Simplified measurement of the orbital angular momentum of single photons

We describe a simplification of a recent approach for sorting photons according to their orbital angular momentum (OAM) [Leach et al., Phys. Rev. Lett. 88 (2002) 257901]. The original cascade of Mach-Zehnder interferometers required optical elements that can increase the OAM per photon of any passing light by a fixed amount, which in practice introduced loss. Our simplification, which we demonstrate experimentally, does not require these lossy elements but instead shifts the phase in one arm of each interferometer. We also point out that an earlier scheme for sorting Hermite-Gaussian (HG) modes [Xue et al., Opt. Lett. 26 (2001) 1746] could be modified to sort photons according to their OAM. (C) 2003 Elsevier Science B.V. All rights reserved.