Page 221 - Optical Communications Essentials

Wavelength Division Multiplexing

Wavelength Division Multiplexing 211

Interleavers are bidirectional, so they can be used as either a multiplexer or
a demultiplexer. For simplicity of discussion here we will consider the demulti-
plexing function. Interleavers are characterized by the designation 1 N,
which indicates one input and N output ports, as shown in Fig. 12.10. For ex-
ample, consider a series of wavelengths separated by 25GHz entering a 1 4
interleaver. The interleaver splits the incoming channels into four sets of 100-GHz
spaced channels. This greatly simplifies further demultiplexing by allowing
optical filtering at 100GHz instead of at the initial 25GHz. Note that for a
1 N demultiplexer, every Nth wavelength is selected to exit a particular port.
For example, in the 1 4 interleaver, wavelengths λ 1 , λ 5 , λ 9 , . . . exit port 1.
Figure 12.11 shows an application of a demultiplexing function for a combin-
ation of 160 C-band and L-band channels spaced by 50GHz. First a wideband
demultiplexer separates the incoming spectrum of wavelengths into 80 C-band
and 80 L-band channels, which still are separated by 50GHz. Next the channels
of each band pass through 1 2 interleavers that separate the wavelengths into
sets of 40 channels with 100-GHz spacings. Further demultiplexing with less
stringent optical filtering then may be carried out.

λ 1 , λ 2 , λ 3 , λ 4 ... 1 × λ 1 , λ 3 , λ 5 ...
1 ⋅ 2 2
Interleaver λ 2 , λ 4 , λ 6 ...
Interleaver

λ 1 , λ 5 , λ 9 ...
λ 1 , λ 2 , λ 3 , λ 4 ...
1 × λ 2 , λ 6 , λ 10 ...
1 ⋅ 4 4
Interleaver λ 3 , λ 7 , λ 11 ...
Interleaver
λ 4 , λ 8 , λ 12 ...
Figure 12.10. Examples of wavelength separations
by two different interleavers.

40 C-band
40 C-band
100-GHz channels
100-GHz channels
80 C-band
80 C-band
50-GHz channels
160 channels 50-GHz channels 40 C-band
40 C-band
spaced by 100-GHz channels
50-GHz 100-GHz channels
Wideband
Wideband
demux
demux
40 L-band
40 L-band
100-GHz channels
100-GHz channels
80 L-band
80 L-band
50-GHz channels
50-GHz channels
40 L-band
40 L-band
100-GHz channels
100-GHz channels
Figure 12.11. An application of an interleaver demultiplexing func-
tion for a combination of 160 C- and L-band channels spaced by
50GHz.
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