reflect off of the grating at different angles. Transmission gratings are like lenses with microscopic grooves, and all diffracted orders transmit through the grating but are offset by angles following Equation 1. Reflection gratings are also commonly known as reflective gratings and transmission gratings are also known as transmissive gratings.
Ruled vs. Holographic Gratings
Both reflection and transmission gratings can be further broken down into ruled or holographic gratings, which differ in the way that the groove profile is created. The grooves in ruled gratings are mechanically scribed, or cut, into the part, while the grooves in holographic gratings are optically introduced. In holographic gratings, a light-sensitive material called photoresist is deposited onto the substrate and exposed to an optical interference pattern which interacts with the photoresist. Chemicals are then used to remove remaining photoresist, leaving behind a grating pattern. Ruled gratings typically have triangular grooves, such as those shown in Figure 1, while holographic gratings generally have sinusoidal grooves, (Figures 3 and 4).
Figure 3: Ruled diffraction gratings typically feature triangular grooves.
Figure 4: Holographic diffraction gratings typically feature sinusoidal grooves.
Echelle gratings feature a higher groove spacing, or lower groove density, than other gratings, typically by around a factor of 10 but sometimes as high as a factor of 100. Illuminating an Echelle grating at a high angle of incidence (α) will result in high dispersion, resolving power, and efficiency with a low dependence on polarization. These gratings are ideal for situations where high resolution is needed, such as sensitive astronomical instruments and systems striving for atomic resolution.
Plane vs. Concave Gratings
All of the above grating types can again be broken down into plane (or plano-) and concave gratings, which describes their overall shape. Plane gratings are flat and much more common. If their grooves are straight and equally spaced, the grating is flat, and incident light is collimated, all of the diffracted light will be collimated. This is beneficial in many applications because the focal properties of the system are wavelength independent. Plane gratings also generally reduce system complexity compared to concave gratings. Concave gratings are curved and therefore either converge or diverge light. This can be useful for reducing the total number of optical components needed in a system, but the focal properties of the system will be wavelength-dependent.