370 CHAPTER 8. MULTICHANNEL SYSTEMS
Several schemes have been proposed for raising the Brillouin threshold [182]–[187].
They rely on increasing either the Brillouin-gain bandwidth∆νBor the spectral width
of optical carrier. The former has a value of about 20 MHz for silica fibers, while the
latter is typically<10 MHz for DFB lasers used in WDM systems. The bandwidth of
an optical carrier can be increased without affecting the system performance by modu-
lating its phase at a frequency much lower than the bit rate. Typically, the modulation
frequency∆νmis in the range of 200–400 MHz. As the effective Brillouin gain is re-
duced by a factor of( 1 +∆νm/∆νB), the SBS threshold increases by the same factor.
Since typically∆νB∼20 MHz, the launched power can be increased by more than a
factor of 10 by this technique.
If the Brillouin-gain bandwidth∆νBof the fiber itself can be increased from its
nominal value of 20 MHz to more than 200 MHz, the SBS threshold can be increased
without requiring a phase modulator. One technique uses sinusoidal strain along the
fiber length for this purpose. The applied strain changes the Brillouin shiftνBby a few
percent in a periodic manner. The resulting Brillouin-gain spectrum is much broader
than that occurring for a fixed value ofνB. The strain can be applied during cabling of
the fiber. In one fiber cable,∆νBwas found to increase from 50 to 400 MHz [182]. The
Brillouin shiftνBcan also be changed by making the core radius nonuniform along the
fiber length since the longitudinal acoustic frequency depends on the core radius. The
same effect can be realized by changing the dopant concentration along the fiber length.
This technique increased the SBS threshold of one fiber by 7 dB [183]. A side effect
of varying the core radius or the dopant concentration is that the GVD parameterβ 2
also changes along the fiber length. It is possible to vary both of them simultaneously
in such a way thatβ 2 remains relatively uniform [185]. Phase modulation induced by
a supervisory channel through the nonlinear phenomenon of cross-phase modulation
(XPM) can also be used to suppress SBS [187]. XPM induced by neighboring chan-
nels can also help [184] but it is hard to control and is itself a source of crosstalk.
In practice, a frequency modulator integrated within the transmitter provides the best
method for suppressing SBS. Threshold levels>200 mW have been realized with this
technique [186].
8.3.5 Cross-Phase Modulation
The SPM and XPM both affect the performance of WDM systems. The effects of SPM
has been discussed in Sections 5.3 and 7.7 in the context of single-channel systems;
they also apply to individual channels of a WDM system. The phenomenon of XPM
is an important mechanism of nonlinear crosstalk in WDM lightwave systems and has
been studied extensively in this context [188]–[199].
As discussed in Section 2.6, XPM originates from the intensity dependence of the
refractive index, which produces an intensity-dependent phase shift as the signal prop-
agates through the optical fiber. The phase shift for a specific channel depends not only
on the power of that channel but also on the power of other channels [59]. The total
phase shift for thejth channel is given by (see Section 2.6)
φNLj =γ
α(
Pj+ 2N∑
m =jPm