Efficient Quantification and Simulation of Modal Dynamics in Multimode Fiber Links

Sudden environmental effects, such as mechanical vibration, wind, and lightning, impart microsecond-timescale changes to the transmission matrix of multimode optical fibers. We introduce a timescale parameter to characterize the rate of channel changes in mode-division-multiplexed (MDM) links. We show how to efficiently generate continuous unitary matrices that form the basis for a simplified, yet general, dynamic channel model incorporating the effects of modal dispersion, mode-dependent loss, and time-varying mode coupling. We show that fast environmental perturbations can be modeled by deriving the unitary matrices from a linear ramp model, where the ramp slopes depend on the timescale parameter. We discuss the implications of channel dynamics on adaptive multiple-input-multiple-output (MIMO) equalization at the receiver in long-haul MDM links using coherent detection and in short-reach MDM links using direct detection. We show that the channel timescale and the rotation rate of the generalized Stokes vector on the generalized Poincaré sphere are both predictive of adaptive MIMO equalizer misadjustment. Our simplified channel model can be applied to optimize adaptive MIMO equalization algorithms for tracking modal dynamics.