Long-Period Fiber Gratings for Mode Coupling in Mode-Division-Multiplexing Systems

In mode-division-multiplexing systems, multi-input multi-output (MIMO) equalization is used to compensate for linear impairments, including modal dispersion (MD) and modal crosstalk. The MIMO equalizer memory length depends on the group delay (GD) spread arising from MD. The GD spread arising from MD can be significantly reduced by introducing strong mode coupling via mode scramblers. We study the design of such mode scramblers implemented as long-period multimode fiber gratings for systems using D = 12 modes (six spatial modes). By optimizing the grating chirp function, we minimize the mode-dependent loss (MDL) of the grating while ensuring full intergroup mode coupling. We find a design yielding MDL and mode-averaged loss in the C band not exceeding 0.36 and 0.45 dB, respectively. We also verify the effect of such mode scramblers on the GD scaling of a long-haul system, demonstrating that the scramblers reduce the scaling of GD spread with length from a linear to a square-root dependence, as expected in the strong coupling regime.