A jumper cable of known good quality that is attached to a light source on one end and a fiber under test on the other end, calibrated for output power, and used for testing optical loss in a fiber optic cable.The launch fiber provides an opportunity for the signal to achieve modal equilibrium, i.e., even power distribution across all modes, and eliminates any possibility that fiber anomalies near the light source can affect the test results.A mandrel wrap sometimes is used with a multi-mode fiber (MMF) launch cable to assist in the achievement of modal equilibrium. Synonymous with injection fiber. See also jumper, mandrel wrapping, MMF, modal equilibrium, optical fiber, power, and source.