Nonlinear Fiber Optics - 4 ed. Agrawal

466 Chapter 12. Novel Nonlinear Phenomena
Figure 12.10: Output spectra recorded for four values of the pump polarization angle from the
slow axis of a photonic crystal fiber that was pumped at 647 nm using 70-ps pulses with 160-W
peak power. (After Ref. [47]; c○2003 OSA.)
0.5 ps and tunable over a 200-nm range were obtained using a ring cavity containing
65 cm of a photonic crystal fiber [57]. The laser was pumped synchronously using 1.3-
ps pulses from a mode-locked ytterbium fiber laser capable of delivering up to 260 mW
of average power. Figure 12.11(a) shows the tuning range of the pump, together with
the range of laser wavelengths for which phase-matching condition is satisfied as pump
wavelength is tuned. This parametric oscillator is, in principle, tunable over 300 nm
by changing the pump wavelength over a 20-nm range. In practice, the walk-off effects
resulting from the group-velocity mismatch limited the tuning range to around
200 nm. The dashed curve in Figure 12.11(b) shows that the walk-off delay between
the pump and laser pulses exceeds 1 ps for a 150-nm detuning. The tuning range of
such parametric oscillators can be increased further by employing wider pump pulses.
As mentioned earlier, microstructured fibers often exhibit two ZDWLs. FWM in
such fibers has been studied and it leads to interesting new features [54]. Equation
(12.2.1) can still be used to calculate the signal and idler wavelengths that satisfy the
phase-matching condition, provided that the actual dispersion curve of the fiber is used
to find the dispersion parameters at each pump wavelength. Figure 12.12 shows the
phase-matched wavelengths for a photonic crystal fiber with a 1.8-μm core diameter for
a CW pump whose wavelength is varied by 20 nm near the first ZDWL. The launched
pump power is assumed to be 1 kW. The measured wavelength dependence of β 2 for
this fiber is also shown; β 2 vanishes at wavelengths of 755 and 1235 nm.
A novel feature of such fibers is that two sets of wavelengths can be phase-matched
simultaneously at a fixed pump wavelength. For example, when the pump light travels
in the normal-GVD regime close to the first ZDWL near 755 nm, the four wavelengths
fall in a range extending from 500 to 1500 nm. The longest wavelength generated
through the FWM process is more than 600 nm away from the pump wavelength in
this specific case. Even larger wavelength shifts can occur for pump wavelengths near