Page 185 - Optical Communications Essentials

Source: Optical Communications Essentials

Chapter
11

Optical Amplifiers

Scattering and absorption mechanisms in an optical fiber cause a progressive
attenuation of light signals as they travel along a fiber. At some point the sig-
nals need to be amplified so that the receiver can interpret them properly.
Traditionally this was done by means of a regenerator that converted the opti-
cal signal to an electrical format, amplified this electric signal, and then recon-
verted it to an optical format for further transmission along the link. The
development of an optical amplifier circumvented the time-consuming function
of a regenerator by boosting the level of a light signal completely in the optical
domain. Thus optical amplifiers now have become indispensable components in
high-performance optical communication links.
This chapter classifies the three fundamental optical amplifier types, defines
their operational characteristics, and describes their basic applications. The
three basic technologies are semiconductor optical amplifiers (SOAs), doped-
fiber amplifiers (DFAs), and Raman amplifiers. SOAs are based on the same
operating principles as laser diodes, whereas the other two types employ a fiber
as the gain mechanism. Among the DFAs, erbium-doped fiber amplifiers
(EDFAs) are used widely in the C- and L-bands for optical communication net-
works. In contrast to an EDFA which uses a specially constructed fiber for the
amplification medium, a Raman amplifier makes use of the transmission fiber
itself.
This chapter first looks at SOA technology since the same operational princi-
ples apply to all types of amplifiers. These discussions include external pump-
ing principles, gain mechanisms, noise effects, and SOA characteristics. Next
we look at the operational principles and applications of EDFA devices. Fol-
lowing this is a discussion of Raman amplification technology and use. Finally,
Sec. 11.6 illustrates how an SOA may be used as a wavelength-converting device
for deployment in optical networks.

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