Page 181 - Optical Communications Essentials

Active Optical Components

Active Optical Components 171

Applications of polarization controllers include polarization mode dispersion
(PMD) compensation, polarization scrambling, and polarization multiplexing.

10.8. Chromatic Dispersion Compensators
A critical factor in optical links operating above 2.5 Gbps is compensating
for chromatic dispersion effects. This phenomenon causes pulse broadening
which leads to increased bit error rates. An effective means of meeting the
strict narrow dispersion tolerances for such high-speed networks is to start
with a first-order dispersion management method, such as a dispersion-com-
pensating fiber (see Chap. 15), that operates across a wide spectral range. Then
fine-tuning can be carried out by means of a tunable dispersion compensator
that works over a narrow spectral band to correct for any residual and variable
dispersion.
The device for achieving this fine-tuning is referred to as a dispersion-
compensating module (DCM). Similar to many other devices, this module can be
tuned manually, remotely, or dynamically. Manual tuning is done by a network
technician prior to or after installation of the module in telecommunications
racks. By using network management software it can be adjusted remotely from
a central management console by a network operator, if this feature is included
in its design. Dynamic tuning is done by the module itself without any human
intervention.
One method of achieving dynamic chromatic dispersion is through the use of
a chirped fiber Bragg grating (FBG), as shown in Fig. 10.8. Here the grating
spacing varies linearly over the length of the grating, which creates what is
known as a chirped grating. This results in a range of wavelengths that satisfy
the Bragg condition for reflection. In the configuration shown, the spacings
decrease along the fiber which means that the Bragg wavelength decreases with
distance along the grating length. Consequently, the shorter-wavelength com-
ponents of a pulse travel farther into the fiber before being reflected. Thereby
they experience greater delay in going through the grating than the longer-
wavelength components. The relative delays induced by the grating on the
different-frequency components of the pulse are the opposite of the delays
caused by the fiber. This results in dispersion compensation, since it compresses
the pulse. Section 15.2.2 has more details on these devices.

Figure 10.8. Dynamic chromatic dispersion compensation is accom-
plished through the use of a chirped fiber Bragg grating.

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