Page 224 - Optical Communications Essentials

Wavelength Division Multiplexing

214 Chapter Twelve

Figure 12.14. Example of a module that provides an absolute multi-
frequency grid over a broad spectrum. (Photo courtesy of DiCOS
Technologies; www.dicostech.com.)

12.4. Summary
Wavelength division multiplexing allows many different wavelengths selected
from the spectral regions ranging from the O-band through the L-band to be
sent along a single fiber simultaneously. A wide variety of passive and active
components are used for WDM. The technologies include thin-film filters
(TFFs), arrayed waveguide gratings (AWGs), Bragg fiber gratings, diffraction
gratings, interleavers, wavelength lockers, controllers for tunable transmitters,
dynamic gain equalizers (DGEs), tunable wavelength filters, and variable optic-
al attenuators (VOAs).
The first ITU-T specification for WDM was Recommendation G.692, Optical
Interfaces for Multichannel Systems with Optical Amplifiers. This document spe-
cifies selecting the channels from a grid of frequencies referenced to 193.100THz
(1552.524nm) and spacing them 100GHz (about 0.8nm at 1550nm) apart.
Suggested alternative spacings in G.692 include 50 and 200GHz. In 2002 the
ITU-T released an updated standard for dense WDM (DWDM). This is
Recommendation G.694.1, which is entitled Dense Wavelength Division
Multiplexing (DWDM). It specifies WDM operation in the S-, C-, and L-bands
and specifies narrow frequency spacings of 100 to 12.5GHz (or, equivalently, 0.8
to 0.1nm at 1550nm).
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 con-
cept 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

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