After every fiber optic cable installation or repair, you need to test for continuity and end-to-end loss. You may also need to troubleshoot installed fiber that’s not performing up to expectations.

To check fiber, you need to test it with a light source and a power meter,  then compare your results with an estimate of what a reasonable loss for that cable or link is. This estimate is called a loss budget and is calculated using typical losses expected for each part of the cable: the fiber itself, connectors, and splices, if any. If the measured loss exceeds the loss budget by a significant amount, there is a problem with the cable, most often at the connectors or a splice rather than with the cable itself.

A fiber light source is used to inject light into a fiber optic cable for the purpose of testing it. They come in two basic varieties: light emitting diodes (LEDs) and laser diodes. They’re further differentiated by the 
wavelength they produce and the type of cable they test.

LEDs are low cost, slower speed, easy to use, multimode-only, and have a wide output pattern. Because LEDs produce a less concentrated light than lasers and have a much lower power output than lasers, they’re difficult to couple into fibers, limiting them to multimode fibers. LEDs have less bandwidth than lasers and can achieve a maximum throughput of 1 Gbps.

Laser diodes are higher cost and faster speed, allow single-mode or multimode, and have a narrow output pattern. Lasers can achieve throughput up to and beyond 10 Gbps.

The three kinds of lasers in use for fiber optic transmission are Fabry-Perot lasers, distributed feedback (DFB) lasers, and vertical cavity surface-emitting lasers (VCSELs). Fabry-Perot lasers are the most versatile, operating over both multimode and single-mode cable. DFB lasers are used for very long-distance applications over single-mode fiber. VCSELs can carry very high speeds. They’re usually used only for multimode fiber, although they can also support 1310 single-mode fiber. 

Because the light source used for testing should work with the fiber being tested, as well as the power meter, it’s important to read the light source’s specifications to ensure that it works with the cable you have (multimode or single-mode) and the wavelength you’re using.

Although fiber optic light sources are usually too low in power to cause much eye damage, some high-powered sources can cause retina damage and blind spots. Never look directly into a light source or into the end of a fiber cable unless you’re sure it’s dark. Always check fiber with a power meter or traffic identifier before looking into it.