Multifocal diffractive lens

A multifocal diffractive lens is a diffractive optical element (DOE) that allows a single incident beam to be focused simultaneously at several positions along the propagation axis.
Principle of operation.
An incident laser beam is deflected by grooved diffraction pattern into axial diffraction orders along its optical axis. The foci appear around the far field position. With an additional focusing lens, foci from multifocal lens will appear at certain distances from the focal point of the lens.
Theory.
The multifocal spots location is a function of refractive focal length "f"Refractive and predetermined diffractive focal length "f"Diffractive The focal spot at the "zero" order refers to the refractive focal length of the lens being used.
The distance between the focal spots can be described by the equation