Mode Coupling and its Impact on Spatially Multiplexed Systems

Mode coupling is a key to overcoming challenges in mode-division-multiplexed transmission systems in multimode fiber. This chapter provides an in-depth description of mode coupling, including its physical origins, its effect on modal dispersion (MD) and mode-dependent loss or gain (MDL), and the resulting impact on system performance and implementation complexity. Strong mode coupling reduces the group delay spread from MD, minimizing the complexity of digital signal processing used for compensating MD and separating multiplexed signals. Likewise, strong mode coupling reduces the variations of MDL arising from transmission fibers and inline optical amplifiers, maximizing average channel capacity. When combined with MD, strong mode coupling creates frequency diversity, which reduces the probability of outage caused by MDL and enables the outage capacity to approach the average capacity. The statistics of strongly coupled MD and MDL depend only on the number of modes and the variances of MD or MDL, and can be derived from the eigenvalue distributions of certain random matrices.