Light often has spin angular momentum, more commonly referred to as left or right circular polarization. Only in the past 30 years have researchers been able to impart orbital angular momentum (OAM; see the article by Miles Padgett, Johannes Courtial, and Les Allen, Physics Today, May 2004, page 35). Instead of the usual flat wavefront, light with nonzero OAM has a helical wavefront that turns like a corkscrew in the direction of propagation. The quantum number m describes how tight the light’s corkscrew motion is. For example, m = 1 means a full clockwise rotation in a wavelength, and m = −2 means two full counterclockwise rotations in a wavelength.
OAM provides additional degrees of freedom for encoding information in optical communications. Modern optical networks rely on wavelength and intensity modulations to send data; the addition of polarization and OAM could pack in more information without boosting traffic....
