Diffractive elements operate through interference and diffraction to produce an arbitrary distribution of light or aid in designing optical systems. Some manufacturers design and fabricate diffractive elements with both binary and analog phase profiles. Typically, diffractive elements operate under collimated, coherent illumination.
Diffractive lenses are made of concentric rings with a saw-tooth profile. They are used for high-quality precision diffractive optics, including replacements for natural lenses in cataracts surgery and for professional-quality photographic camera lenses. Diffractive lenses are very thin elements that have total depth eight equal to λ/(n – 1), where λ is the operating wavelength and n is the index of refraction.
Diffractive diffusers offer controlled illumination for some specialized applications like lithographic illumination systems. The main benefit of these elements compared to refractive ones is the sharp intensity fall-off. Diffractive diffusers can be implemented with analog or binary phase functions with efficiencies of up to 90%-95% and 80% respectively.
Diffractive diffusers are ideal for laser applications, in which a certain laser beam shape is required to refract homogenizers. Also, they let users realize uniform performance for a certain range at a defined distance from the light source. Some diffusers feature a strictly controlled beam angle that ensures high levels of efficiency. They are not sensitive to alignments and have no impact on the input beam’s polarization. As a result, they are ideal for applications that require rapid sensing of big areas, such as in the fields of remote sensing or LIDAR/LADAR.
Beam Splitters (Spot Arrays)
A diffractive element is commonly used in splitting a laser beam into an array of spots. This involves the separation of a generally collimated beam incident on the elements into an array, either 1D or 2D. A grating is an element that produces a beam splitter with some complicated shape that generates the spots’ desired distribution. To produce a diffractive beam splitter, the binary and analog solutions are used. The binary solution is used for producing a beam splitter if the desired spot distribution is centrosymmetric. Analog design solutions can reach more than 90% theoretical efficiency; however, they require more sophisticated production techniques.
Some applications require a certain wavefront at some point in the optical system. But, no matter the reason, the actual wavefront doesn’t show the desired form. If the wavefront consistently deviates from the ideal, a corrector plate can be used to correct the wavefront. It works by inducing the appropriate phase delay at different points of the aperture to produce the desired wavefront.