Because the majority of network cabling is copper based, most of the tools designed to test cabling are designed for copper-based cabling. However, when you test fiber-optic cable, you need an optical tester.
An optical cable tester performs the same basic function as a wire media tester, but on optical media. Unlike wire cables, the most common problem with an optical cable is a break in the cable that prevents the signal from reaching the other end. Because of the extended distances that can be covered with fiber-optic cables, degradation is rarely an issue in a fiber-optic LAN environment.
Ascertaining whether a signal reaches the other end of a fiber-optic cable is a relatively easy task, but when you determine that there is a break, the problem becomes locating the break. That's when you need a tool called an optical time-domain reflectometer (OTDR). By using an OTDR, you can locate how far along in the cable the break occurs. The connection on the other end of the cable might be the source of the problem, or perhaps there is a break halfway along the cable. Either way, an OTDR can pinpoint the problem.
Unless you work extensively with fiber-optic cable, you're unlikely to have an OTDR or even a fiber-optic cable tester in your toolbox. Specialized cabling contractors will have them, though, so knowing that they exist is important.updated