T1600 Attenuation And Dispersion In Fiber-Optic Cable - Juniper T1600 Hardware Manual

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T1600 Attenuation and Dispersion in Fiber-Optic Cable

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results. Together these factors limit the transmission distance of multimode fiber
compared to single-mode fiber.
Single-mode fiber is so small in diameter that rays of light can reflect internally through
one layer only. Interfaces with single-mode optics use lasers as light sources. Lasers
generate a single wavelength of light, which travels in a straight line through the
single-mode fiber. Compared with multimode fiber, single-mode fiber has higher
bandwidth and can carry signals for longer distances. It is consequently more expensive.
For information about the maximum transmission distance and supported wavelength
range for the types of single-mode and multimode fiber-optic cable used by PICs on the
T1600 router, see the T1600 Core Router PIC Guide. Exceeding the maximum transmission
distances can result in significant signal loss, which causes unreliable transmission.
T1600 Calculating Optical Power Budget for Fiber-Optic Cable on page 367
T1600 Calculating Optical Power Margin for Fiber-Optic Cable on page 368
Correct functioning of an optical data link depends on modulated light reaching the
receiver with enough power to be demodulated correctly. Attenuation is the reduction in
power of the light signal as it is transmitted. Attenuation is caused by passive media
components, such as cables, cable splices, and connectors. While attenuation is
significantly lower for optical fiber than for other media, it still occurs in both multimode
and single-mode transmission. An efficient optical data link must have enough light
available to overcome attenuation.
Dispersion is the spreading of the signal in time. Two types of dispersion can affect an
optical data link:
Chromatic dispersion—Spreading of the signal in time resulting from the different
speeds of light rays.
Modal dispersion—Spreading of the signal in time resulting from 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 rather than modal dispersion limits 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 less than 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.
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