Nonlinear Fiber Optics - 4 ed. Agrawal

12.3. Supercontinuum Generation 469
Figure 12.14: (a) Measured output spectra showing FWM-induced sidebands for a 624.5-nm
quasi-CW pump, linearly polarized at 45 ◦ from the slow axis of a 4-m-long microstructured
fiber. (b) Theoretical gain curve calculated using actual fiber parameters. (After Ref. [44];
c○2002 OSA.)
(resulting from an effective mode area of only 1.7 μm 2 ) and a relatively large birefringence
(∼10 −4 ) associated with the microstructured fiber used in the experiment.
As discussed in Section 10.6.3, FWM in highly nonlinear fibers provides a simple
way to generate correlated photon pairs useful for applications related to quantum
cryptography and quantum computing. However, the quality of such a photon-pair
source is severely deteriorated by spontaneous Raman scattering if the signal and idler
are copolarized with the pump. This problem can be solved to a large extent by using
a configuration in which the signal and idler photons are polarized orthogonally
to the pump [61]. The reason is that the Raman gain almost vanishes for an orthogonally
polarized pump, whereas the FWM efficiency is only reduced by a factor of 1/3
in an isotropic fiber with no birefringence. In a FWM process that is phase-matched
through fiber birefringence, it is possible to reduce the impact of Raman gain without
sacrificing the FWM efficiency much.
12.3 Supercontinuum Generation
When optical pulses propagate through an optical fiber, their temporal as well as spectral
evolution is affected not only by a multitude of nonlinear effects such as SPM,
XPM, FWM, and SRS but also by the dispersive properties of the fiber. All of these
nonlinear processes are capable of generating new frequencies within the pulse spectrum.
It turns out that, for sufficiently intense pulses, the pulse spectrum becomes so
broad that it may extend over a frequency range exceeding 100 THz. Such extreme
spectral broadening is referred to as supercontinuum generation, a phenomenon first
observed around 1970 in solid and gaseous nonlinear media [62]–[64].
In the context of optical fibers, supercontinuum was first observed in 1976 by
launching Q-switched pulses (width ∼10 ns) from a dye laser into a 20-m-long fiber
with a core diameter of 7 μm [65]. The output spectrum spread as much as over 180 nm