Page 172 - Optical Communications Essentials

Active Optical Components

162 Chapter Ten

Tunable optical sources, which are described in Chap. 6, allow their emission
wavelength to be tuned precisely to a particular optical frequency (or, equiva-
lently, to a particular wavelength) by some external control mechanism.
Wavelength lockers are important devices in WDM systems to maintain the
output from a laser diode at a predefined ITU-T frequency with a precision of
1GHz (or 8 pm). More details on this device are given in Chap. 12 on WDM.
External modulators are described in Chap. 6. Such a device can be in the
form of a separate external package, or it can be integrated into the laser
diode package. These components allow the optical output to be modulated
external to the light source at rates greater than 2.5Gbps without significant
distortion.
Photodetectors are described in Chap. 7. A photodetector acts upon an optical
signal by sensing the light signal falling on it and converting the variation of
the optical power to a correspondingly varying electric current.
Optical amplifiers are described in Chap. 11. These devices operate com-
pletely in the optical domain to boost the power level of optical signals. They
work over a broad spectral range and boost the amplitudes of independent sig-
nals at all wavelengths in this band simultaneously. The fundamental optical
amplification mechanisms are based on semiconductor devices, erbium-doped
optical fibers, and the Raman effect in standard transmission fibers.
Variable optical attenuators (VOAs) are used in multiple-wavelength links to
adjust the power levels of individual wavelengths so that they closely have the
same value. This chapter describes the construction and operation of VOAs.
Tunable optical filters are key elements in a WDM system where one needs
the flexibility to be able to select a specific wavelength for data receipt or per-
formance monitoring. This chapter describes the construction and operation
of tunable optical filters. Chapter 12 on WDM presents further details con-
cerning the applications of these tunable filters.
Dynamic gain equalizers, also called dynamic channel equalizers or dynamic
spectral equalizers, provide dynamic gain equalization or blocking of individ-
ual channels across a given spectral band within a link in a WDM system. This
chapter describes the construction and operation of some representative
devices.
Optical add/drop multiplexers (OADM) can be passive or active devices. Their
function is to add or drop one or more selected wavelengths at a designated
point in an optical network. This chapter describes an active OADM, and
Chap. 17 describes switching applications in a network.
Polarization controllers offer high-speed real-time polarization control in a
closed-loop system that includes a polarization sensor and control logic. These
devices dynamically adjust any incoming state of polarization to an arbitrary
output state of polarization. This chapter describes their construction and
operation.

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