Efficient generation and multiplexing of optical orbital angular momentum modes in a ring fiber by using multiple coherent inputs Yan Yan, 1, * Yang Yue, 1 Hao Huang, 1 Jeng-Yuan Yang, 1 Mohammad R. Chitgarha, 1 Nisar Ahmed, 1 Moshe Tur, 2 Samuel. J. Dolinar, 3 and Alan E. Willner 1 1 Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, USA 2 School of Electrical Engineering, Tel Aviv University, Ramat Aviv 69978, Israel 3 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA *Corresponding author: yanyan@usc.edu Received June 21, 2012; accepted July 11, 2012; posted July 12, 2012 (Doc. ID 171052); published August 28, 2012 We propose an approach to efficiently generate and multiplex optical orbital angular momentum (OAM) modes in a fiber with a ring refractive index profile by using multiple coherent inputs from a Gaussian mode. By controlling the phase relationship of the multiple inputs, one can selectively generate OAM modes of different states l. By con- trolling both the amplitude and phase of the multiple inputs, multiple OAM modes can be generated simultaneously without additional loss coming from multiplexing. We show, by simulation, the generation of OAM modes (OAM state jlj < 3) with mode purity greater than 99%. The power loss of generating and multiplexing seven modes is about 35%. A transmitter for an OAM-based mode-division multiplexing system is proposed based on the discrete Fourier transform between the data carried by the multiple inputs and the data carried by the OAM modes. The experimental implementation of the proposed approach could be achieved by integrating ring fiber, multicore fiber, and photonic integrated circuit technology. © 2012 Optical Society of America OCIS codes: 060.2380, 050.4865. An optical beam having a helical wavefront phase of 2lπ is associated with an optical orbital angular momentum of lℏ of a photon [ 1]. Recently, orbital angular momen- tum (OAM) modes have gained much interest for increas- ing transmission capacity and spectral efficiency in optical communication systems because of their ability to carry independent data streams on orthogonal modes [ 2], which could be used in mode-division multiplexing (MDM) in free-space and fiber communication systems [ 3]. The efficient generation and multiplexing of OAM modes has been a key challenge. Generation has typically been accomplished by using spatial-light modulators (SLMs) [ 4] or other free-space components [ 5], which are generally bulky and expensive. The multiplexing of multiple spatial modes is usually achieved by using beam splitters [ 2, 3], which is inherently lossy. One of the pro- mising approaches to efficiently generate and multiplex multiple OAM modes without significant loss is to use the interference of multiple coherent Gaussian modes in the far field [ 6, 7]. However, the mode purity of the gen- erated OAM mode will not be high enough due to the discontinuity of the input conditions, which may not be preferable to some applications such as optical com- munications that require low crosstalk among the spatial channels. In this Letter, we propose and show, by using simula- tion, a new approach to generate OAM modes of high purity in a ring fiber from multiple coherent inputs of Gaussian mode. We further show that, in an OAM-based MDM system, there is a discrete Fourier transform (DFT) relationship between the data modulation of the input Gaussian mode channels and the output OAM channels. Compared to the existing methods of generating OAM modes in an optical fiber [ 8, 9], this approach is able to generate and multiplex multiple different OAM modes simultaneously with fairly low loss. Figure 1(a) illustrates the proposed approach to gen- erate and multiplex OAM modes by using multiple coher- ent inputs (which could be from a multicore fiber [ 10] or a grating coupler [ 11]) followed by a fiber with a ring refractive index profile [ 12]. The linear-polarized modes in the ring fiber can be expressed as OAM l;p  R p r expilφ, in which l is the OAM charge number, repre- senting an azimuthal phase change of 2πl, and p is the radial index, indicating there are p intensity peaks in the radial direction. OAM l;p modes with different values of p or l are spatially orthogonal to one another. Figure 1(b) shows the refractive index of the ring fiber. The back- ground refractive index is n 1  1.46 and the refractive index difference is Δn  0.12% × n 1 . The N inputs to the ring fiber are fundamental Gaussian modes in multi- ple single-mode fibers of the diameter d s and of the same linearly polarized state. They are evenly distributed around a circle of diameter D d in  d out . The de- signed ring fiber should have two properties: (i) only OAM l;p modes with p  1 can be supported by this fiber, Fig. 1. (Color online) (a) Generation of OAM modes of charge number l in a ring fiber with N coherent Gaussian inputs. (b) The refractive index of the ring fiber, which is designed to support OAM l;1 modes with l  −3 ∼ 3. September 1, 2012 / Vol. 37, No. 17 / OPTICS LETTERS 3645 0146-9592/12/173645-03$15.00/0 © 2012 Optical Society of America