Nonlinear Fiber Optics - 4 ed. Agrawal

488 Chapter 12. Novel Nonlinear Phenomena
Figure 12.29: Measured spectra as a function of launched power at the output of a 5-m-long
fiber when 30-fs input pulses were propagated (a) without and (b) with weak second-harmonic
pulses. The white rectangle indicates the region covered in part (a) on the left. (After Ref. [125];
c○2005 OSA.)
visible region [121]. Clearly, XPM can be used as a tool to tailor the properties of the
generated supercontinuum.
12.4.4 Polarization Effects
The numerical model based on Eq. (12.4.1) assumes that the state of polarization (SOP)
of the input pulse is preserved during its propagation inside the fiber. As we have seen
in Chapter 6, this assumption is often not valid in practice. In the case of a highly
birefringent fiber, the SOP may remain preserved if the input pulse is launched with
its polarization oriented along a principal axis of the fiber. However, if it is initially
tilted at an angle, the input pulse will split into two orthogonally polarized modes of
the fiber that would begin to separate from each other because of their different group
velocities. Even in an ideal isotropic fiber, nonlinear polarization rotation induced by
XPM-induced coupling between the two orthogonally polarized fiber modes can affect
the process of soliton fission. The important question is how such phenomena affect
the generation of a supercontinuum in highly nonlinear fibers [126]–[133].
Microstructured fibers with a cladding containing multiple air holes often do not
have a circular core. The polarization dependence of supercontinuum in such fibers
has been observed in several experiments [126]–[130]. In a 2002 experiment, photonic
crystal fibers with a hexagonal symmetry of air holes in its cladding were employed
[127]. The birefringence should be relatively weak in these fibers because of a nearly
circular core. Nevertheless, it was found that the supercontinuum changed with the
input SOP. Moreover, different parts of the supercontinuum exhibited different SOPs,
indicating a nonlinear coupling between the two orthogonally polarized pulses within
the fiber. Figure 12.30 shows changes in the recorded supercontinuum spectra with
±30 ◦ rotations in the SOP of a linearly polarized input pulse, realized by rotating a 41-
mm-long fiber with the 2.5-μm core diameter. In this experiment, 18-fs input pulses
from a Ti:sapphire laser operating at 790 nm were launched into the fiber. It is clear