Optical Vortex - Creation

Creation

Hypergeometric-Gaussian modes can be created with a spiral phase plate, computer-generated holograms, mode conversion, q-plate or a spatial light modulator.

Static spiral phase plates (SPPs) are spiral-shaped pieces of crystal or plastic that are engineered specifically to the desired topological charge and incident wavelength. They are efficient, yet expensive. Adjustable SPPs can be made by moving a wedge between two sides of a cracked piece of plastic.

Computer-generated holograms (CGHs) are the calculated interferogram between a plane wave and a Laguerre-Gaussian beam which is transferred to film. The CGH resembles a common Ronchi linear diffraction grating, save a "fork" dislocation. An incident laser beam creates a diffraction pattern with vortices whose topological charge increases with diffraction order. The zero order is Gaussian, and the vortices have opposite helicity on either side of this undiffracted beam. The number of prongs in the CGH fork is directly related to the topological charge of the first diffraction order vortex. The CGH can be blazed to direct more intensity into the first order. Bleaching transforms it from an intensity grating to a phase grating, which increases efficiency.

Mode conversion requires Hermite-Gaussian (HG) modes, which can easily be made inside the laser cavity or externally by less accurate means. A pair of astigmatic lenses introduces a Gouy phase shift which creates an LG beam with azimuthal and radial indices dependent upon the input HG.

A spatial light modulator is a computer-controlled electronic device which can create dynamic vortices, arrays of vortices and other types of beams.

At radio frequencies it is trivial to produce a (non optical) vortex. Simply arrange a half wavelength diameter ring of antennas such that the phase shift of the broadcast antennas varies an integral multiple of 2π around the ring.