Using optical time domain reflectometer testing, you'll measure the length of the fiber optic cable, attenuation, and any events occurring on that fiber segment. Events are splices, stress points, or breaks that cause unacceptable amounts of attenuation on the length of the fiber. The three standard methods for testing fiber optic cabling are a visible light source, power meter and light source, and optical time domain reflectometer (OTDR). Optical Power Meter (OPM) and Light Source (LS) What they do: These work as a pair. Why Does Fiber Optic Testing Matter? Fiber internet offers better speed and performance than copper options, but the cables are very sensitive to bending, contamination, and physical.