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differential mode delay (DMD)

Differential Mode Delays (DMD) are signal distortions due to different propagation times of modes in optical fibers. They are also called multimode group delays because lower and higher order modes propagate differently in multimode fibers and thus have different propagation times.

Differential mode delays can also be caused by partial excitation of laser light injected into a multimode fiber.

10 Gigabit signal

10 Gigabit signal

deformed
by dispersion

by dispersion

When light signals are transmitted in optical fibers, they are deformed by profile dispersion, which is due to production tolerances in the manufacture of optical fibers. As a result, the transmitted light pulses are broadened and flattened, leading to the corresponding DMD delays.

In Gigabit Ethernet and in even faster Ethernet variants, the DMD distortions can affect the transmission specifications. They are therefore specified.

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Englisch: differential mode delay - DMD
Updated at: 28.11.2016
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