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.

中文翻译：

---

弱耦合 FMF LP 模式内色散的分析和测量


基于超低模串扰少模光纤（FMF）中线性偏振（LP）模式的弱耦合模分复用（MDM）技术被认为是短距离传输应用容量增强的有希望的候选者。与单模光纤 (SMF) 中的偏振模色散 (PMD) 类似，弱耦合 FMF 中的信号也会受到 LP 模内色散 (ILMD) 的影响。本文首先分析了ILMD的产生机制，并计算了典型阶跃折射率圆芯FMF中的平均ILMD。然后进行数值模拟来研究其影响，严重的性能下降表明它可能是强度调制/直接检测（IM/DD）MDM 系统的主要传输损伤之一。此外，提出了一种用于ILMD测量的改进固定分析器方法（IFAM），基于该方法测量了支持6种LP模式的弱耦合FMF。实验结果表明，每个圆对称 LP 模式的 ILMD 行为与 SMF 中的 PMD 类似，而非圆对称 LP 模式的 ILMD 可能大数十倍。这项工作首次评估了 ILMD 的影响，有利于弱耦合 FMF 和 IM/DD MDM 系统的设计。