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 interface into the. Optical fiber red light pen (i. It's a cost-effective and. The visual fault locator is a commonly used optical fiber detection tool for troubleshooting network faults. Tool sends visible light over a fiber strand with a 10mW power, good enough to reach.