Juniper PTX10001-36MR - Page 76

Signal Loss in Mulmode and Single-Mode Fiber-Opc Cable
Mulmode ber is large enough in diameter to allow rays of light to reect internally (bounce o the
walls of the ber). Interfaces with mulmode opcs typically use LEDs as light sources. However, LEDs
are not coherent sources. They spray varying wavelengths of light into the mulmode ber, which
reects the light at dierent angles. Light rays travel in jagged lines through a mulmode ber, causing
signal dispersion. When light traveling in the ber core radiates into the ber cladding, higher-order
mode loss results. Together these factors limit the transmission distance of mulmode ber compared
with single-mode ber.
Single-mode ber is so small in diameter that rays of light can reect internally through one layer only.
Interfaces with single-mode opcs use lasers as light sources. Lasers generate a single wavelength of
light, which travels in a straight line through the single-mode ber. Compared with mulmode ber,
single-mode ber has higher bandwidth and can carry signals for longer distances.
Exceeding the maximum transmission distances can result in signicant signal loss, which causes
unreliable transmission.
Aenuaon and Dispersion in Fiber-Opc Cable
Correct funconing of an opcal data link depends on modulated light reaching the receiver with
enough power to be demodulated correctly.
Aenuaon is the reducon in power of the light signal as it
is transmied. Aenuaon is caused by passive media components such as cables, cable splices, and
connectors. Although aenuaon is signicantly lower for opcal ber than for other media, it sll
occurs in both mulmode and single-mode transmission. An ecient opcal data link must have enough
light available to overcome aenuaon.
Dispersion is the spreading of the signal over me. The following two types of dispersion can aect an
opcal data link:
• Chromac dispersion—Spreading of the signal over me, resulng from the dierent speeds of light
rays.
• Modal dispersion—Spreading of the signal over me, resulng from the dierent propagaon modes
in the ber.
For mulmode transmission, modal dispersion—rather than chromac dispersion or aenuaon—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, chromac dispersion rather than modal
dispersion limits maximum link length.
An ecient opcal data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specicaons. In addion, the total dispersion must be less than the limits
specied for the type of link in Telcordia Technologies document GR-253-CORE (Secon 4.3) and
Internaonal Telecommunicaons Union (ITU) document G.957.
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