Page 235 - Optical Communications Essentials

Constructing the WDM Network Puzzle

Constructing the WDM Network Puzzle 225

1 ×
1 ⋅ 2 2
λ 1 , λ 2 , λ 3 , ... λ 8 λ 1 , λ 3 , λ 5 , λ 7
Interleaver
Interleaver
λ 2 , λ 4 , λ 6 , λ 8
Figure 13.7. A 1 2 interleaver separates eight
wavelengths into two sets of four wavelengths.

λ 1 , λ 5 , λ 9 ...
λ 1 , λ 2 ,… λ 16 λ 2 , λ 6 , λ 10 ...
1 × 1 ×
1 ⋅ 4 4
1 ⋅ 4 4
Interleaver Interleaver
Interleaver
Interleaver
λ 3 , λ 7 , λ 11 ...
λ 4 , λ 8 , λ 12 ...

Dropped Added
wavelengths wavelengths
Figure 13.8. An interleaver can route or drop a group of channels
while allowing all other wavelengths to pass through the device.

The remainder of the wavelengths then pass through and travel to the next
node. These through-going wavelengths are known as express traffic. Figure 13.8
shows an example of this. Here 16 wavelengths enter a 1 4 deinterleaver that
separates them into four sets. A second identical 1 4 interleaver then can
recombine them onto a single fiber. Wavelengths λ 4 , λ 8 , λ 12 , and λ 16 are chosen
as the add/drop channels. After being dropped, separated, and processed, these
four wavelengths can be multiplexed again and inserted into the second inter-
leaver to rejoin the express wavelengths. Although the majority of the traffic is
express, the transmission through an interleaver/deinterleaver pair can place
high demands on the power budget and the performance requirements to min-
imize channel mixing and crosstalk.

13.2.5. Wideband optical amplifiers
An amazingly rapid development of various configurations of optical amplifiers
for use in WDM systems occurred since 1990. Whereas EDFA devices originally
dominated for operation in the C-band (1530 to 1560nm), Raman amplifiers for
operation in any of the optical fiber transmission bands also are in use now. The
use of optical amplifiers has spawned a whole new area of both passive and
active optical components. Most importantly are the pump lasers that are
required for signal amplification.
When EDFAs first started being used, their operation was limited to the
1530- to 1560-nm C-band in which a standard erbium-doped fiber has a high
gain response. By adding a Raman amplification mechanism, the gain response

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.