Nonlinear Fiber Optics - 4 ed. Agrawal

11.4. Microstructured Fibers 443
Effective Cladding Index
(a)
λ/Λ
10
(b)
λ/Λ
d/Λ
Figure 11.13: (a) Effective cladding index of a PCF plotted as a function of λ/Λ for values
of d/Λ in the range of 0.2 to 0.9. (b) Three operational regimes of a PCF. The solid curve
corresponds to the condition V eff = 2.405. The empty and filled circles show the numerical and
experimental data, respectively. (After Ref. [84]; c○2005 IEEE.)
ZDWL of such fibers can be varied over a wide range extending from 0.5 to 1.5 μm
with changes in d and Λ. A remarkable feature of Figure 11.14 is that such fibers exhibit
two ZDWLs, one falling in the visible region and the other beyond 1 μm, such that
GVD is anomalous in the spectral region between them. It will be seen in Chapter 12
that this feature can be exploited for observing a variety of novel nonlinear effects.
The tapering technique discussed in Section 11.3 has been employed for reducing
the core diameter of PCFs [85]–[87]. The main advantage is that both the dispersive
and nonlinear properties of a PCF can be altered with tapering, provided care is taken
to ensure that air holes within the cladding do not collapse. In practice, the core size
as well as the diameter of air holes can be reduced by a factor of 2 or so without
collapsing the air holes. Even such small changes modify the characteristics of most
microstructured fibers significantly because their dispersion is extremely sensitive to
the size and spacing of air holes.
Dispersion [ps/(km-nm)]
(a)
(b)
Wavelength (μm)
Wavelength (μm)
Figure 11.14: Variation of dispersion D with wavelength λ for PCFs designed with different
values of d/Λ when Λ = 1 and 2 μm. (After Ref. [84]; c○2005 IEEE.)