Inter-Mode Interference Effect in Sparse-MDM System Over Weakly-Coupled FMF

Recently, sparse mode-division multiplexing (MDM) scheme has been proposed and experimentally verified to support long-haul MDM transmission without intermodal multiple-input-multiple-output (MIMO) processing, in which only multiple non-adjacent linearly-polarized (LP) modes in weakly-coupled few-mode fibers (FMFs) are utilized as active channels to achieve ultralow modal crosstalk. However, suffering from inevitable coupling among active modes and idle modes, signals may be exposed to distributed impairment in form of inter-mode interference (IMI). In this article, the generation mechanism of IMI effect is firstly theoretically analyzed. Then numerical simulations are carried out to investigate the impact of IMI accompanied with distributed modal crosstalk (DMC) in sparse-MDM systems. Experimental setup for measuring IMI intensities of 3 non-degenerate LP modes (LP 01 , LP 02 and LP 03 ) in a weakly-coupled 10-LP-mode fiber is established and the measurement results agree well with the theoretical square-law relationship. Finally, the optical signal-to-noise ratio (OSNR) penalty related to IMI effect is experimentally investigated for 60 km 3-mode FMF transmission, the experimental results of which show that IMI effect may be a major impairment of sparse-MDM systems. This work is beneficial to practical applications of long-haul MDM techniques.