A Different Angle on Light Communications

Can “twisted” light beams enhance optical communication systems? A conventional propagating free-space laser beam typically has an approximately flat phase front and exhibits an intensity profile that decreases as a function of the radial distance from the beam center. However, in the 1990s, it was shown that propagating light waves can contain an interesting property known as orbital angular momentum (OAM) (1). OAM implies that the light wave's phase front is twisting along the direction of propagation. This twisting of the phase front into a corkscrew shape results in a doughnut-like ring intensity profile (2). It was then demonstrated that a given optical beam with OAM can be encoded with data (3–5). Moreover, a radiowave field trial showed that a free-space radio data link can use OAM for transmission (6). Recent work has demonstrated a 2.56 terabits per second (Tbit/s) free-space optical data transfer using OAM (7).