Conventional optical lenses designed following the guidelines of geometrical or ray optics. At the heart of this design discipline lies the well-known principle of light refraction that predicts the exact bending that rays undergo when going from one medium to another. Optical glass surfaces then grinded to give them a certain shape. So that the final curvature induces the necessary local change in ray direction to form an overall focus at some distance away from the lens.
There is another type of lens that is less frequently encountered and that offers some unique properties. Such a type of lens is the diffractive lens that works by employing the principles of light diffraction, as opposed to light refraction. Diffraction is the interference phenomena that arises among many waves and thus when designing a diffractive optical element, it is the wave nature of light that is predominant over the ray description of light.
What are Diffractive Lens?
A diffractive lens is very much related to the Fresnel lenses that used in lighthouses. The design of the Fresnel lens starts by calculating the geometrical optical lens counterpart. Once the shape of this lens that meets the intended application requirements obtained, the lens partitioned into radial zones.
The starting sag of each zone collapsed into an input plane such that each zone starts at the same coordinate along the optical axis. This process will result in a lens that is thinner than the starting lens. This important fact permits having a relatively lightweight element operating at the lighthouse.
A diffractive lens goes one step further. In the Fresnel lens each radial zone has a continuous curvature. The partitioning into rings done at some arbitrary height. With each ring serving as a refractive surface, resulting in shaping and other artifacts that reduce the lens quality.
In a diffractive lens the curvature sampled and then quantised into rings. Whose height is a single full wave of delay for the design wavelength. Thus the lens has no artifacts and has perfect imaging quality. Often, the diffractive rings divided to a discrete number of step-wise height levels. To facilitate manufacturing by conventional lithographic techniques.
Advantage of a Diffractive Lens
The main advantage of a diffractive lens is that it occupies much less volume and is lightweight and thin. Thus, it inserted into places where a conventional lens would not fit or where the total mass constrained. Or when using ultra-short lasers where thick material causes temporal dispersion.
Another remarkable feature is that the chromatic dispersion of the diffractive lens. Although higher than in a conventional lens. It goes in the opposite direction so it used as a colour corrector. For more information about diffractive lenses please visit https://www.holoor.co.il/diffractive-optics-lenses/