Increasing the data density of free-space optical communications using orbital angular momentum Graham Gibson* a , Johannes Courtial a , Mikhail Vasnetsov a , Stephen Barnett b , Sonja Franke-Arnold b and Miles Padgett a a Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK b Department of Physics and Applied Physics, University of Strathclyde, Glasgow, G4 0NG, UK ABSTRACT Laguerre-Gaussian (LG) light beams possess discrete values of orbital angular momentum (OAM) of lh per photon, where l is the azimuthal index of the mode. In principle l can take on any integer number, resulting in an unlimited amount of information that can be carried by any part of the beam - even a single photon. We have developed a technology demonstrator that uses OAM to encode information onto a light beam for free-space optical communications. In our demonstrator units both the encoding and decoding of the orbital angular momentum states is achieved using diffractive optical components (holograms). We use 9 different OAM values; one value is used for alignment purposes, the others carry data. Keywords: orbital angular momentum, optical communications, free-space optics, bandwidth 1. INTRODUCTION That light carries a spin angular momentum has been known since the time of Maxwell, and macroscopically this corresponds to circularly polarized light. Also understood - but less well appreciated - is that light beams carry orbital angular momentum (OAM) associated with their phase structure. In 1992 Allen et al. identified that beams with helical phase fronts described by an azimuthal phase term exp( ilφ ) possess an OAM of lh per photon 1 . The origin of this angular momentum is readily appreciated from the fact that the helical nature of the phase fronts implies that the linear momentum of the light has an azimuthal component. Although it had been suspected for a while, it was not until 2001 that experimental evidence was obtained that showed that orbital angular momentum was also a valid description of light at the level of single photons 2 . What was actually demonstrated is that parametric down conversion produces signal/idler photon pairs, whose orbital angular momenta add up to that of the pump photon. This work opened the door for the exploration of OAM in the quantum regime. A standard measurement of the spin angular momentum of a single photon gives one of two possible results, corresponding to the photon being either right- or left-circularly polarized. If data were to be encoded onto single photons using their spin angular momentum, their capacity is therefore one bit per photon. By contrast, a measurement of the OAM of a single photon has in principle infinitely many possible outcomes. Therefore the OAM of a single photon can in principle carry an unlimited amount of data 3 . In practice the number of different OAM states that can be set or controlled is limited by two factors. Firstly for a given aperture of optical system beams with a higher value of l diffract more quickly, limiting the range of different l values that can be efficiently coupled through any optical system 4 . The product of numerical aperture and aperture of any optical system required to efficiently transmit a set of beams of differing l values scales with the square root of the highest value of | l |. Secondly, the technology to create and measure these states gets more problematic as the number of states increases. In this work we demonstrate an optical system that uses OAM of light to transmit data through free space. The system uses spatial light modulators acting as reconfigurable diffractive optical elements (holograms) to both create and measure the OAM states and commercial telescopes to transmit and receive the beams. * g.gibson@physics.gla.ac.uk ; phone +44 141 330 6432; www.physics.gla.ac.uk/Optics/ Free-Space Laser Communications IV, edited by Jennifer C. Ricklin, David G. Voelz, Proceedings of SPIE Vol. 5550 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 doi: 10.1117/12.557176 367 Downloaded From: http://spiedigitallibrary.org/ on 04/15/2014 Terms of Use: http://spiedl.org/terms