Diffractive Optical lenses

Diffractive Optical lenses are lenses that work by diffraction.

12:33 18 February 2021

As in the case of a normal, conventional, lens, a diffractive optical lens focuses the light from a collimated beam onto a focal plane. The difference resides in that in a diffractive lens the wave nature of light is fully exploited. This is not the case in a conventional lens in which a focal spot is created by virtue of light refraction only. The curvature of the refractive element is calculated in such a way that rays from a collimated beam are bent locally towards a focal point. The resulting curvature is continuous and the shorter the focal length, the thicker the lens will be as it needs to make up for the steeper curvature required.

 

In comparison, a diffractive optical lens can be very thin as the surface is composed of small pixels or modulating elements in which an equivalent radius of curvature is encoded. This process of encoding the focusing function into a transparent substrate starts with the calculation of the curvature, just as if the design were for a refractive lens. Then, the slope is quantized so that the sag is set to zero every time the height goes above a phase delay of one full wave of the design wavelength. This results in uneven concentric zones, known as Fresnel zones, each having a piece of a lens or a diffractive “tooth” whose height goes for zero to one full wave delay.

 

The next step, which is common to all diffractive optical elements, is to quantise the resulting curvature profile on each Fresnel zone. Depending on the number of phase levels, the slope will be approximated to a certain degree of accuracy. But even in the case of a binary phase quantisation, the resulting element will still retain the focusing capabilities, with reduced efficiency. After quantization, diffractive lenses are often referred to as Fresnel Zone plates.

 

Diffractive optical lenses are a perfect alternative for those applications in which mass or volume are of paramount importance due to their lightweight nature. In principle they should only be used with monochromatic light, but diffractive optical lenses can be used to counteract the effects of chromatic dispersion in conventional lens systems.