Group delay statistics and management in mode-division multiplexing

Reduction of the group delay (GD) spread is crucial for minimizing multi-input multi-output (MIMO) signal processing complexity in mode-division multiplexing (MDM) systems using multi-mode fibers (MMFs). In this paper, we focus on GD statistics and management techniques. First, we review the generalized Jones and Stokes representations for modeling propagation in MMF, and describe key GD properties derived using the two representations. Then, we describe three approaches for GD management: (a) optimized fiber design, (b) mode coupling and (c) GD compensation. For approach (a), we explain design principles for minimizing the GD spread. We review experimental results to date, showing that fabrication non-idealities significantly increase the GD spread, and this approach alone may not achieve sufficiently low GD spread. For approach (b), we describe mechanisms for inducing intragroup and intergroup coupling. We describe mode scrambler designs based on photonic lanterns or long-period fiber gratings, both of which can ensure strong intergroup coupling. For approach (c), we review GD-compensated system design principles and show that GD compensation is only partially effective in the presence of random intragroup or intergroup coupling. We provide design guidelines for minimizing MIMO signal processing complexity in MDM systems.