This procedure describes light launch conditions to the test fiber for the purpose of achieving a uniform overfilled launch with a laser diode or other light sources. While FOTP-54 can be used to establish overfilled launching conditions for the measurement of various fiber parameters, it is principally used in conjunction with FOTP-30 or FOTP-51 for measuring information carrying capacity. Light launch conditions are established through the use of a mode scrambler. The mode scrambler is positioned between the light source and test fiber to produce a radiation distribution overfilling the test fiber core and numerical aperture, irrespective of the spatial radiation properties of the light source.

For many mode scrambler designs, however, the launching conditions produced depend on t h e source/mode scrambler alignment and the interaction with any intermediary optics such as connectors or optical imaging systems. If the source or any component in the optical system is changed, the qualification tests must be repeated. When applied to information transmission capacity measurements, the overfilled launch gives good measurement reproducibility; it is not intended to necessarily give the best bandwidth prediction for concatenated lengths. Also, a particular light source/mode scrambler combination may be satisfactory for one size core diameter and numerical aperture test fiber, but not for another.