Space Division Multiplexing Using Few-Mode Fibers

  • Description

    The ultimate capacity of single-mode fiber (SMF) has been shown to be limited by fiber nonlinearities [1]. One promising approach to further increase the capacity of fiber-based transmission systems is Space Division Multiplexing (SDM) [2]. SDM can be realized using, for instance, fiber ribbons, multicore (MC) fibers or few-mode fibers (FMF). In this example, we discuss a SDM system (Figure 1) based on a FMF supporting six LP modes (LP01, LP11, LP21, LP02, LP31 & LP12). Because modes are degenerated (e.g., LP11aX, LP11aY, LP11bX and LP11bY), a total of 10 dual-polarization channels can be supported by the fiber. Each channel (25-Gbaud DP QPSK) is coupled into a fiber mode using a mode coupler which can emulate the behavior of a realistic device such as a photonic lantern. At the receiver side, the output of each mode is detected using a coherent polarization-diversity receiver. As modes travelling at different velocities couple during propagation [3], the outputs of all modes must be processed jointly using a multiple-input multiple-output (MIMO) filter [4].

  • Typical Results

    Under ideal conditions, mode coupling takes place within mode groups, i.e., between modes with similar velocity. However, mechanical stress (such as microbending) and splices can lead to mode-group coupling. The impulse response (optical pulse coupled into all modes) of 50 km of the considered FMF is displayed in Figure 2 for the cases when coupling takes place only within or between mode groups. As the size and complexity of the MIMO filter directly depends on the channel response, it is highly desirable to avoid intermode group coupling in FMF-based systems.
    The constellation of the channels launched in the LP01X, LP21aX, LP02X, LP31aX and LP12bY are displayed after MIMO processing (600 taps memory) in Figure 3 for the case where only intragroup coupling and PMD are considered.

  • Further Information

    Keywords: Space Division Multiplexing, SDM, few-mode fiber, MIMO, DSP, DGD, DMD, mode coupling

    Similar demonstrations are available in VPItransmissionMaker Optical Systems and on the VPIphotonics Forum.

    [1] Essiambre, R-J. et al. "Capacity limits of optical fiber networks." Lightwave Technology, Journal of 28, no. 4 (2010): 662-701.
    [2] Richardson, D. J., J. M. Fini, and L. E. Nelson. "Space-division multiplexing in optical fibres." Nature Photonics 7.5 (2013): 354-362.
    [3] Marcuse, D. "Pulse Propagation in Mutlimode Dieletric Waveguides", The Bell System Technical Journal, vol 51, No 6, July-August 1972
    [4] Randel, S. et al. "6 56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6 6 MIMO equalization." Optics Express 19.17 (2011): 16697-16707.

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