Diffractive Optical Elements are unique optical components that are used to perform complex transformations on any input beam. They work by harnessing those effects that can arise only from the wave nature of light. Thus, the description of light as merely rays that travel in a straight direction along a given medium is certainly not sufficient to explain the changes that a beam undergoes when traversing a diffractive optical element.
A diffraction grating, for example, is a type of diffractive optical element whose behaviour cannot be explained by geometrical optics. Once a beam passes through a diffraction grating, the wavefront of the beam is said to be modulated by the periodic structure of the diffraction grating. This modulation gives rise to other smaller waves or wavelets that interfere among themselves that in turn originate a new wavefront envelope. This process is what is known as diffraction. In the case of the diffraction grating, two main new wave fronts are generated and each one travels in a different angular direction. If the input beam is monochromatic, the diffraction grating acts as a beam splitting element.
In general a diffractive optical element can be designed to perform any type of beam splitting operations, not just a pair of beams. The output distribution can be an array of basically any number of beams and arranged in any arbitrary pattern.
Another light transformation that can be obtained with a diffractive optical element is the generation of geometrical shapes. This kind of beam transformation can be very useful in many practical fields of interest such as structured illumination or materials processing.
A diffractive optical element is also very often used to convert the Gaussian radiance, that is present in many laser beams, into a radiance that is more uniform and that has clear boundaries. A diffractive optical element that does this type of transformation is referred to as Top Hat or Flat Top diffractive optical element.
Diffractive optical elements work then by harnessing the fact that diffraction can be used to alter completely the shape of an output wavefront. To design a diffractive optical element, the first step is to define the target which is the desired output beam distribution at a plane sufficiently far away from the element. Then, the relation between the input plane, which is the plane of the diffractive optical element, and the output plane is given by the so called diffraction integral which in turn is given by a Fourier transform using the appropriate coordinates.
You can find more information about diffractive optical elements here:
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