Chapter 6
Optical Amplifiers
As seen in Chapter 5, the transmission distance of any fiber-optic communication sys-
tem is eventually limited by fiber losses. For long-haul systems, the loss limitation
has traditionally been overcome using optoelectronic repeaters in which the optical
signal is first converted into an electric current and then regenerated using a transmit-
ter. Such regenerators become quite complex and expensive for wavelength-division
multiplexed (WDM) lightwave systems. An alternative approach to loss management
makes use of optical amplifiers, which amplify the optical signal directly without re-
quiring its conversion to the electric domain. Several kinds of optical amplifiers were
developed during the 1980s, and the use of optical amplifiers for long-haul lightwave
systems became widespread during the 1990s. By 1996, optical amplifiers were a part
of the fiber-optic cables laid across the Atlantic and Pacific oceans. This chapter is
devoted to optical amplifiers. In Section 6.1 we discuss general concepts common
to all optical amplifiers. Semiconductor optical amplifiers are considered in Section
6.2, while Section 6.3 focuses on Raman amplifiers. Section 6.4 is devoted to fiber
amplifiers made by doping the fiber core with a rare-earth element. The emphasis is
on the erbium-doped fiber amplifiers, used almost exclusively for 1.55-μm lightwave
systems. System applications of optical amplifiers are discussed in Section 6.5.
6.1 Basic Concepts
Most optical amplifiers amplify incident light through stimulated emission, the same
mechanism that is used by lasers (see Section 3.1). Indeed, an optical amplifier is
nothing but a laser without feedback. Its main ingredient is theoptical gainrealized
when the amplifier is pumped (optically or electrically) to achievepopulation inversion.
The optical gain, in general, depends not only on the frequency (or wavelength) of the
incident signal, but also on the local beam intensity at any point inside the amplifier.
Details of the frequency and intensity dependence of the optical gain depend on the
amplifier medium. To illustrate the general concepts, let us consider the case in which
the gain medium is modeled as a homogeneously broadened two-level system. The