Nonlinear Fiber Optics - 4 ed. Agrawal

474 Chapter 12. Novel Nonlinear Phenomena
SPM-broadened pulse spectrum by producing the two Stokes bands located at 666
and 686 nm (through cascaded SRS). The anti-Stokes bands are also created near 612
and 629 nm through a process known as coherent anti-Stokes Raman scattering. The
spectrum is relatively narrow at the 100-W level because the pulse is propagating in
the normal-dispersion region of the fiber [D = −20 ps/(nm-km)]. At a power level
of 230 W, two FWM-induced side bands appear near 525 and 950 nm. As they were
found to be orthogonally polarized, phase matching for them appears to be provided by
the fiber birefringence.
The important question is what mechanism produces in Figure 12.17 a nearly symmetric
broad supercontinuum at power levels above 200 W. The answer turned out to
be a strong FWM-type coupling between the Stokes and anti-Stokes bands [83]. One
should also note that some Stokes components fall in the anomalous-GVD regime of
the fiber. The energy within these spectral sidebands can appear in the form of solitons.
At high pump powers, their peak power becomes large enough that FWM sidebands are
generated through modulation instability. The location of FWM-induced sidebands depends
on the pump power such that they are shifted further from the pump as its power
increases. Multiple spectral bands also interact with each other through XPM and SRS
and produce a nearly flat supercontinuum extending over 500 nm for P 0 = 400 W.
12.3.2 Continuous-Wave Pumping
The use of picosecond pulses is not a requirement for supercontinuum generation. As
mentioned earlier, 10-ns pulses were used for this purpose in the original 1976 experiment
[65]. In a 2003 experiment, 42-ns pulses from a Q-switched Nd:YAG laser were
launched into a 2-m-long microstructured fiber (with a random hole pattern) to produce
a wide supercontinuum at 10-kW power levels [84]. Somewhat surprisingly, it turned
out that even CW lasers can produce a supercontinuum at sufficiently high power levels
[85]–[95].
The two most important ingredients for any recipe of supercontinuum generation
are a high-power source and a highly nonlinear fiber so that the product γP 0 L exceeds
30 or so. This condition can be satisfied for a 1-km-long fiber with γ = 10 W −1 /km
at a pump-power level of a few Watts. Such power levels are easily available from
modern high-power fiber lasers. The other requirement for creating a supercontinuum
is that a suitable FWM process be phase-matched. In the case of a CW pump laser,
modulation instability can generate the FWM sidebands if one ensures that the pump
light experiences anomalous dispersion inside the fiber. This instability converts the
CW beam into a train of picosecond pulses. Once that occurs, supercontinuum can be
generated through the same mechanism discussed in the preceding subsection.
In a 2004 experiment, three highly nonlinear fibers of lengths 0.5, 1, and 1.5 km
were used for supercontinuum generation by launching a CW beam at 1486 nm from
a Raman fiber laser [88]. The ZDWL of the fibers was close to 1480 nm to ensure
anomalous GVD at the pump wavelength. Figure 12.18 shows the measured spectra
for three fibers at power levels ranging from 0.4 to 4 W. In each case, the output
spectrum broadens considerably when the pump power is close to 4 W. It is also
highly asymmetric with much more power on the long-wavelength side. As discussed