Nonlinear Fiber Optics - 4 ed. Agrawal

476 Chapter 12. Novel Nonlinear Phenomena
Figure 12.19: Supercontinuum generated in a 75-cm-long microstructured fiber when 100-fs
pules with 0.8 nJ energy are launched close to the ZDWL near 767 nm. The dashed curve shows
for comparison the spectrum of input pulses. (After Ref. [96]; c○2000 OSA.)
width. Similar features were observed in another 2000 experiment in which a 9-cmlong
tapered fiber with 2-μm core diameter was employed [97]. Figure 12.20 shows
the output spectra of 100-fs pulses observed at average power levels ranging from 60
to 380 mW. It is important to note that the optical power in Figures 12.15 to 12.20
is plotted on a decibel scale; the spectra would appear much more nonuniform on a
linear scale. How should one characterize the spectral width of such a supercontinuum
because the concept of a FWHM cannot be applied? A common approach is to employ
the 20-dB spectral width, defined as the frequency or wavelength range over which the
power level exceeds 1% of the peak value. The 20-dB spectral width exceeds 1000 nm
in Figure 12.19 as the supercontinuum extends over two octaves.
It is evident from the superbroad and nearly symmetric nature of the supercontinuum
spectra in Figures 12.19 and 12.20 that a different physical mechanism is at play
in the case of femtosecond pulses. At high peak power levels and anomalous dispersion
used in the experiments, input pulses correspond to a higher-order soliton whose
Figure 12.20: Optical spectra at the output of a 9-cm-long tapered fiber with a 2-μm waist. The
average power was 380, 210, and 60 mW (from top to bottom) for the three curves. The input
spectrum is also shown for comparison. (After Ref. [97] c○2000 OSA.)