Optical fiber red light pen (i., optical fiber fault detector, optical fiber fault test pen) is a 650nm (± 20nm) semiconductor laser as a light-emitting device, which emits stable red light through a constant current source drive, and connects with the. Optical fiber red light pen (i. A VFL is used to detect faults, breaks, or bends in fiber optic cables by emitting a bright red light that is visible even through the fiber's jacket. It's a cost-effective and. Hobbes Fiber Checker Pro is a Visual Fault Locator (VFL) used to accurately check and determine defects of a fiber optic cable. It's often used during installation or repair because it.