Page 63 - Optical Communications Essentials

Optical Fibers

Optical Fibers 53

the maximum transmission distance possible between a transmitter and a receiver
(or an amplifier) before the signal power needs to be boosted to an appropriate
level above the signal noise for high-fidelity reception. The degree of the attenu-
ation depends on the wavelength of the light and on the fiber material.
Figure 4.5 shows a typical attenuation versus wavelength curve for a silica
fiber. The loss of power is measured in decibels (see Chap. 1), and the loss within
a cable is described in terms of decibels per kilometer (dB/km).
Example Suppose a fiber has an attenuation of 0.4dB/km at a wavelength of
1310nm. Then after it travels 50km, the optical power loss in the fiber is 20dB (a fac-
tor of 100).

Figure 4.5 also shows that early optical fibers had a large attenuation spike
between 900 and 1200nm due to the fourth-order absorption peak from water
molecules. Another spike from the third-order water absorption occurs between
1350 and 1480nm for commonly fabricated fibers. Because of such absorption
peaks, three transmission windows were defined initially. The first window
ranges from 800 to 900nm, the second window is centered at 1310nm, and the
third window ranges from 1480 to 1600nm. Since the attenuation of low-water-
peak fibers makes the designation of these windows obsolete, the concept of
operational spectral bands arose for the 1260- to 1675-nm region, as described
in Chap. 1. Figure 4.6 shows the attenuation as a function of wavelength for a
low-water-peak fiber in the region covered by the six operational bands.

Figure 4.5. A typical attenuation versus wavelength curve for a silica
fiber. Early fibers had a high loss spike around 1100nm. Full-spectrum
(low-water-content) fibers allow transmission in all spectral bands.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.