Nonlinear Fiber Optics - 4 ed. Agrawal

382 Chapter 10. Four-Wave Mixing
Figure 10.7: Phase-matching diagrams near the zero-dispersion wavelength for three values of
the pump wavelength λ 1 . Dotted, dashed, and solid lines show respectively Δk M , Δk W , and their
sum. (After Ref. [15]; c○1981 OSA.)
by up to 46 dB while, at the same time, three pairs of Stokes and anti-Stokes lines were
observed at the fiber output. These equally spaced lines (spacing 3.3 THz) originate
from a cascade of FWM processes in which successive lines interact with each other to
generate new frequencies. In a later experiment, FWM occurred spontaneously without
an input signal [15]. Mode-locked input pulses at 1.319 μm, with peak powers ∼1kW,
were propagated through a 50-m-long fiber. Their peak power was large enough to
exceed the Raman threshold. Figure 10.8 shows the spectrum observed at the fiber
output. The Stokes and anti-Stokes lines at 1.67 and 1.09 μm, respectively, originate
from FWM. The huge frequency shift (ν s ≈ 48 THz) is comparable to that achieved
in multimode fibers. Similar experiments show that ν s can be varied in the range 3–50
THz by changing the core diameter from 7.2 to 8.2 μm [16]. This scheme is useful for
realizing new optical sources pumped by a 1.319-μm Nd:YAG laser.
Phase Matching Due to Self-Phase Modulation
When the pump wavelength lies in the anomalous-GVD regime and deviates considerably
from λ 0 , Δk M exceeds significantly from Δk W and it becomes difficult to achieve
phase matching (see Figure 10.7). However, because Δk M +Δk W is negative, it is possi-
Figure 10.8: Output spectra showing Stokes and anti-Stokes bands generated by FWM. The
Raman band is also shown. (After Ref. [15]; c○1981 OSA.)