Equalization of Modal Dispersion


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Equalization of Modal Dispersion in Multimode Fiber using Spatial Light Modulators Elad Alon Vladimir Stojanovi Joseph M Kahn Stephen Boyd and Mark Horowitz Department of Electrical Engineering Stanford University CA 94305 USA Abstract Intersymbol interference ISI due to modal dispersion is the dominant limitation to the bit rate distance product in multimode fiber optic communication systems If the light launched into the fiber excites only the desired principal modes modal dispersion can be eliminated We can achieve this by using spatial light modulators SLMs to perform adaptive spatial filtering on the electric fields of the light In this paper we develop an optimization framework for setting the SLMs to obtain an upper bound on the achievable performance and develop heuristics that nearly reach this upper bound Using this framework we show that both a sophisticated semidefinite programming based algorithm and a simple adaptive algorithm achieve performance close to the upper bound Performance and system complexity tradeoff curves are constructed showing that a 20 20 array of SLM pixels with binary phase control performs within 15 of more complex implementations Finally we extend the framework and present preliminary results showing the promise of further increases in the capabilities of multimode fiber by using the fiber as a multiple input multiple output MIMO transmission medium I INTRODUCTION Multimode fiber is the dominant type of fiber used for data communications in current local area networks these fibers have several non idealities that limit the achievable signaling rates for a given length of fiber The dominant limiting factor in multimode links is the ISI caused by modal dispersion 1 Light can travel through a multimode fiber in many different modes the modes in which light propagates depend upon the launch conditions at the input of the fiber The existence of multiple modes makes multimode fiber easier to launch light into and therefore less

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