Nonlinear Fiber Optics - 4 ed. Agrawal

292 Chapter 8. Stimulated Raman Scattering
are chosen such that they form three cavities for three Raman lasers operating at wavelengths
1.117, 1.175, and 1.24 μm that correspond to the first-, second-, and third-order
Stokes lines of a 1.06-μm pump. All three lasers are pumped using a diode-pumped
Nd-fiber laser through cascaded SRS. The 1.24-μm laser then pumps the Raman amplifier
to provide signal amplification in the 1.3-μm region. The same idea of cascaded
SRS was used to obtain a 39-dB gain at 1.3 μm by using WDM couplers in place of
fiber gratings [89]. In a different approach, the core of silica fiber is doped heavily
with germania. Such a fiber can be pumped to provide 30-dB gain at a pump power of
only 350 mW [90]. Such pump powers can be obtained by using two or more semiconductor
lasers. A dual-stage configuration has also been used in which a 2-km-long
germania-doped fiber is placed in series with a 6-km-long dispersion-shifted fiber in
a ring geometry [93]. Such a Raman amplifier, when pumped by a 1.24-μm Raman
laser, provided a 22-dB gain in the 1.3-μm wavelength region with a noise figure of
about 4 dB.
Raman amplifiers can be used to extend the bandwidth of WDM systems operating
in the 1.55-μm region [94]–[96]. Erbium-doped fiber amplifiers used commonly in
this wavelength regime, have a bandwidth of under 40 nm. Moreover, a gain-flattening
technique is needed to use the entire 40-nm bandwidth. Massive WDM systems typically
require optical amplifiers capable of providing uniform gain over a 70 to 80-nm
wavelength range. Hybrid amplifiers made by combining erbium doping with Raman
gain have been developed for this purpose. In one implementation of this idea [96],
a nearly 80-nm bandwidth was realized by combining an erbium-doped fiber amplifier
with two Raman amplifiers, pumped simultaneously at three different wavelengths
(1471, 1495, and 1503 nm) using four pump modules, each module launching more
than 150 mW of power into the fiber. The combined gain of 30 dB was nearly uniform
over a wavelength range of 1.53 to 1.61 μm.
Starting around 2000, distributed Raman amplification was used for compensation
of fiber losses in long-haul WDM systems [97]–[102]. In this configuration, in place of
using erbium-doped fiber amplifiers, relatively long spans (80 to 100 km) of the transmission
fiber are pumped bidirectionally to provide Raman amplification. In a 2000
demonstration of this technique, 100 WDM channels with 25-GHz channel spacing,
each operating at a bit rate of 10 Gb/s, were transmitted over 320 km [97]. All channels
were amplified simultaneously by pumping each 80-km fiber span in the backward
direction using four semiconductor lasers. By 2004, 128 channels, each operating at
10 Gb/s, could be transmitted over 4000 km with the use of distributed Raman amplification
[101].
8.2.4 Raman-Induced Crosstalk
The same Raman gain that is beneficial for making fiber amplifiers and lasers is also
detrimental to WDM systems. The reason is that a short-wavelength channel can act
as a pump for longer-wavelength channels and thus transfer part of the pulse energy to
neighboring channels. This leads to Raman-induced crosstalk among channels that can
affect the system performance considerably [103]–[113].
Consider first a two-channel system with the short-wavelength channel acting as a
pump. The power transfer between the two channels is governed by Eqs. (8.1.2) and