Contents Introduction
Mode mixing significantly reduces transmission impairments in space-division multiplexed systems employing multiple-input multiple-output (MIMO) processing. Ensuring that each transmitted signal spends equal time on all modes can mitigate mode-dependent loss (MDL) which causes capacity loss [1] and reduce differential group delay (DGD) which reduces the complexity of the equalizer [2]. Strong mode mixing can prevent nonlinearities from coherently accumulating. For instance, the coupled-core fiber which has built-in strong mode mixing has better nonlinear tolerance than an equivalent single-mode fiber [3]. Most SDM fibers do not benefit from strong mode mixing and will require external mode mixers to improve their performance. MMF has strong mixing within mode groups, but only weak mixing between mode groups. Additionally, the lowest order modes have less propagation loss than the higher order modes. The ideal mode mixer should be able to controllably mix the modes to minimize impairments without introducing MDL. For MMF, this might be to swap the lowest order modes with the highest order modes.
