15 March 2016 Free-space optical communications using encoding of data on different orbital-angular-momentum modes
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Abstract
Free-space optical communications can play a significant role in line-of-sight links. In general, data can be encoded on the amplitude, phase, or temporal position of the optical wave. Importantly, there are environments for which ever-more information is desired for a given amount of optical energy. This can be accomplished if there are more degrees-of-freedom that the wave can occupy to provide higher energy efficiency for a given capacity (i.e., bits/photon). Traditionally, free-space optical links have used only a single beam, such that there was little opportunity for a wave to occupy more than one spatial location, thereby not allowing the spatial domain to be used for data encoding. Recently, space- and mode-multiplexing has been demonstrated to simultaneously transmit multiple data-carrying free-space beams. Each spatially overlapping mode was orthogonal to other modes and carried a unique amount of orbital-angular-momentum (OAM). In this paper, we consider that OAM modes could be a data-encoding domain, such that a beam could uniquely occupy one of many modes, i.e., 4 modes would provide 4 possible states and double the bits of information for the same amount of energy. In the past, such OAM-based encoding was shown at kHz data rates. We will present the architecture and experimental results for OAM-based data encoding for a free-space 1.55-μm data link under different system parameters. Key features of the results include: (a) encoding on several modes is accomplished using a fast switch, and (b) low bit-error-rates are achieved at >Gbit/s, which is orders-of-magnitude faster than previous results.
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Asher J. Willner, Yongxiong Ren, Guodong Xie, Long Li, Yinwen Cao, Zhe Zhao, Peicheng Liao, Zhe Wang, Yan Yan, Nisar Ahmed, Cong Liu, Moshe Tur, Alan E. Willner, "Free-space optical communications using encoding of data on different orbital-angular-momentum modes", Proc. SPIE 9739, Free-Space Laser Communication and Atmospheric Propagation XXVIII, 97390J (15 March 2016); doi: 10.1117/12.2225375; https://doi.org/10.1117/12.2225375
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