Nonlinear Fiber Optics - 4 ed. Agrawal

310 Chapter 8. Stimulated Raman Scattering
Figure 8.22: Pulse spectra at the output of a 1-km-long fiber for several peak powers of 0.83-ps
input pulses. The input wavelength of 1.341 μm lies in the anomalous-GVD region of the fiber.
(After Ref. [155]; c○1987 IEEE.)
λ p = 1.317 μm, the Stokes bands are more intense because SRS is more effective in
transferring pulse energy toward low frequencies.
Spectral changes for input pulses launched well into the anomalous-GVD regime
are shown in Figure 8.22 for λ p = 1.341 μm and a fiber length of 1 km. Pulse spectra
change considerably with an increase in the input peak power. Three Stokes bands
appear for P 0 = 530 W (topmost curve). An anti-Stokes band carrying 5 to 10% of the
input energy also appears. In the time domain, a separate Raman pulse is associated
with each of the Stokes bands. The autocorrelation measurements show that the widths
of these Raman solitons are ∼300 fs [155]. The width depends considerably on the
fiber length; the smallest width was 55 fs for 150-ps wide input pulses. When input
peak power was increased to 5 kW, multiple Stokes bands could be generated in a 20-
m-long fiber. The fourth-order Stokes band near 1.3 μm served as a pump to generate a
continuum that extended up to 1.7 μm. Using a two-stage compression configuration,
the pulse width could be reduced to 18 fs, a pulse consisting of only three optical
cycles. This 1987 experiment constitutes an early example of the formation of Raman
solitons and supercontinuum generation. This phenomenon has attracted considerable
attention in recent years [170]–[172] with the advent of highly nonlinear fibers and is
discussed in detail in Chapter 12.
Raman solitons have also been generated using 100-ps pump pulses from a modelocked
Nd:YAG laser operating at 1.32 μm. In one experiment, the use of a conventional
fiber with the zero-dispersion wavelength near 1.3 μm led to 100-fs Raman