Hermite–Gauss vortex beams
In 1992, Allen  showed that the Laguerre–Gauss (LG) modes have an orbital angular momentum (OAM). All optical vortices or singular laser beams with a phase feature or a wave front dislocation have the same moment . In such beams, the Umov–Poynting vector (power flux) rotates in a spiral around the points of the phase singularity. The first generation of a laser beam with a phase singularity was carried out in 1979 . Two Hermite–Gauss (HG) modes HG01 and HG10 interfering in the resonator of a krypton ion laser, were shown to form the LG01 mode. In 1989, the term optical vortex was introduced in . In 1990, Soskin  formed an optical vortex using an amplitude diffraction grating with a ‘fork’, and in 1992 a singular laser beam was formed using a spiral phase plate .