Diffractive Optical lenses

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Diffractive Optical lenses

Diffractive optical lenses, also known as Fresnel zone plates, are focusing optical components which use the wave nature of light to perform their optical function, unlike conventional refractive lenses which are based on geometrical ray optics concepts. Therefore, the diffractive optical lenses are flat and can be very thin, regardless of their focal length and overall diameter. To put this in perspective, in a conventional lens the diameter and the focal length will constraint the required minimum thickness that is necessary for the lens to be feasible.  Thus, for a lens with small focal length and big diameter, the resulting lens can end up being very thick.

The design of the diffractive lens starts by calculating the parameters from a normal lens that will accomplish the same function in terms of focal length distance or magnification. The resulting curvature is then sampled at equidistant points. With the data on the local curvature the next step is to collapse the entire curvature into a plane, such that the sag of each local area, the sampling points, starts at the same location along the optical axis. The resulting structure is referred to as Fresnel lens and is the kind of lens that can be found in lighthouses. Going one step further in the simplification of the new structure, the local curvature can be substituted by the average slope in that specific area. This will result in a structure that is easier to manufacture. Now, for making a diffractive optical lens, the local curvature can be approximated by discrete steps of light modulation, so the end structure is pixelated as in the case of all diffractive optical elements.

Diffractive Optical lenses are a very practical solution for applications in which a lightweight element is necessary. For the same reason, diffractive optical lenses are a befitting component for miniaturisation. Also, given the small volume required, the thermal effects on effective focal length are considerably less than for an equivalent conventional lens. As with all diffractive optical elements they work best with narrowband sources so they can be used more effectively with laser sources or even with LEDs. Given that any curvature information can be encoded into a diffractive optical lens, highly aspheric profiles can be used with no penalty whatsoever in terms of manufacturability. This is in contrast to conventional lenses that need to have an aspheric departure. In that case, the correct tooling had to be placed in beforehand and the limitations had to be well understood. Furthermore, the properties that can be encoded into a diffractive optical lens go beyond an aspheric profile for correcting aberrations. Some other functions like top hat beam generation can also be added.

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