Mode coupling effects in mode-division-multiplexed systems

Mode coupling is a key to overcoming major challenges in mode-division-multiplexed (MDM) long-haul systems using coherent detection. Strong mode coupling reduces the modal group delay spread, minimizing the complexity of digital multi-input multi-output (MIMO) signal processing. Likewise, strong mode coupling mitigates the mode-dependent gain of optical amplifiers, maximizing average channel capacity. When combined with modal dispersion, strong mode coupling creates frequency diversity, dramatically reducing outage probability. Remarkably, the statistical distributions of strongly coupled modal group delays or gains depend only on the number of modes and the variances of accumulated delay or gain, and can be derived from the eigenvalue distributions of certain random matrices. Insight into mode coupling suggests alternative approaches for MDM based on optical MIMO signal processing, which may offer reduced power consumption in short-reach direct-detection systems.