Page 153 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
9

Passive Optical Components

In addition to fibers, light sources, and photodetectors, many other components
are used in a complex optical communication network to split, route, process, or
otherwise manipulate light signals. The devices can be categorized as either
passive or active components. Passive optical components do not hum or wink
or blink, since they require no external source of energy to perform an opera-
tion or transformation on an optical signal. Just as a filter in a coffee pot or a
sprayer head in a shower just sit there while performing very important func-
tions, passive components carry out their unique processes without any physi-
cal or electrical action. For example, a passive optical filter will allow only a
certain wavelength to pass through it while absorbing or reflecting all others,
and an optical splitter divides the light entering it into two or more, smaller opti-
cal power streams. 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 flexibility.
Although optical fibers and connectors are passive elements, one usually con-
siders them separately from other passive optical components. Some basic pas-
sive functions and the devices which enable them are as follows:

■ Transfer energy: optical fibers
■ Attenuate light signals: optical attenuators, isolators
■ Influence the spatial distribution of a light wave: directional coupler, star coup-
ler, beam expander
■ Modify the state of polarization: polarizer, half-wave plates, Faraday rotator
■ Redirect light: circulator, mirror, grating
■ Reflect light: fiber Bragg gratings, mirror
■ Select a narrow spectrum of light: optical filter, grating
■ Convert light wave modes: fiber gratings, Mach-Zehnder interferometer

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