Attenuation And Dispersion In Fiber-Optic Cable - Juniper EX9214 Hardware Manual

Attenuation and Dispersion in Fiber-Optic Cable

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Copyright © 2015, Juniper Networks, Inc.
Single-mode fiber is so small in diameter that rays of light 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 to multimode fiber, single-mode fiber has a higher bandwidth and can carry
signals for longer distances. It is consequently more expensive.
Exceeding the maximum transmission distances can result in significant signal loss, which
causes unreliable transmission. For information about the maximum transmission distance
and supported wavelength range for the types of single-mode and multimode fiber-optic
cables that are used on different EX Series switches see Pluggable Transceivers Supported
on EX Series Switches.
An optical data link functions correctly provided that modulated light reaching the receiver
has enough power to be demodulated correctly. Attenuation is the reduction in strength
of the light signal during transmission. Passive media components such as cables, cable
splices, and connectors cause attenuation. Although 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 transmit enough light to overcome
attenuation.
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.
Calculating the EX Series Switch Fiber-Optic Cable Power Budget on page 84
Calculating the EX Series Switch Fiber-Optic Cable Power Margin on page 84
Chapter 8: Transceiver and Cable Specifications
117