Page 169 - Optical Communications Essentials

Passive Optical Components

Passive Optical Components 159

Figure 9.13. Illustration of the meanings of several filter parame-
ters given in Table 9.6.

which means that a range of different wavelengths will be reflected by the FBG.
This is the basis of what is known as a chirped grating. Section 15.2 describes
the construction and application of such a device in greater detail.

9.5. Summary
Passive components include optical couplers, isolators, circulators, filters, grat-
ings, and wavelength multiplexers. Optical couplers perform functions such as
splitting a light signal into two or more streams, combining two or more light
streams, tapping off a small portion of optical power for monitoring purposes,
or transferring a selective range of optical power from one fiber to another. In
general, an N M coupler has N 2 input ports and M 2 output ports. For
example, a device with two inputs and two outputs is called a 2 2 coupler.
Optical isolators are devices that allow light to pass through them in only one
direction. This is important in a number of instances to prevent scattered or
reflected light from traveling in the reverse direction. One common application
of an optical isolator is to keep such light from entering a laser diode and pos-
sibly causing instabilities in the optical output. Table 9.3 lists some typical
parameter values of commercially available isolators.
An optical circulator is a nonreciprocal multiport passive device that directs
light sequentially from port to port in only one direction. The operation of a cir-
culator is similar to that of an isolator except that its construction is more com-
plex. A variety of circulators are available commercially. These devices have low
insertion loss, high isolation over a wide wavelength range, minimal polarization-
dependent loss (PDL), and low polarization mode dispersion (PMD). Table 9.4
lists some operational characteristics of commercially available circulators.
A dielectric thin-film filter (TFF) is used as an optical bandpass filter. This
means that it allows a particular very narrow wavelength band to pass straight
through it and reflects all others. The basis of these devices is a classical Fabry-
Perot filter structure. In practice a TFF consists of a stack of several dielectric

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