Editing Mode scrambler (section)

== Overview ==
If multimode fiber bandwidth is measured using a laser diode directly coupled to its input, the resulting measurement can vary by as much as an order of magnitude. This measurement variability is due to the combination of differences in laser output characteristics (emitted mode power distribution) and the differential mode delay of the fiber. Differential mode delay is the difference in the time delays amongst the fiber's propagating modes caused by imperfections or nonideality of the fiber refractive index profile.

The primary purpose of a mode scrambler is to create a uniform, [[overfill]]ed launch condition that can be easily reproduced on multiple measurement systems, so that measurement systems have essentially the same launch conditions and can measure approximately the same bandwidth despite having different laser sources. These were used for this purpose in the first U.S. NIST round-robins on multimode fiber.<ref name="nist">{{Cite web |url=http://nvlpubs.nist.gov/nistpubs/sp958-lide/297-299.pdf |title=Archived copy |access-date=2012-02-08 |archive-date=2013-02-17 |archive-url=https://web.archive.org/web/20130217073201/http://nvlpubs.nist.gov/nistpubs/sp958-lide/297-299.pdf |url-status=dead }}</ref> The overfilled launch (OFL) was created to reduce measurement variability, and improve concatenation estimates for multimode fibers, used at that time for telecom 'long haul' (e.g., 7–10&nbsp;km 850&nbsp;nm or 20–30&nbsp;km 1300&nbsp;nm) systems.<ref name="nist"/>

When the telecom industry converted to near-exclusive use of single-mode fiber ca. 1984, multimode fiber was re-purposed for use in LANs, such as Fiber Distributed Data Interface (FDDI), then under development.  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.  On average, but not in every case, the OFL bandwidth measured using a mode scrambler is lower than that produced by excitation of a partial mode volume (restricted mode launch or RML), such as occurs with directly coupled laser diodes.