Nonlinear Fiber Optics - 4 ed. Agrawal

12.2. Four-Wave Mixing 467
Figure 12.11: (a) Phase matching curves as a function of pump wavelength for a photonic crystal
fiber having its ZDWL at 1038 nm. Tuning curve of the pump laser is also shown. (b) Walk-off
delay (dashed curve) as a function of pump wavelength and a typical output spectrum showing
the signal and idler bands on opposite sides of the pump. (After Ref. [57]; c○2005 OSA.)
900 nm. This type of multiwave generation was first predicted in a 1989 study [59] and
was also found in 1995 to occur in dispersion-flattened fibers with two ZDWLs [60].
12.2.2 Effects of Fiber Birefringence
It was seen in Section 6.4 that fiber birefringence affects the FWM process behind
the phenomenon of modulation instability. Moreover, it can also lead to the so-called
polarization instability. As one would expect, both instabilities change considerably for
highly nonlinear fibers with unusual dispersion characteristics. In one study, dispersion
terms up to 12th order were included for calculating the instability gain, while carrying
Figure 12.12: Phase-matching curves as a function of pump wavelength for a photonic crystal
fiber whose dispersion curve shown on the right exhibits two ZDWLs near 755 and 1235 nm.
Arrows mark the four wavelengths phase-matched simultaneously when λ p = 752 nm. (After
Ref. [54]; c○2004 OSA.)