Attenuation And Dispersion In Fiber-Optic Cables - Juniper PTX10004 Hardware Manual

index material in close contact with a core material of higher refractive index), higher-order mode loss
occurs. Together, these factors reduce the transmission distance of multimode fiber compared to that of
single-mode fiber.
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.
For information about the maximum transmission distance and supported wavelength range for the types
of single-mode and multimode fiber-optic cables that are connected to the PTX Series, see
Compatibility Tool. Exceeding the maximum transmission distances can result in significant signal loss,
which causes unreliable transmission.

Attenuation and Dispersion in Fiber-Optic Cables

An optical data link functions correctly if the 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 transmissions. 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 caused by the different speeds of light rays.
Modal dispersion, which is 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 the 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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