Dispersion is the spreading of the signal over time. The following two types of dispersion can affect signal
transmission through an optical data link:
•
Chromatic dispersion, which is the spreading of the signal over time caused by the different speeds of
light rays.
•
Modal dispersion, which is the spreading of the signal over time caused by the different propagation
modes in the fiber.
For multimode transmission, modal dispersion, rather than chromatic dispersion or attenuation, usually
limits the maximum bit rate and link length. For single-mode transmission, modal dispersion is not a factor.
However, at higher bit rates and over longer distances, chromatic dispersion limits the maximum link length.
An efficient optical data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specifications. In addition, the total dispersion must be within the limits
specified for the type of link in Telcordia Technologies document GR-253-CORE (Section 4.3) and
International Telecommunications Union (ITU) document G.957.
When chromatic dispersion is at the maximum allowed, its effect can be considered as a power penalty in
the power budget. The optical power budget must allow for the sum of component attenuation, power
penalties (including those from dispersion), and a safety margin for unexpected losses.
Understanding QFX Series Fiber-Optic Cable Signal Loss, Attenuation, and
Dispersion
IN THIS SECTION
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables | 151
Attenuation and Dispersion in Fiber-Optic Cable | 152
To determine the power budget and power margin needed for fiber-optic connections, you need to
understand how signal loss, attenuation, and dispersion affect transmission. The QFX Series uses various
types of network cables, including multimode and single-mode fiber-optic cables.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables
Multimode fiber is large enough in diameter to allow rays of light to reflect internally (bounce off the walls
of the fiber). Interfaces with multimode optics typically use LEDs as light sources. However, LEDs are not
coherent light sources. They spray varying wavelengths of light into the multimode fiber, which reflect
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