Nonlinear Fiber Optics - 4 ed. Agrawal

12.3. Supercontinuum Generation 475
Figure 12.18: Output spectra at three power levels for highly nonlinear fibers of length (a) 0.5,
(b) 1.0, and (c) 1.5 km. In each case, a CW beam is launched into the anomalous-GVD region
of the fiber. (After Ref. [88]; c○2004 OSA.)
earlier, this asymmetry is due to the phenomenon of SRS that selectively amplifies only
the long-wavelength components.
As the growth of the supercontinuum is seeded by the phenomenon of modulation
instability, the dispersion of the fiber plays an important role. In the case of Figure
12.18, the dispersion slope of the fiber was kept relatively small, resulting in a value
of β 2 close to −0.2 ps 2 /km at the pump wavelength. As seen from Eq. (10.3.9), the
sidebands created through modulation instability are spaced further apart for smaller
values of |β 2 |. At a power level of 4 W, they are located about 25 nm away on each side
of the pump wavelength under the experimental conditions. The sideband on the longwavelength
side is amplified through SRS, which also creates other standard Stokes
sidebands (spaced apart by 100 nm or so) through a cascaded SRS process. Since most
of the pump power is transferred toward the long-wavelength side, the supercontinuum
becomes asymmetric and exhibits the shape seen in Figure 12.18. On the shortwavelength
side, the power is generated through a process discussed later in the context
of femtosecond pulses. The reason is that the onset of modulation instability creates
a train of ultrashort pulses that form solitons. Indeed, much less power is generated
on the short-wavelength side when the pump wavelength lies in the normal-dispersion
region of the fiber [89] because solitons are not formed under such conditions.
12.3.3 Pumping with Femtosecond Pulses
The use of femtosecond pulses became practical with the advent of highly nonlinear
fibers whose ZDWL lies close to 800 nm, the wavelength region around which tunable
Ti:sapphire lasers operate. Starting in 2000, femtosecond pulses from these and other
mode-locked lasers were used for supercontinuum generation, and their use has become
relatively common in recent years [96]–[112].
In an early 2000 experiment, 100-fs pulses at 790 nm were launched into a 75-cm
section of a microstructured fiber exhibiting anomalous dispersion [96]. Figure 12.19
shows the spectrum observed at the fiber output when 100-fs pulses centered at 770 nm
were launched close to the ZDWL near 767 nm (peak power ≈7 kW). Even for such
a short fiber, the supercontinuum not only is extremely broad, extending from 400 to
1600 nm, it is also relatively flat (on a logarithmic power scale) over the entire band-