Page 228 - Optical Communications Essentials

Constructing the WDM Network Puzzle

218 Chapter Thirteen

WDM WDM Optical
mux Optical fiber demux receivers

λ 1 λ 1
• •
OADM
• OADM •
λ N λ N
In-line Preamplifier
Fixed or amplifier Tunable
tunable Added/dropped optical
sources wavelengths filters
Figure 13.1. The major parts of a generic WDM link.

onto a single fiber. Within the link there may be optical amplifiers, add/drop
multiplexers for inserting or subtracting individual wavelengths along the path,
and other devices to enhance the link performance. At the end of the link there
is a demultiplexing device for separating the wavelengths into independent sig-
nal streams and an array of tunable optical receivers.
A major application of WDM technology is to increase the capacity of long-
haul links. Owing to the large amount of traffic carried on these long links,
high-performance wideband components are required. Metro WDM links have
a different set of applications which point to the need for lower-cost narrowband
components. One important point is that WDM-based networks are bit-rate- and
protocol-independent, so they can carry various types of traffic at different
speeds concurrently.

13.1.1. Wideband long-haul WDM network
Wideband long-haul networks are essentially a collection of point-to-point
trunk lines with one or more optical add/drop multiplexers (OADMs) for insert-
ing and extracting traffic at intermediate points. Standard transmission dis-
tances in long-haul terrestrial WDM links are 600km with 80km between
optical amplifiers. Since a primary desire is to have a high link capacity, mod-
ern systems can carry 160 channels running at 2.5 or 10Gbps each (OC-48/
STM-16 or OC-192/STM-64, respectively). By boosting the transmission capacity,
long-haul DWDM networks lower the cost per bit for high-rate traffic.
If the 160 channels are separated by 50GHz, then the frequency span is
8THz (8000GHz or a wavelength band of about 65nm). This shows that oper-
ation is required over both the C- and L-bands simultaneously. As a result, the
various active and passive components must meet high performance require-
ments, such as the following:
■ The optical amplifiers need to operate over a wide spectral band (e.g., over
both the C- and L-bands).
■ High-power pump lasers are needed for the optical amplifiers in order to
amplify a large number of channels.

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