Structured light, especially beams carrying orbital angular momentum (OAM), has gained much interest due to its unique amplitude and phase structures. In terms of communication systems, multiple orthogonal OAM beams can be potentially utilized for increasing link capacity in different scenarios. This review describes challenges, advances, and perspectives on different aspects of the OAM-based optical communications, including (a) OAM generation/detection and (de)multiplexing, (b) classical free-space optical communication links, (c) fiber-based communication links, (d) quantum communication links, (e) OAM-based communications in different frequency ranges, (f) OAM-based communications using integrated devices, and (g) novel structured beams for communications.
Novel forms of beam generation and propagation based on orbital angular momentum (OAM) have recently gained significant interest. In terms of changes in time, OAM can be manifest at a given distance in different forms, including: (1) a Gaussian-like beam dot that revolves around a central axis, and (2) a Laguerre-Gaussian (LG ';p) beam with a helical phasefront rotating around its own beam center. Here we explore the generation of dynamic spatiotemporal beams that combine these two forms of orbital-angular-momenta by coherently adding multiple frequency comb lines. Each line carries a superposition of multiple LG ';p modes such that each line is composed of a different ' value and multiple p values. We simulate the generated beams and find that the following can be achieved: (a) mode purity up to 99%, and (b) control of the helical phasefront from 2π-6π and the revolving speed from 0.2-0.6 THz. This approach might be useful for generating spatiotemporal beams with even more sophisticated dynamic properties.
Beams carrying orbital-angular-momentum (OAM) have gained much interest due to their unique amplitude and phase structures. In terms of communication systems, each of the multiple independent data-carrying beams can have a different OAM value and be orthogonal to all other beams. This paper will describe the use of multiplexing and the simultaneous transmission of multiple OAM beams for enhancing the capacity of communication systems. We will highlight the key advances and technical challenges in the areas of (a) free-space and fiber communication links, (b) mitigation of modal coupling and channel crosstalk effects, (c) classical and quantum systems, and (d) optical and radio frequency beam multiplexing.
In this paper, we propose and demonstrate a blind chromatic dispersion (CD) estimation method based on fractional Fourier transformation (FrFT). Through numerical simulations, the proposed CD estimation method is shown to be robust against amplified spontaneous emission (ASE) noise and nonlinear interference. Only 2048 samples are required for reliable CD estimation for single carrier 28 GBaud DP-QPSK or 32 GBaud DP-16QAM signals and the standard deviation can be as low as 98.9 ps/nm and 103.6 ps/nm respectively. The feasibility of the proposed CD estimation method has been experimentally verified using 28 GBaud DP-QPSK and 14 GBaud DP-16QAM signals over various transmission distances with CD ranges of 46332 ps/nm ~ 77220 ps/nm and 5148 ps/nm ~ 51480 ps/nm respectively. Compared with some other CD estimation methods, the method based on FrFT has advantages in the aspects of less computation complexity and robustness to transmission impairments.
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