Fiber Optics – Light Brigade Practice Test

An OTDR uses time-domain reflectometry: it sends a short light pulse into the fiber and records the backscattered and reflected light as it returns. The trace is plotted with distance along the fiber on the horizontal axis, so you can see where losses occur, where splices or connectors sit, and where a fault might be. But the distance it shows to the end of the fiber is an inferred value, not a direct ruler measurement. That distance comes from the time it takes for light to travel to the end and back, converted using the fiber’s propagation speed (which depends on its refractive index). If the fiber end doesn’t produce a clear reflection, or if the index varies along the length or multiple events are present, the endpoint distance can be uncertain. So an OTDR characterizes length by timing to events rather than measuring the total cable length with a direct measurement. That’s why the statement is false.