Nonlinear Fiber Optics - 4 ed. Agrawal

472 Chapter 12. Novel Nonlinear Phenomena
Figure 12.16: Supercontinuum spectra at the output of two dispersion-shifted fibers having γ of
2.3 and 9.9 W −1 /km when 1.3-ps input pulses were launched into relatively short fibers. (After
Ref. [82]; c○2000 IEEE.)
and it generates signal and idler bands that are shifted by >10 THz. It is this FWM
process that can produce a wideband supercontinuum in the normal-GVD region of an
optical fiber. In fact, a 280-nm-wide (at 10-dB points) nearly flat supercontinuum was
generated when 0.5-ps chirp-free pulses, obtained from a mode-locked fiber laser, were
propagated in a dispersion-flattened fiber with a small positive value of β 2 . Even when
2.2-ps pulses from a mode-locked semiconductor laser were used, supercontinuum as
broad as 140 nm could be generated, provided pulses were initially compressed to
make them nearly chirp-free. Figure 12.15 shows the pulse spectra obtained at several
power levels when a 1.7-km-long dispersion-flattened fiber with β 2 = 0.1 ps 2 /km at
1569 nm was used for supercontinuum generation [77]. Input spectrum is also shown
as a dashed curve for comparison. The nearly symmetric nature of the spectra indicates
that the Raman gain played a relatively minor role in this experiment. The combination
of SPM, XPM, and FWM is responsible for most of spectral broadening seen in Figure
12.15. In a 1999 experiment such a supercontinuum spectrum was used to produce
10-GHz pulse trains at 20 different wavelengths, with nearly the same pulse width in
each channel [78].
From the standpoint of its application as a multichannel WDM source, the supercontinuum
generated in the 1.55-μm spectral region does not have to extend over
a very wide range. What is really important is the flatness of the supercontinuum
over its bandwidth. In a 2001 experiment, a dispersion-flattened fiber (polarizationmaintaining
kind) was used to produce 150 channels spaced apart by 25 GHz [79].
The total bandwidth of all channels occupied only a 30-nm bandwidth. Of course, the
number of channels can be increased to beyond 1000 if the supercontinuum bandwidth
were to increase beyond 200 nm. Indeed, it was possible to create a WDM source with
4200 channels spaced apart by only 5 GHz when the supercontinuum bandwidth was
close to 200 nm [80]. By 2003, such a WDM source provided 50-GHz-spaced channels
over the spectral range extending from 1425 to 1675 nm, thus covering the three main
(S, C, and L) telecommunication bands [81].
Until 2000, most experiments employed long fibers (length ∼1 km) for generating
a supercontinuum. However, short fibers with lengths