Diffractive Optical Elements, or simply DOEs, are optical components that perform complex transformations on light beams. Unlike more conventional optical components such as lenses and mirrors that work under the laws of refraction and reflection, DOEs work on the principle of diffraction in which the wave nature of light is dominant.
Diffraction is defined as light interference among many waves that originate from a single input wave. These other subwaves, or wavelets, start appearing as a consequence of obstacles or delays in the path of the input wave. The overall interference of this set of waves gives rise to a diffraction pattern that can be observed if a screen is placed at some distance from the location of the obstacles. The obstacles can be of any sort. They can consist of limiting apertures , blocking elements or different delays.
There is a mathematical formalism that describes to a very good accuracy the overall diffraction pattern if the input wave is known as well as the shape and size of the obstacles or apertures on its path. This opens the possibility of obtaining a target diffraction pattern by tuning the shape and arrangement of the obstacles on the path according to back projection algorithms based on the mathematical models of diffraction. A structure obtained via this process is referred to as a diffractive optical element.
A diffractive optical element usually consists of an array of blocking elements arranged in a predefined way such that by means of diffraction they give rise to a desired diffraction pattern, also referred to as the far field of the diffraction optical element.
Now, for increased throughput, the array of blocking obstacles can be substituted by an array of transparent obstacles with different optical paths. Thus, each discrete area of the element can impart a path difference that is different from neighbouring areas. This is accomplished by having different areas on a transparent substrate like glass or acrylic. Each area has a different height, creating a delay that is proportional to the desired path difference once the refractive index of the material is considered.
The far field, or diffraction pattern of a diffractive optical element, can be the result of a great variety of transformations to the input beam. It can be used to generate different geometrical shapes, different irradiance distributions or beam splitting operations, to name a few. In some contexts, DOEs are also referred to as computer generated holograms, or CGHs.
