Nonlinear Fiber Optics - 4 ed. Agrawal

12.1. Intrapulse Raman Scattering 461
Figure 12.6: (a) X-FROG spectrogram of trapped pulses at the output of a 140-m-long fiber.
(b) Measured wavelengths of slow (circles) and fast (squares) polarization components for three
different fiber lengths. Filled circles show power of the fast component at the central wavelength.
(After Ref. [32]; c○2002 OSA.)
the peak power of the amplified pulse becomes comparable to that of the soliton [32].
Such a trapping mechanism can be used for ultrafast optical switching by selectively
dragging one pulse from a pulse train [33].
The preceding approach can be used to create more than two Raman solitons. The
basic idea consists of creating multiple pulses with different peak powers from a single
pump pulse. For example, if the pulse is first passed through a high-birefringence fiber
of short length and large core diameter so that nonlinear effects are negligible inside it,
the pulse will split into two orthogonally polarized pulses with a temporal spacing set
by the birefringence of this fiber. If these two pulses are then launched into a highly
nonlinear birefringent fiber whose principal axes are oriented at an angle to the first
fiber, each pulse would create two Raman solitons, resulting in the formation of four
solitons at four different wavelengths. Time-domain multiplexing or polarization-mode
dispersion inside a fiber can be used to make multiple copies of one pulse with different
amplitudes, each of which then creates a Raman soliton at a different wavelength in the
same highly nonlinear fiber [34].
12.1.4 Suppression of Raman-Induced Frequency Shifts
One may ask whether the RIFS of solitons can be suppressed under some conditions.
This question attracted attention soon after this phenomenon was discovered, and several
techniques have been proposed for answering it [35]–[39]. Bandwidth-limited
amplification of solitons for RIFS suppression was suggested in 1988 [35] and verified
in a 1989 experiment [37]. More recently, the unusual dispersive properties of photonic
crystal fibers have been utilized for this purpose [39]. In this section we focus on this
later scheme.
The GVD parameter β 2 of some microstructured fibers vanishes at two wavelengths,
one lying in the visible region and the other located in the infrared region.
The dispersion slope, governed by the TOD parameter β 3 , is positive near the first zero
but becomes negative near the second one. This change in the sign of β 3 changes the