The ultimate capacity of single-mode fiber (SMF) has been shown to be
limited by fiber nonlinearities . One promising approach to further increase the capacity
of fiber-based transmission systems is Space Division Multiplexing (SDM) . 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
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 , the outputs of all modes must be processed jointly using a
multiple-input multiple-output (MIMO) filter .
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
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
for the case
where only intragroup coupling and PMD are considered.
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.
 Essiambre, R-J. et al. "Capacity limits of optical fiber networks." Lightwave Technology, Journal of 28, no. 4 (2010): 662-701.
 Richardson, D. J., J. M. Fini, and L. E. Nelson. "Space-division multiplexing in optical fibres." Nature Photonics 7.5 (2013): 354-362.
 Marcuse, D. "Pulse Propagation in Mutlimode Dieletric Waveguides", The Bell System Technical Journal, vol 51, No 6, July-August 1972
 Randel, S. et al. "6x 56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6x 6 MIMO equalization." Optics Express 19.17 (2011): 16697-16707.