Experimental Evaluation of Photonic Lanterns for Lossless Mode-Multiplexing

We theoretically investigate lossless adiabatic mode-converters to couple into and out of multi-mode fiber's supporting 15 spatial modes without modal dependent loss. Measurements of a 7 mode demultiplexer shows 1 dB coupling and mode dependent losses. Introduction In coherent optical spatial-division multiplexing (SDM), N separate information channels are mapped onto an orthogonal combination of the M modes of a multi-mode fiber (MMF). Fig. 1(a) illustrates a coherent SDM system consisting of a mode-multiplexer, MMF transmission, a mode-demultiplexer, an array of coherent receivers, and electrical multiple-input multipleoutput (MIMO) processing. Maximal capacity occurs only when N = M in systems with minimal mode-dependent losses (MDL) since MDL is equivalent to reducing the available modes [1]. Typically, the mode-multiplexers map to the fiber eigenmodes (the simplest orthogonal combination of modes). However, exciting the MMF eigenmodes often requires large bulk-optics setups incorporating phase-masks or spatial-light modulators to produce the mode profiles and beam-combiners to place the modes into the fiber [2­4]. Passive beam-combining increases the coupling loss (CPL) proportionally to the number of modes. Alternatively, since large mode scrambling/coupling is inevitable in MMFs [5] it is not necessary to excite individual MMF eigenmodes ­ a mode-coupler that excites an orthogonal combination of eigenmodes is (a) Coherent SDM with MIMO processing (a) LP01 LP11 LP21 LP02 LP31 LP12 LP41 LP22 LP03