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Understanding QFX Series Fiber-Optic Cable Signal Loss, Attenuation, and
Dispersion
IN THIS SECTION
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables | 94
Attenuation and Dispersion in Fiber-Optic Cable | 94
To determine the power budget and power margin needed for fiber-optic connections, you need to
understand how signal loss, attenuation, and dispersion affect transmission. The QFX Series uses various
types of network cables, including multimode and single-mode fiber-optic cables.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables
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 light sources. They spray varying wavelengths of light into the multimode fiber, which reflect
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 (layers of lower refractive
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 QFX Series, see the Hardware
Compatibility Tool . Exceeding the maximum transmission distances can result in significant signal loss,
which causes unreliable transmission.
Attenuation and Dispersion in Fiber-Optic Cable
An optical data link functions correctly provided that 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
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