Analysis and Measurement of Intra-LP-Mode Dispersion for Weakly-Coupled FMF

Weakly-coupled mode division multiplexing (MDM) technique based on linearly-polarized (LP) modes in ultralow-modal-crosstalk few-mode fibers (FMF) has been considered as a promising candidate for capacity enhancement of short-reach transmission applications. Similar to the polarization mode dispersion (PMD) in single-mode fibers (SMF), signals in weakly-coupled FMFs suffer from the influence of intra-LP-mode dispersion (ILMD). In this paper, the generation mechanism of ILMD is firstly analyzed and the mean ILMDs in a typical step-index circular-core FMF are calculated. Then numerical simulations are carried out to investigate its impact and the severe performance degradation indicates that it may be one of the major transmission impairments for intensity-modulation/direct-detection (IM/DD) MDM systems. Moreover, an Improved Fixed Analyzer Method (IFAM) is proposed for ILMD measurement, based on which a weakly-coupled FMF supporting 6 LP modes is measured. Experimental results show that the ILMD for each circularly-symmetric LP mode behaviors like PMD in a SMF, while it may be up to tens of times larger for the non-circularly-symmetric LP modes. This work evaluates for the first time the ILMD impact and is beneficial for the design of weakly-coupled FMFs and IM/DD MDM systems.