Nonlinear Fiber Optics - 4 ed. Agrawal

12.3. Supercontinuum Generation 471
Figure 12.15: Supercontinuum spectra measured at average power levels of (a) 45 mW, (b)
140 mW, and (c) 210 mW. Dashed curve shows the spectrum of input pulses. (After Ref. [77];
c○1998 IEEE.)
power is not large enough to reach the Raman threshold, SRS can amplify the pulse
spectrum on the long-wavelength side as soon as SPM broadens it by 5 nm or more.
Clearly, SRS would affect any supercontinuum by enhancing it selectively on the longwavelength
side, and thus making it asymmetric. However, SRS cannot generate any
frequency components on the short-wavelength side.
FWM is the nonlinear process that can create sidebands on both sides of the pulse
spectrum, provided a phase-matching condition is satisfied, and it is often behind a supercontinuum
generated using optical fibers. FWM is also the reason why dispersive
properties of the fiber play a critical role in the formation of a supercontinuum. Indeed,
because of a large spectral bandwidth associated with any supercontinuum, the GVD
parameter β 2 cannot be treated as a constant over the entire bandwidth, and its wavelength
dependence should be included through higher-order dispersion parameters in
any theoretical modeling. Moreover, the process of supercontinuum generation may be
improved if dispersion is allowed to change along the fiber length. As early as 1997,
numerical simulations showed that the uniformity or flatness of the supercontinuum
improved considerably if β 2 increased along the fiber length such that the optical pulse
experienced anomalous GVD near the front end of the fiber and normal GVD close
to the output end [74]. By 1998, a 280-nm-wide supercontinuum could be generated
by using a special kind of fiber in which dispersion not only decreased along the fiber
length but was also relatively flat over a 200-nm bandwidth in the wavelength region
near 1.55 μm [75]. Dispersion flattening turned out to be quite important for supercontinuum
generation. In a 1998 experiment, a supercontinuum extending over a 325-nm
bandwidth (at 20-dB points) could be generated when 3.8-ps pulses were propagated
in the normal-dispersion region of a dispersion-flattened fiber [76].
At first sight, it appears surprising that supercontinuum can be produced in a fiber
exhibiting normal GVD (β 2 > 0) along its entire length. However, one should note,
that even though β 3 is nearly zero for a dispersion-flattened fiber, the fourth-order
dispersion governed by β 4 plays an important role. As discussed in Section 10.3.3 and
seen in Figure 10.9, FWM can be phase-matched even when β 2 > 0 provided β 4 < 0,