Page 45 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes
Figure 3.6: Glass Fiber Optic Cable, Side View, and Cross Section

The light pulse travels down the center core of the glass fiber. Surrounding the inner core is a
layer of glass cladding, with a slightly different refractive index. The cladding serves to reflect
the light waves back into the inner core. Surrounding the cladding is a layer of protective plastic
coating that seals the cable and provides mechanical protection. This is shown in Figure 3.6.
Typically, multiple fibers are housed in a single sheath, which may be heavily armored.
Light propagates along the optical fiber core in one of the following ways depending on the
type and width of core material used.
Multimode Fiber

Here, the core diameter is relatively large compared to a wavelength of light. Core diameter
ranges from 50 micrometers (µm) to 1,000 µm, compared to the wavelength of light of about
1 µm. This means that light can propagate through the fiber in many different ray paths, or
modes, hence the name multimode. Multimode fiber is less expensive to produce and inferior in
performance because of the larger diameter of the inner core. When the light rays travel down
the fiber, they spread out due to a phenomenon known as modal dispersion. Although reflected
back into the inner core by the cladding, they travel different distances and, therefore, arrive at
different times. The received signal thus has a wider pulse width than the input signal with a
corresponding decrease in the speed of transmission. As a result, multimode fiber is relegated to
applications involving relatively short distances and lower speeds of transmission, for example,
LANs and campus environments. Two basic types of multimode fibers exist. The simpler and
older type is a “step index” fiber, where the index of refraction (the ability of a material to bend
light) is the same all across the core of the fiber.

Figure 3.7: Multimode Step Index Fiber

This is shown in Figure 3.7. With all these different ray paths or modes of propagation, different
rays travel different distances, and take different amounts of time to transit the length of a fiber.

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