VFL are also called light pens, pen-type red light sources, visible light detection pens, optical fiber fault detectors, and optical fiber fault locators. Most of them are used to detect fiber breakpoints. According to their shortest detection distance, they are divided into: 5Km, 10Km, 15Km, 20Km, 25Km, 30Km, 35Km, 40Km and so on. Driven by a constant current source, a stable red light is emitted, which is connected to the optical interface to enter the optical fiber, thereby realizing the optical fiber fault detection function.