Nonlinear Fiber Optics - 4 ed. Agrawal

12.1. Intrapulse Raman Scattering 463
Figure 12.8: Measured spectra as a function of average input power when 110-fs pulse at wavelengths
(a) 1400, (b) 1510, and (c) 1550 nm were launched into a fiber with its second ZDWL
near 1510 nm (horizontal dashed line). A darker shading represents higher power. Right panel
shows the spectra at the 9-mW power level. (After Ref. [42]; c○2005 Elsevier.)
lost to the NSR affects the soliton itself [26]. The requirement of energy conservation
dictates that a soliton would become slightly wider as it loses energy to NSR, as N = 1
must be maintained. This broadening is relatively small in practice if solitons lose energy
slowly. Much more important is the requirement of momentum conservation. It
dictates that, as NSR is emitted in the normal-GVD regime, the soliton should “recoil”
further into the anomalous-GVD regime. This spectral recoil mechanism is responsible
for the suppression of RIFS. Near the first ZDWL, spectral recoil is negligible because
the RIFS forces soliton spectrum to shift away from this wavelength, and the loss of
energy decreases as β 2 increases. However, the exact opposite occurs near the second
ZDWL, where the RIFS moves the soliton closer to it, and the rate of energy loss to
NSR increases drastically. The resulting spectral recoil reduces the RIFS until a steady
state is reached in which spectral recoil is just large enough to nearly cancel the RIFS.
The X-FROG technique as well as numerical simulations confirm the preceding physical
scenario [41]. Thus, we can conclude that the unusual dispersive properties of some
highly nonlinear fibers are responsible for the suppression of RIFS in combination with
the emission of NSR induced by higher-order dispersive effects.
The pulse-propagation scenario seen in Figure 12.7 depends on the input wavelength
of pulses because it sets the initial values of the dispersion parameters. In
one study, 110-fs pulses were launched at wavelengths close to the second ZDWL
(around 1510 nm) of a 1-m-long photonic crystal fiber [42]. Figure 12.8 shows the
measured spectra at the fiber output as a function of the launched average power (at