Juniper MX2010, Hardware Manual

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Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
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 sources. 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 results.
Together these factors limit the transmission distance of multimode fiber compared with 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.
Exceeding the maximum transmission distances can result in significant signal loss, which causes unreliable
transmission.
Attenuation and Dispersion in Fiber-Optic Cable
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. 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 have enough light
available to overcome attenuation.
Dispersion is the spreading of the signal over time. The following two types of dispersion can affect an
optical data link:
•
Chromatic dispersion—Spreading of the signal over time resulting from the different speeds of light rays.
•
Modal dispersion—Spreading of the signal over 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.
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