Page 210 - Optical Communications Essentials

Wavelength Division Multiplexing

200 Chapter Twelve

spectral band ∆λ 80nm in the center of the O-band. Similarly, ∆ν 15THz
for a usable spectral band ∆λ 120nm in the low-loss region running from
near the beginning of the S-band to almost the end of the L-band. This yields a
total available fiber bandwidth of about 30THz in the two low-loss windows.
The insert in Fig. 12.1 shows how a series of 100-GHz channel separations fits
into the C-band.
Prior to about 2000, the peak wavelengths of adjacent sources typically were
restricted to be separated by 0.8 to 1.6nm (100 to 200GHz). This was done to
take into account possible drifts of the peak wavelength due to aging or tem-
perature effects, and to give both the manufacturer and the user some leeway
in specifying and choosing the precise peak emission wavelength. As described
in Sec. 12.1.3, the next generation of WDM systems specified both narrower and
much wider channel spacings depending on the application and on the wave-
length region being used. The much narrower spacings thus require strict wave-
length control of the optical source, as discussed in Sec. 12.3. On the other hand,
the wider wavelength separations offer inexpensive WDM implementations
since wavelength control requirements are relaxed significantly.
Example If one takes a spectral band of 0.8nm (or, equivalently, a mean frequency
spacing of 100GHz) within which narrow-linewidth lasers are transmitting, then one
can send about 36 independent signals in the 1530- to 1560-nm C-band on a single
fiber.

12.1.2. WDM standards
Since WDM is essentially frequency division multiplexing at optical carrier fre-
quencies, the WDM standards developed by the International Telecommunication
Union (ITU) specify channel spacings in terms of frequency. A key reason for
selecting a fixed-frequency spacing, rather than a constant-wavelength spacing,
is that when a laser is locked to a particular operating mode, it is the frequency
of the laser that is fixed. The first ITU-T specification for WDM was Recom-
mendation G.692, Optical Interfaces for Multichannel Systems with Optical
Amplifiers. This document specifies selecting the channels from a grid of fre-
quencies 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, which correspond to spectral widths of 0.4 and 1.6nm,
respectively, at 1550nm.
The literature often uses the term dense WDM (DWDM), in contrast to con-
ventional or regular WDM. Historically this term was used somewhat loosely to
refer to channel spacings such as those denoted by ITU-T G.692. In 2002 the
ITU-T released an updated standard aimed specifically at 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
for high-quality, high-rate metro-area network (MAN) and wide-area network
(WAN) services. It calls out for narrow frequency spacings of 100 to 12.5GHz
(or, equivalently, 0.8 to 0.1nm at 1550nm). This implementation requires the

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