Page 237 - Optical Communications Essentials

Constructing the WDM Network Puzzle

Constructing the WDM Network Puzzle 227

C-band/980-nm Erbium-doped
power combiner fiber Tap
Isolator coupler

Pump-reflection
Pump laser Monitor
Pump laser
Monitor
(980 nm) filter photodiode
photodiode
(980 nm)
Figure 13.10. Schematic of a narrowband EDFA for use in metro
networks.
EDFA where it would create output instabilities. The monitor photodiode veri-
fies that the pump laser output is providing the desired EDFA gain.

13.2.7. Optical add/drop multiplexer
An optical add/drop multiplexer (OADM) allows the insertion or extraction of a
wavelength from a fiber at a point between terminals. An OADM can operate
either statically or dynamically. Some vendors call a dynamic device a reconfig-
urable OADM (R-OADM). A static version obviously is not as flexible and may
require a hardware change if a different wavelength needs to be dropped or
added. For example, a static OADM might use two optical circulators in con-
junction with a series of fixed-wavelength fiber Bragg gratings. A dynamic or
reconfigurable OADM results if the gratings are tunable. Although a dynamic
feature adds greater flexibility to a network, this versatility also requires more
careful system design. In particular, tunable (wavelength-selectable) optical fil-
ters may be needed at the drop receivers, and the optical signal-to-noise ratio
for each wavelength must be analyzed more exactly.
Depending on whether an engineer is designing a metropolitan-area network
(MAN) or a long-haul network, different performance specifications need to be
addressed when implementing an OADM capability in the network. In general,
because of the nature of the services provided, changes in the add/drop config-
uration for a long-haul network tend to occur less frequently than in a MAN. In
addition, the channel spacing is much narrower in a long-haul network, and the
optical amplifiers which are used must cover a wider spectral band. For an
interesting analysis of the EDFA performance requirements and the link power
budgets used for an OADM capability in a MAN environment, the reader is
referred to the paper by Pan et al.

13.2.8. Chromatic dispersion compensation
Chromatic dispersion and polarization-mode dispersion are the two principal
signal-distorting mechanisms in optical fiber links. To mitigate these effects,
different techniques usually are implemented in separate modules for each dis-
persion type at the end of a fiber or following an EDFA. Chromatic dispersion
occurs because any optical pulse contains a spectrum of wavelengths. Since each

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