Overview

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 operation 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 functions, passive components carry out their unique processes without any physical 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 optical 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 considers them separately from other passive optical components. Some basic passive 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 coupler, beam expander

• Modify the state of polarization: polarizer, half-wave plates, Faraday rotator

• Redirect light: circulator, mirror, grating