Mode scrambler

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.

Prior to the IEEE 802.3 10GBASE-LRM Ethernet standard, which requires a specific range of mode power distribution be excited in fiber by semiconductor lasers, mode scramblers were used primarily 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, which is unacceptable for trade and commerce. Multimode optical fiber exhibits differential time delays amongst its propagating modes, differential mode delay, which is a normally unpredictable residue caused by imperfections or nonideality of the fiber refractive index profile. Semiconductor lasers can differ in emitted field distribution, modal content and emission region size, which results in variation of the mode power distribution excited in the fiber. In conjunction with the fiber's differential mode delay, the result is variation and non-reproducibility of the measured fiber bandwidth.

The mode scrambler has two purposes. (Determination of an "LED bandwidth" is not one of them.) First, it diminishes the effect of output variability amongst lasers, and has ameliorated the lack of standards for such characteristics. Second, it more uniformly distributes the optical power among the all the fiber's modes, effectively averaging the differential mode delays and reducing excitation variability caused by minor misalignment between source and fiber. On average, but not in every case, the bandwidth measured using a mode scrambler is lower than that produced by excitation of a partial mode volume, such as occurs with directly-coupled laser diodes.

One common type of mode scrambler is the "Step-Graded-Step" (S-G-S) fiber assembly, a fusion-spliced concatenation of three fibers. The first and third fibers are step-index profile; the middle section is graded-index. Typically, each segment is approximately 1 meter long. These have been used in fiber manufacturing test systems since about 1980.

Other forms of mode scrambler include corregated or rough-surfaced plates squeezed or pressed against a fiber to create microbending, which enhances mode coupling. The reproducibility of microbend-induced mode scramblers is not as good as that of the S-G-S type, since S-G-S properties are determined only by the index profile characteristics of the fibers, joined with stable fusion splices.

A mode scrambler can be characterized by measuring its output intensity (nearfield) distribution as a small mode volume source, such as a laser or single-mode fiber, is placed or imaged at each of several positions across the core of the mode scrambler input endface. Uniformity and invariance of the output intensity distribution can be used to define a figure of merit for the mode scrambler. Practical mode scramblers only approximate the ideal result.

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