Router; Signal Loss In Multimode And Single-Mode Fiber-Optic Cable For The M7I Router - Juniper M7i Hardware Manual

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Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the M7i Router

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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. The following 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.
Fiber-Optic and Network Cable Specifications for the M7i Router on page 91
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the M7i Router on
page 92
Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 93
Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 94
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. LEDs are not coherent sources, however. They spray varying wavelengths
of light into the multimode fiber, which reflects the light at different angles. Light rays
travel in jagged lines through a multimode fiber, causing signal dispersion. When light
traveling in the fiber core radiates into the fiber cladding, higher-order mode loss (HOL)
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
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