Mode scrambler

In telecommunications, a mode scrambler or mode mixer is a device for inducing mode coupling in an optical fiber, or a device that, itself, exhibits a uniform output intensity profile independent of the input mode volume or modal excitation condition. 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. Mode scramblers are primarily used to improve reproducibility of multimode fiber bandwidth measurements.

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, overfilled launch condition that can be easily reproduced on multiple measurement systems, so that these measurement systems have essentially the same launch conditions and can produce roughly the same bandwidth measurements despite having different laser sources. This overfilled launch (OFL) was originally created in an effort to replicate the launch conditions that were produced by the LED sources used in early datacom links. 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.

There are two common types of mode scramblers: the "Step-Graded-Step" (S-G-S) and the "Step Index with Bends". The S-G-S mode scrambler is actually an assembly, a fusion-spliced concatenation of a step-index profile, a graded-index profile and another step-index profile fiber. Typically, each segment is approximately 1 meter long. This type of mode scrambler was used in early fiber manufacturing test systems, but due to difficulty complying with revised qualification standards have largely been replaced in commercial bandwidth test equipment by "Step Index with Bends" mode scramblers. 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.

A mode scrambler can be characterized and qualified by measuring its near field and far field distributions, as well as by measuring one of these distributions while restricting the other. 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).

See also

• Equilibrium mode distribution
• Mandrel wrapping

References

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).