Dead Zone Fault Detection Optimization Method for Few-Mode Fiber Links Based on Unexcited Coupled Higher-Order Modes

The traditional single-mode fiber (SMF) optical time domain reflectometer (OTDR) may not be able to accurately detect and locate fault events in the dead zone of few-mode fiber (FMF) links. This paper introduces the concept of higher-order spatial mode detection dimensions unique to FMF, combined with the spatial mode coupling characteristics between modes. The Fresnel reflection from the end face of the fiber, the interior of the circulator, and the connector only occurs in the spatial mode of the injected optical pulse. The Rayleigh backscattering, which reflects the fault distribution characteristics of FMF links, can be detected by non-excited higher-order spatial modes. The proposed method can completely overcome the traditional OTDR dead zone. In this paper, the six-mode fiber is taken as an example for experimental verification. The detection optical pulse is injected into the fundamental mode LP01, and the Rayleigh backscattering of LP11a, LP11b, LP21a, LP21b, and LP02 higher-order spatial mode are collected and analyzed to accurately detect and locate the fusion splice fault event at 100 m and 500 m in the dead zone.