Cisco Cisco DWDM Transceiver Modules 白皮書
White Paper
© 2008 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information.
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The “cladding” is the layer completely surrounding the core. The difference in refractive index
between the core and cladding is less than 0.5 percent. The refractive index of the core is higher
than that of the cladding, so that light in the core strikes the interface with the cladding at a
bouncing angle, gets trapped in the core by total internal reflection, and keeps traveling in the
proper direction down the length of the fiber to its destination.
Surrounding the cladding is usually another layer, called a “coating,” which typically consists of
protective polymer layers applied during the fiber drawing process, before the fiber contacts any
surface. “Buffers” are further protective layers applied on top of the coating.
Figure 1. Basic View of an Optical Fiber
Types of Fiber and Various Parameters
Fibers come in several different configurations, each ideally suited to a different use or application.
Early fiber designs that are still used today include single-mode and multimode fiber. Since Bell
Laboratories invented the concept of application-specific fibers in the mid-1990s, fiber designs for
specific network applications have been introduced. These new fiber designs – used primarily for
the transmission of communication signals – include Non-Zero Dispersion Fiber (NZDF), Zero
Water Peak Fiber (ZWPF), 10-Gbps laser optimized multimode fiber (OM3), and fibers designed
specifically for submarine applications. Specialty fiber designs, such as dispersion compensating
fibers and erbium doped fibers, perform functions that complement the transmission fibers. The
differences among the different transmission fiber types result in variations in the range and the
number of different wavelengths or channels at which the light is transmitted or received, the
distances those signals can travel without being regenerated or amplified, and the speeds at which
those signals can travel.
A number of key parameters impact how optical fibers perform in transmission systems. The
specifications for each parameter will vary by fiber type, depending upon the intended application.
Two of the more important fiber parameters are attenuation and dispersion. Attenuation is the
reduction in optical power as it passes from one point to another. In optical fibers, power loss
results from absorption and scattering and is generally expressed in decibels (dB) for a given
length of fiber, or per unit length (dB/km) at a specific transmission wavelength. High attenuation
limits the distance a signal can be sent through a network without adding costly electronics to the
system. Figure 2 illustrates the variation of attenuation with wavelength taken over an ensemble of
fiber optic cable material types. The three principal windows of operation, propagation through a
cable, are indicated. These correspond to wavelength regions where attenuation is low and
matched to the ability of a transmitter to generate light efficiently and a receiver to carry out
detection. Hence, the lasers deployed in optical communications typically operate at or around 850
nanometers (nm) (first window), 1310 nm (second window), and 1550 nm (third and fourth
windows).