Nonlinear Fiber Optics - 4 ed. Agrawal

12.4. Temporal and Spectral Evolution 489
Figure 12.30: Supercontinuum spectra at the output of a photonic crystal fiber for three different
orientations of linearly polarized 18-fs input pulses. (After Ref. [127]; c○2002 OSA.)
from Figure 12.30 that spectral details of a supercontinuum are quite sensitive to the
input SOP of the pulse.
In another experiment, a highly birefringent microstructured fiber with an elliptical
core was employed [128]. Figure 12.31 shows changes in the supercontinuum spectra
observed as the angle φ (that the SOP direction of the input pulse makes from the slow
axis) was varied. This experiment launched 200-fs pulses with 56-mW average power
(at a 80-MHz repetition rate) at a wavelength near 732 nm into a 5-m-long fiber with
the effective mode area of 2.3 μm 2 . The ZDWL of the fiber was 635 and 675 nm for
light polarized along the fast and slow axes of the fiber, respectively. When light is
polarized along these two axes (φ = 0or90 ◦ ), only one of the orthogonally polarized
modes is excited. Differences observed in the supercontinuum spectra in these two
cases are due to different dispersive properties of the fiber experienced by the pulse
because of a large birefringence.
In the case (b) in Figure 12.31 (φ = 45 ◦ ), both modes are excited with the same
peak powers because the pulse energy splits into two equal parts. The two orthogonally
polarized pulses travel at different group velocities because of birefringence. However,
they separate from each other in