Mode scramblers are used to provide a modal distribution that is independent of the optical source for purposes of laboratory, manufacturing, or field measurements or tests.
The output modal power distribution of a mode scrambler is similar to the surface-emitters used in those first LAN transmitters, but this was fortuitous coincidence.
There are two common types of mode scramblers: the "Step-Graded-Step" (S-G-S) and the "step index with bends".
Step Index with Bend mode scramblers are created simply by routing a specially designed step-index multimode fiber through a series of small radius bends, or by compressing fiber against surfaces with specific roughness.
Guidelines for constructing a mode scrambler and qualifying its output can be found in the ANSI/TIA/EIA-455-54 fiber optic test procedure (FOTP).