Page 209 - Optical Communications Essentials

Wavelength Division Multiplexing

Wavelength Division Multiplexing 199

12.1.1. WDM operating regions
To see the potential of WDM, let us first examine the characteristics of a high-
quality optical source. As an example, a distributed-feedback (DFB) laser has a
frequency spectrum on the order of 1MHz, which is equivalent to a spectral
linewidth of 10 5 nm. With such spectral-band widths, simplex systems make
use of only a tiny portion of the transmission bandwidth capability of a fiber.
This can be seen from Fig. 12.1, which depicts the attenuation of light in a sil-
ica fiber as a function of wavelength. The curve shows that the two low-loss
regions of a standard single-mode fiber extend over the O-band wavelengths
ranging from about 1270 to 1350nm (originally called the second window) and
from 1480 to 1600nm (originally called the third window).
We can view these regions either in terms of spectral width (the wavelength
band occupied by the light signal) or by means of optical bandwidth (the fre-
quency band occupied by the light signal). To find the optical bandwidth corres-
ponding to a particular spectral width in these regions, we use the fundamental
relationship c λν, which relates the wavelength λ to the carrier frequency ν,
where c is the speed of light. Differentiating this, we have
c
∆ν ∆λ (12.1)
λ 2
where the deviation in frequency ∆ν corresponds to the wavelength deviation ∆λ
around λ.
Now suppose we have a fiber that has the attenuation characteristic shown
in Fig. 12.1. From Eq. (12.1) the optical bandwidth is ∆ν 14THz for a usable

2.0 100-GHz (0.8-nm) channel
spacing for ITU-T standard

1.5
Attenuation (dB/km) 1.0 1535 to
1562 nm

0.5 14 THz 15 THz Third
window
Second
window
0
900 1100 1300 1500 1700
Wavelength (nm)
Figure 12.1. The transmission bands of a standard single-mode fiber
in the O-band (second window) and from 1480 to 1600nm (third win-
dow) allow the use of many simultaneous channels. The first ITU-T
standard for WDM specified channels with 100-GHz spacings.

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.