Nonlinear Fiber Optics - 4 ed. Agrawal

306 Chapter 8. Stimulated Raman Scattering
Figure 8.18: (a) Evolution toward a parabolic shape of a “sech” input pulse over the 5.3-km
length of the Raman amplifier. (b) Intensity and chirp (solid lines) profiles of the output pulse
together with parabolic and linear fits, respectively (circles). The bottom part shows the same
results on a logarithmic scale together with the Gaussian (dashed curve) and “sech” (dotted
curve) fits. (After Ref. [147]; c○2003 OSA.)
Raman amplifier [147]. The shape of the output pulse can be fitted quite well with a
parabola but the fit is relatively poor when the shape is assumed to be either Gaussian
(dashed curve) or hyperbolic secant (dotted curve). The chirp is approximately linear.
Both the predicted pulse shape and the chirp profile agree well with the experimental
data. It was also confirmed experimentally that the final pulse characteristics were independent
of input pulse parameters and were determined solely by the Raman amplifier.
Both the width and the chirp changed when the gain of the amplifier was changed,
but the pulse shape remained parabolic, and the frequency chirped across the pulse
remained nearly linear.
8.4 Soliton Effects
When wavelengths of the pump and Raman pulses lie in the anomalous-GVD region
of a fiber, the soliton effects become important (see Section 5.2.1). Such effects have
attracted considerable attention, both theoretically and experimentally [150]–[166].
When the Raman pulse propagates as a soliton, it is common to refer to it as a Raman
soliton [165]. These solitons must be distinguished from the pair of bright and
dark solitons formed during transient SRS in molecular gases. In that case, it is necessary
to include the dynamics of the vibrational mode participating in the process of
transient SRS [167]–[169]. In contrast, Raman solitons discussed here occur in the
stationary SRS regime.
8.4.1 Raman Solitons
In the anomalous-dispersion region of an optical fiber, under suitable conditions, almost
all of the pump-pulse energy can be transferred to a Raman pulse that propagates