Dispersion; Signal Loss In Multimode And Single-Mode Fiber-Optic Cable; Attenuation And Dispersion In Fiber-Optic Cable - Juniper T640 Hardware Manual

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Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
Copyright © 2013, Juniper Networks, Inc.
Calculating Power Budget and Power Margin for Fiber-Optic Cables on page 446
Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion on page 445
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable on page 445
Attenuation and Dispersion in Fiber-Optic Cable on page 445

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. 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 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. It is consequently more expensive.
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. While 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 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.
Appendix E: T640 Router Cable and Wire Guidelines and Specifications
445