Diffractive Optical Element

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Diffractive Optical Element

Diffractive optical elements are unique optical elements that can be used in almost all current laser applications. They can introduce rather complex transformations to an input beam. In spite of this remarkable fact, they can be as thin and lightweight as conventional spectral filter windows.

The kind of complex transformations that can be accomplished with diffractive optical elements extends across many optical beam types. They can be used to alter the smooth Gaussian radiance pattern, that is intrinsic to many laser systems, into, for example, a Top Hat radiance pattern. This Top hat radiance pattern is much more convenient for any laser application, especially for those in which the laser is used for ablation, or material processing in general. A Top Hat radiance pattern will increase the uniformity of the laser spot irradiance and will prevent light falling into adjacent areas.

 Another type of optical transformation is beam splitting. In this case the diffractive optical element is used as a beam array generator. The laser array is created from a single input beam and each new beam on the array will have the same beam characteristics as the original one. This is a very versatile optical transformation as it can be used in structured illumination, for instance, and without having to increase the optical system envelope substantially.

There are many other optical transformations that can be performed with diffractive optical elements. These elements are capable of achieving all this because they exploit the wave nature of light and harness the diffractive effects, which is basically just interference happening among many waves. This is in stark contrast to conventional refractive elements such as lenses or the even more complex micro lens arrays. In these refractive components, the diffraction effects are seen as a degrading aspect and, when possible, the lens design aims at reducing them by increasing the clear apertures of the optical system. 

Diffractive optical elements consist of an array of very small modulating elements, with length scales that can be as small as a micrometre. Each modulating element imparts a local phase delay to a corresponding small zone on the overall beam’s wavefront. That phase delay is achieved by simply adjusting the thickness of the substrate material (usually glass) , which has a higher refractive index compared to air. With all these phase delays, from every pixel, acting locally at once on the input wavefront, diffraction phenomena take place and a new output wavefront emerges.

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