STh1F.6.pdf CLEO:2015 © OSA 2015 Demonstration of Distance Emulation for an Orbital-Angular-Momentum Beam Nisar Ahmed 1,* , Martin P. J. Lavery 1,2 , Peicheng Liao 1 , Guodong Xie 1 , Hao Huang 1 , Long Li 1 , Yongxiong Ren 1 , Yan Yan 1 , Zhe Zhao 1 , Zhe Wang 1 , Nima Ashrafi 3,4 , Solyman Ashrafi 4 , Roger D. Linquist 4 , Moshe Tur 5 , and Alan E. Willner 1 1 Dept. of Electrical Engineering, University of Southern California, Los Angeles CA 90089, USA. 2 University of Glasgow, Glasgow, G12 8QQ, UK. 3 University of Texas at Dallas, Richardson, TX 75080, USA. 4 NxGen Partners, Dallas, TX 75219, USA. 5 School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, ISRAEL. * nisarahm@usc.edu Abstract: We design and experimentally demonstrate a free-space distance emulator for propagating OAM beams over long distances in a lab environment. The performance of the system is assessed by measuring spot radius and radius of curvature of propagated beams. © 2015 Optical Society of America OCIS codes: 080.4865, 350.5500. 1. Introduction Orbital-angular-momentum (OAM) beams have gained interest over the past several years due partially to the potential applications to several fields, including communications, microscopy and sensing [1–3]. OAM beams can form an orthonormal basis set, such that different beams of various OAM values can be orthogonal to each other [4]. Such orthogonality enables the efficient multiplexing at a transmitter and demultiplexing at a receiver of several independent beams. For example, in a communication system in which each beam carries an independent data stream, the total system capacity and spectral efficiency can potentially be significantly increased [5]. The amount of OAM carried by a beam can be identified by the number of 2π phase shifts, l, that occur across the wavefront, such that the phase is twisting in a helical fashion as it propagates [4]. The beam itself has an interesting structure, such that: (i) the intensity forms a doughnut ring with little power in the center and that grows with larger l, (ii) the phase changes in an azimuthal fashion according to the l number, and (iii) the beam itself diverges faster with a larger l. All these properties mean that the beam is complex and exhibits unique behavior. For some applications, e.g., communications, it is important to experimentally show how the beam evolves under free-space propagation. Unfortunately, this is difficult in a lab environment for anything more than a few meters, and yet many important types of experiments require extensive lab equipment. Therefore, it is a laudable goal to have a free-space emulator for lab use that can correctly reproduce the intensity and phase characteristics of a propagating beam over longer distances. In this paper, we demonstrate a free-space emulator that can emulate free-space propagation of different distances for OAM beams. To confirm our design, we propagate Laguerre Gaussian (LG) beams of different mode orders through the emulator and measure the spot size and curvature of the beams at the output. 2. Emulator Design and Experiment Setup Figs. 1(a)–(b) show the concept of free-space emulator, a system whose output field matches both in intensity and phase to a beam propagated over a specific distance in free-space. Fig. 1(c) shows the experiment setup. We begin with LG beam (l = 0) having a beam waist w 0 . Due to the divergence, the beam waist at a distance z can be given by w(z)= w 0 (1 +(z/z r ) 2 ) 0.5 . We define the ratio of w(z) to w 0 as the magnification M of the free-space length. The divergence angle θ of such a beam can be calculated by using the relation θ = 2 tan -1 (w(z) - w 0 )/(2z). Using these M M BS BS Reference Beam Free-space Emulator M d f 1 f 2 SLM 2x HWP Col. λ 1550 nm Camera EDFA Free-space distance of length L Transmitted OAM Beam Received OAM Beam Free-space Emulator Input OAM Beam Output OAM Beam (a) (b) (c) Fig. 1: Concept of free-space emulator and experiment setup. (a) Optical beam propagated in free-space of distance L; (b) A bench-top free-space emulator to perform propagation experiments over long distances; (c) Experiment setup. Col. Collimator; HWP: Half-wave plate; BS: Beamsplitter; SLM: spatial light modulator; M: Mirror; f: lens; L: distance between two lenses.