Delay Spread Reduction in Mode-Division Multiplexing: Mode Coupling Versus Delay Compensation

Reduction of the group delay (GD) spread is crucial for minimizing signal processing complexity in mode-division multiplexing. Strong mode coupling and GD compensation (concatenating different fibers with opposing GD ordering) are two approaches for reducing the end-to-end GD spread. In this paper, we study the GD behavior in systems where mode coupling and GD compensation are both present. Using a propagation model in generalized Stokes space, we describe the evolution of the GD variance by coupled differential equations. By integration of these equations, we evaluate the GD variance in GD compensated systems with different mode coupling lengths and GD compensation lengths. When the mode coupling length is much longer than the GD compensation length, a low GD variance can be obtained as a result of GD compensation. By contrast, when the mode coupling length is much shorter than the GD compensation length, GD compensation becomes ineffective, but a low GD variance can be obtained as a result of strong mode coupling. The largest GD variance is obtained when the mode coupling length is comparable to the GD compensation length.