Nonlinear Fiber Optics - 4 ed. Agrawal

Chapter 12
Novel Nonlinear Phenomena
The development of highly nonlinear fibers discussed in Chapter 11 has led to the observation
of novel nonlinear effects such as the generation of extremely broad optical
spectra (supercontinuum generation) and large Raman-induced frequency shifts that allow
wavelength tuning of mode-locked lasers. These nonlinear phenomena are finding
a multitude of new applications for optical fibers in fields as diverse as high-precision
metrology and optical coherence tomography. This chapter focuses on several nonlinear
effects that have been attracting attention in recent years. We revisit in Section
12.1 the topic of Raman-induced frequency shifts discussed earlier in Section 5.5. It
turns out that such spectral shifts can be enhanced as well as suppressed with a proper
design of highly nonlinear fibers. Section 12.2 discusses how dispersion tailoring in
highly nonlinear fibers can be used to enhance the four-wave mixing (FWM) effects
for practical applications. Supercontinuum generation is covered in Section 12.3 with
emphasis on the physical processes behind its creation. Section 12.4 focuses on the
temporal, spectral, polarization, and coherence properties of a supercontinuum and
employs numerical simulations to understand the underlying physics. Section 12.5
is devoted to the topic of harmonic generation in optical fibers with emphasis on the
second and third harmonics.
12.1 Intrapulse Raman Scattering
As discussed in Section 5.5.4, the spectrum of an ultrashort pulse shifts toward longer
wavelengths because of intrapulse Raman scattering. Such a shift was first noticed
around 1985 in the context of solitons and was called the soliton self-frequency shift
[1]. The term Raman-induced frequency shift (RIFS) was used in Section 4.4.3 because
a spectral shift can occur even in the normal-GVD regime of an optical fiber where
solitons are not formed [2]. It was discovered during the 1990s that the RIFS of solitons
can become quite large in highly nonlinear fibers. Later experiments revealed that it
can also be suppressed under certain conditions. This section focuses on such novel
aspects of intrapulse Raman scattering.
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