Page 301 - Fiber Optic Communications Fund

282 Fiber Optic Communications

6.8 Raman Amplifiers
Distributed Raman amplifiers have become a viable alternative to EDFAs because of their relatively lower
ASE [10, 13–15]. Raman amplifiers are based on stimulated Raman scattering, which occurs in fibers at high
powers (see Chapter 10 for more details). As an intense pump beam of frequency propagates down the
p
fiber, an optical wave of lower frequency is generated due to SRS. The frequency difference, − =Ω,
s
s
p
is known as Stokes’s shift. If a signal field of frequency (Stokes wave) is incident at the input of the fiber
s
along with the pump beam, the signal field gets amplified due to SRS. As shown in Fig. 6.20, the pump photons
cause transitions to the excited level 3 from level 1, and silica molecules relax to one of the vibrational levels
in band 2; the energy difference ℏ( − ) appears as molecular vibrations or optical phonons. If a signal
s
p
photon corresponding to the energy difference between level 3 and one of the levels in band 2 is present, the
molecules are stimulated to emit signal photons of the same kind, leading to the amplification of the signal
photons, which is known as SRS. The silica molecule could also make a transition to band 2 from level 3
by spontaneous emission, whether or not the signal beam is present. This is known as spontaneous Raman
scattering and is the source of noise in Raman amplifiers. Band 2 is a collection of vibrational states of silicon
molecules. In other words, part of the pump energy is converted into signal energy and the rest is dissipated as
molecular vibrations. Quantum mechanically, a pump photon of energy ℏ is annihilated to create a signal
p
photon of lower energy ℏ and an optical phonon of energy ℏΩ. A semiclassical description of the Raman
s
scattering is provided in Section 10.11. Fig. 6.21 shows the typical Raman gain spectrum as a function of
the frequency shift for a silica-core single-mode fiber. The frequency shift shown in Fig. 6.21 refers to the
frequency deviation of the Stokes wave from the pump. The Raman gain curve has a peak around a frequency
shift, Ω of about 14 THz. In amorphous materials such as fused silica, molecular vibrational states form a
continuum [14] shown by crossed lines in Fig. 6.20 and, therefore, the Raman gain occurs over a broad range
of frequencies up to 40 THz. Fig. 6.22 shows a schematic of the Raman amplifier with co-propagating pump.

Level 3
ħω s
ħω p
Band 2
(vibrational levels)
ħΩ
Level 1
Figure 6.20 Energy levels of silica.

Normalized Raman gain coefficient

0 14 THz 24 THz Frequency shift

Figure 6.21 Typical Raman gain spectrum of silica fibers.