6.3. RAMAN AMPLIFIERS 243
Figure 6.10: Schematic of a fiber-based Raman amplifier in the forward-pumping configuration.
It is clear that SOAs suffer from several drawbacks which make their use as in-line
amplifiers impractical. A few among them are polarization sensitivity, interchannel
crosstalk, and large coupling losses. The unsuitability of SOAs led to a search for
alternative amplifiers during the 1980s, and two types of fiber-based amplifiers using
the Raman effect and rare-earth dopants were developed. The following two sections
are devoted to these two types of amplifiers. It should be stressed that SOAs have found
many other applications. They can be used for wavelength conversion and can act as a
fast switch for wavelength routing in WDM networks. They are also being pursued for
metropolitan-area networks as a low-cost alternative to fiber amplifiers.
6.3 Raman Amplifiers
A fiber-based Raman amplifier usesstimulated Raman scattering(SRS) occurring in
silica fibers when an intense pump beam propagates through it [27]–[29]. The main
features of SRS have been discussed in Sections 2.6. SRS differs from stimulated emis-
sion in one fundamental aspect. Whereas in the case of stimulated emission an incident
photon stimulates emission of another identical photon without losing its energy, in the
case of SRS the incident pump photon gives up its energy to create another photon
of reduced energy at a lower frequency (inelastic scattering); the remaining energy is
absorbed by the medium in the form of molecular vibrations (optical phonons). Thus,
Raman amplifiers must be pumped optically to provide gain. Figure 6.10 shows how
a fiber can be used as a Raman amplifier. The pump and signal beams at frequencies
ωpandωsare injected into the fiber through a fiber coupler. The energy is transferred
from the pump beam to the signal beam through SRS as the two beams copropagate in-
side the fiber. The pump and signal beams counterpropagate in the backward-pumping
configuration commonly used in practice.
6.3.1 Raman Gain and Bandwidth
The Raman-gain spectrum of silica fibers is shown in Figure 2.18; its broadband nature
is a consequence of the amorphous nature of glass. The Raman-gain coefficientgRis
related to the optical gaing(z)asg=gRIp(z), whereIpis the pump intensity. In terms
of the pump powerPp, the gain can be written as
g(ω)=gR(ω)(Pp/ap), (6.3.1)