Page 46 - DCAP406_DCAP_207_Computer Networks

Unit 3: Physical Layer

This being the case, if a short pulse of light is injected into a fiber, the various rays emanating Notes
from that pulse will arrive at the other end of the fiber at different times, and the output pulse
will be of longer duration than the input pulse. This phenomenon is called “modal dispersion”
(pulse spreading), and limits the number of pulses per second that can be transmitted down a
fiber and still be recognizable as separate pulses at the other end. This, therefore, limits the bit
rate or bandwidth of a multimode fiber. For step index fibers, wherein no effort is made to
compensate for modal dispersion, the bandwidth is typically 20 to 30 MHz over a length of one
kilometer of fiber, expressed as “MHz - km”.

Graded Index Multimode Fiber

In the case of a graded index multimode fiber, the index of refraction across the core is gradually
changed from a maximum at the center to a minimum near the edges, hence the name graded
index. This design takes advantage of the phenomenon that light travels faster in a low-index-
of-refraction material than in a high-index material. If a short pulse of light is launched into the
graded index fiber, it may spread some during its transit of the fiber, but much less than in the
case of a step index fiber. Therefore, dispersion can be reduced using a core material that has a
variable refractive index. In such multimode graded index fiber light is refracted by an increasing
amount as it moves away from the core as shown in Figure 3.8. This has the effect of narrowing
the pulse width of the received signal compared with stepped index fiber, allowing a
corresponding increase in the speed of transmission. They therefore can support a much higher
bit rate or bandwidth.

Did u know? Typical bandwidths of graded index fibers range from 100 MHz-km to well
over 1GHz-km. The actual bandwidth depends on how well a particular fiber’s index
profile minimizes modal dispersion, and on the wavelength of light launched into the
fiber.
Figure 3.8: Multimode Granded Index

Monomode/Singlemode fiber: This has a thinner inner core. In this case, the core diameter of
about 9 µm is much closer in size to the wavelength of light being propagated, about 1.3 µm.
This limits the light transmission to a single ray or mode of light to propagate down the core of
the fiber as shown in Figure 3.9. All the multiple-mode or multimode effects described above
are eliminated. However, one pulse-spreading mechanism remains. Just as in the multimode
fibers, different wavelengths of light travel at different speeds, causing short pulses of light
injected into the fiber to spread as they travel. This phenomenon is called “chromatic dispersion”.

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