Page 171 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
10

Active Optical Components

Active components require some type of external energy either to perform their
functions or to be used over a wider operating range than a passive device,
thereby offering greater application flexibility. In that sense, optical sources,
external modulators, and optical amplifiers can be considered as falling into the
broad area of active devices. However, these are examined in separate chapters
since they constitute major elements in an optical link. In addition, to get a full
appreciation of the functions and applications of a particular active device, one
needs a detailed discussion of how it is used in its intended system place.
Therefore, in a number of cases we will only briefly look at the characteristics
of a certain component and will defer a detailed description of its functions to a
later chapter. Section 10.1 specifies which devices fall into this category.
The active devices described in this chapter include variable optical attenua-
tors, tunable optical filters, dynamic gain equalizers, optical add/drop multi-
plexers, polarization controllers, and dispersion compensators. Many types of
active optical components are based on using microelectromechanical systems
(MEMS) technology. This is the topic of Sec. 10.2. The remainder of the chapter
describes various active devices. Sometimes one can purchase a module that
contains many different components but which has one particular function
(e.g., dispersion compensators, add/drop multiplexers). These will be treated as
active components for the purposes of this chapter.

10.1. Overview of Major Components
Before diving into device details, we first take an introductory look at various
types and categories of active components to get an overview of the different
functions they perform. In some cases the device characteristics are examined
in either previous or forthcoming chapters. In other cases the devices will only
be highlighted briefly in this chapter and will be described in greater detail in
later chapters when examining their application.
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