What are Diffractive Optical Elements?

Diffractive Optical Elements are a unique family of optical elements that are used to manipulate and alter the characteristics of a beam such as a shape, radiance profile, or even the depth of focus. The applications for diffractive optical elements are thus wide and varied.

To accomplish all these transformations on optical beams, diffractive optical elements act on the wave nature of light by harnessing the diffraction effects. Diffraction, in essence, is the interference effect that takes place among many different beams. The wave description of light states that a beam’s wavefront is the result of the superposition of many smaller waves or wavelets. Thus, a diffractive optical element accomplishes the desired beam transformation by modulating the overall wavefront locally. Thus, the surface of a diffractive optical element is composed of an array of modulating elements, which can be likened to pixels, that impart a constant phase change or retardance to the corresponding zone on the beam. The phase value on each pixel is not random but is calculated by one of the various algorithms available for diffractive optics.

Diffractive Optical Elements can be used in many applications. One application is to perform beam splitting. In this application, a single beam is used to create an array of any number of beams. Each new beam can share the same characteristics as the original beam. This beam splitting, or fan out, the operation can be used for metrology or in skin aesthetic treatments.

Another common application for diffractive optical elements is changing the original radiance profile of a beam to an entirely different profile. The input beam, for instance, is usually a Gaussian beam as it is intrinsic to many laser systems. The output beam can be a Top Hat, which is a radiance profile in which there is an area with constant energy with very steep edges. This kind of transformation can be achieved with a single Diffractive Optical element.

Along the same line, the output beam can be changed from a circle to any other geometrical shape. This operation can be harnessed in many industrial fields that require structured illumination or more precise target areas for laser ablation, for instance.

Diffractive optical elements can also be used to increase the depth of focus of an imaging system by reproducing the behavior of a refractive axicon. Indeed, a diffractive axicon can offer many advantages over the refractive counterpart, as it has no undefined central area and maintains a high-quality wavefront profile. 

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