Diffractive elements are referred to as thin phase elements that run with the help of interference and diffraction to obtain random distributions of light or to help in the design of optical system. The experts design and fabricate the diffractive elements with the binary as well as the analog phase profiles. The binary elements produce efficiencies around 80 percent excluding the surface losses and usually represent the cost effective solutions if the feature sizes are tiny for analog fabrication and if the desired patter has centrosymmetry. It means that if l(m,n) is representing the intensity at the diffraction order (m,n) then l(-m,-n) = l(m,n). If the pattern to be produced has centrosymmetry then it can be made with a binary phase mask or else it needs to be continuous. For instance, you can use a binary element to obtain a square diffuser but not a triangular one. The continuous elements can obtain efficiencies at over 90 percent and give away the production of more general patterns.
Diffractive optics or elements usually run under collimated, coherent illumination in the geometry with an optional focusing lens. The problem is to measure the diffractive element surface structure considering the desired intensity distribution in the image plane. When you have a rigorous design and modeling codes ready, you can optimize the diffractive element to maximize the performance for the particular manufacturing method. Here are some of the diffractive elements:
- Diffractive lenses
This can be used to bring down the number of elements in the conventional lens system and give away the need for exotic materials for rectifying the chromatic aberrations. These lenses are very thin elements that have a total depth height tantamount to λ/(n – 1). A diffractive lens has a series of zones that get finer towards the edge of the lens. As a general rule, these elements work best at single wavelength or else the image contrast and efficiency are reduced.
- Beam splitters
The very common use here is the splitting of the laser beam into an array of spots. Here a generally collimated beam incident on the element is segregated into an array, in 1D or 2D. The diffractive element that gives a beam splitter is usually a grating with a complex shape that gives the desired distribution of spots. There are two basic designs being used: binary and analog solutions. The former is a useful approach to produce a beam splitter is a desired spot distribution is centrosymmetric. For analog, laserwriting technique is used on a continuously varying surface that gives the desired spot distribution.