Page 211 - Optical Communications Essentials

Wavelength Division Multiplexing

Wavelength Division Multiplexing 201

use of stable, high-quality, temperature-controlled and wavelength-controlled
(frequency-locked) laser diode light sources. For example, the wavelength drift
tolerances for 25-GHz channels are 0.02nm.
Table 12.1 lists part of the ITU-T G.694.1 DWDM frequency grid for 100- and
50-GHz spacings in the L- and C-bands. The column labeled “50-GHz offset”
means that for the 50-GHz grid one uses the 100-GHz spacings with these 50-GHz
values interleaved. For example, the 50-GHz channels in the L-band would be
at 186.00, 186.05, 186.10THz, and so on. Appendix C gives a more complete fre-
quency table for the L- and C-bands. Note that when the frequency spacings are
uniform, the wavelengths are not spaced uniformly because of the relationship
given in Eq. (12.1).
With the production of full-spectrum (low-water-content) fibers, the develop-
ment of relatively inexpensive VCSEL optical sources, and the desire to have
low-cost optical links operating in metro- and local-area networks came the
concept of coarse WDM (CWDM). In 2002 the ITU-T released a standard aimed
specifically at CWDM. This is Recommendation G.694.2, which is entitled Coarse
Wavelength Division Multiplexing (CWDM). The CWDM grid is made up of 18
wavelengths defined within the range of 1270 to 1610nm (O- through L-bands)
spaced by 20nm with wavelength drift tolerances of 2nm. This can be
achieved with inexpensive VCSEL light sources that are not temperature-con-
trolled. The targeted transmission distance for CWDM is 50km on single-mode
fibers, such as those specified in ITU-T Recommendations G.652, G.653, and
G.655. Chapter 13 gives more details on CWDM applications.

TABLE 12.1. Sample Portion of the ITU-T G.694.1 DWDM Grid for 100- and 50-GHz Spacings
in the L- and C-Bands
L-band C-band
100-GHz 50-GHz offset 100-GHz 50-GHz offset

Unit THz nm THz nm THz nm THz nm
1 186.00 1611.79 186.05 1611.35 191.00 1569.59 191.05 1569.18
2 186.10 1610.92 186.15 1610.49 191.10 1568.77 191.15 1568.36
3 186.20 1610.06 186.25 1609.62 191.20 1576.95 191.25 1567.54
4 186.30 1609.19 186.35 1608.76 191.30 1567.13 191.35 1566.72
5 186.40 1608.33 186.45 1607.90 191.40 1566.31 191.45 1565.90
6 186.50 1607.47 186.55 1607.04 191.50 1565.50 191.55 1565.09
7 186.60 1606.60 186.65 1606.17 191.60 1564.68 191.65 1564.27
8 186.70 1605.74 186.75 1605.31 191.70 1563.86 191.75 1563.45
9 186.80 1604.88 186.85 1604.46 191.80 1563.05 191.85 1562.64
10 186.90 1604.03 186.95 1603.60 191.90 1562.23 191.95 1561.83

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.