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 attenuators, tunable optical filters, dynamic gain equalizers, optical add/drop multiplexers, 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.