Diffractive Optics lens

Diffractive Optics lenses are a unique type of optical lenses that in some situations offer many advantages compared to conventional, or refractive, optical lenses. Diffractive optical lenses can be very thin and lightweight, have no wavefront error (perfect diffraction limited performance) and can be easily thoric, aspheric, or combine complex aberration correction functions. They are also much less susceptible to thermal effects, such as thermal lensing.

With these important characteristics in mind, diffractive optics lenses are a perfect solution for those applications that have volume and mass constraints such as miniature optical sensors or laser beam delivery systems, as well as high end applications where precision is paramount.

Diffractive optics lenses work, as their name implies, by using the principles of diffraction. Hence, it is the wave nature of light that dominates the behaviour over the ray optics description of light. Given that diffraction is the predominant effect, the chromatic dispersion is substantially higher than the one encountered in conventional lenses, thus they are mostly suitable for laser applications where light is monochromatic. The chromatic dispersion can be beneficial by noticing that the dispersion due to diffraction goes in the opposite direction that the dispersion due to refractive effects. With this remarkable fact, a compound lens system that combines a conventional lens with a diffractive optic lens can result in a system with zero chromatic dispersion.

Another important feature of diffractive optical lenses, and that is lesser known, is that when forming an image, the geometrical distortion is much smaller than with a conventional lens.

A diffractive optical lens is considered to be a diffractive optical element but it has its origins in another similar component referred to as a Fresnel zone plate. In a Fresnel zone plate a radial phase modulation is imparted to an incoming wavefront. Then, as a result of interference, that modulation gives rise to a bright spot at a specific distance away from the element.

Another similar component which is much more efficient in generating bright spots, but has much lower imaging quality is the Fresnel lens. In this component the curvature profile of a conventional lens is sampled at specific radial zones. The sag of each radial zone is then brought back to the origin such that the sag of each radial zone starts from the same point which is the apex from the original refractive lens profile. This process will result in a lens that is thinner than the original. Finally, a diffractive optical lens is then created from this surfacer profile by quantising the phase on the resulting curvature. The number of phase steps will depend on the manufacturing techniques, but it can be as small as only three phase steps.

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