The Optical Time Domain Reflectometer (OTDR) is useful for testing the integrity of fiber optic cables. It can verify splice loss, measure length and find faults. It is used to characterize and troubleshoot optical fibers by measuring the loss in a fiber link and pinpointing locations of potential issues such as breaks and splice losses. By sending. To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant.