Page 213 - Optical Communications Essentials

Wavelength Division Multiplexing

Wavelength Division Multiplexing 203

a 10dB level is not sufficient whereas a level of 30dB is acceptable. In princi-
ple, any optical demultiplexer also can be used as a multiplexer. For simplicity,
the word multiplexer is used as a general term to refer to both combining and
separating functions, except when it is necessary to distinguish the two devices
or functions.

12.2. Multiplexers for WDM
A key WDM component is the wavelength multiplexer. The function of this
device is to combine independent signal streams operating at different wave-
lengths onto the same fiber. Many different techniques using specialized com-
ponents have been devised for combining multiple wavelengths onto the same
fiber and separating them at the receiver. Each of these techniques has certain
advantages and of course various limitations. These include thin-film filters,
arrayed waveguide gratings, Bragg fiber gratings, diffraction gratings, and
interleavers. The performance demands on these components are increasing
constantly with the desire to support higher channel counts and longer dis-
tances between terminals.
Whereas prior to 2000 the standard wavelength spacing was 100GHz for
2.5-Gbps DWDM links, the current move is toward 10-Gbps ultradense systems
operating with channels that are spaced 25 or 12.5GHz apart. An even further
squeezing of the channels is seen in the hyperfine WDM products that have sep-
arations down to 3.125GHz. For 40-Gbps systems the channels nominally are
spaced 50 or 100GHz apart because of the greater impact from nonlinear dis-
persion effects at these higher data rates. The expansion of WDM channels
beyond the C-band into the S- and L-bands has allowed the possibility of send-
ing 320 wavelengths spaced 25GHz apart in the combined C- and L-band with
10-Gbps transmission rates per channel. This is in contrast to the earlier 96
maximum 10-Gbps channels that were separated by 50GHz in the C-band.
Multiplexers for CWDM applications have less stringent performance demands
for certain parameters such as center wavelength tolerance, its change with
temperature, and the passband sharpness. However, they still need to have a
good reflection isolation, a small polarization-dependent loss, and low insertion
losses. These CWDM devices can be made with thin-film filter technology.

12.2.1. Thin-film filters
Section 9.3 discusses the operational principles of thin-film optical filters. Here
we will look at how to use them in a WDM system to create a multiplexer. As
Sec. 9.3 describes, a thin-film filter allows only a very narrow slice of spectral
width to pass through it and reflects all other light outside this band. To create
a wavelength multiplexing device for combining or separating N wavelength
channels, one needs to cascade N 1 thin-film filters.
Figure 12.3 illustrates a multiplexing function for the four wavelengths λ 1 , λ 2 ,
λ 3 , and λ 4 . Here the filters labeled TFF 2 , TFF 3 , and TFF 4 pass wavelengths λ 2 ,

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