Nonlinear Fiber Optics - 4 ed. Agrawal

12.5. Harmonic Generation 495
Figure 12.35: Comparison of (a) filtered spectra near 580 nm and (b) intensity noise for three
types of fibers. HF and taper denote holey and tapered fibers of constant core diameters. (After
Ref. [148]; c○2005 OSA.)
experimental spectra results from a time averaging taking place during measurements.
A natural question to ask is whether it is possible to improve the coherence properties
of a supercontinuum. In an interesting approach, known as dispersion micromanagement
(DMM), dispersion is made nonuniform across fiber length by tapering
the fiber appropriately [148]. In the experiment, supercontinuua were generated using a
80-cm-long holey fiber with 2.6-μm core, a 6-cm-long tapered fiber with 2.7-μm core,
and a 2.3-cm-long PCF with a 1-cm tapering region where core diameter decreased
from 3.3 to 2.6 μm. The measured spectrum was smoother in the last case because
of DMM. Figure 12.35 shows the output when an optical filter centered at 580 nm
and having a 25-nm bandwidth was used to slice each supercontinuum. The intensity
noise spectrum centered around the repetition rate of mode-locked pulses shows that
the noise floor is reduced to the background level in the DMM case for which the spectrum
is relatively smooth. The physical region behind the observed low noise is related
to the frequency at which solitons generate the NSR. More specifically, the wavelength
at which NSR is phase-matched to a soliton shifts in the 1-cm-long tapered region at
the output end of fiber.
12.5 Harmonic Generation
Harmonic generation is a common nonlinear phenomenon used to generate light in
the visible and ultraviolet regions. This section is devoted to a brief discussion of the
creation of the second and third harmonics when an intense pump beam is launched
into an optical fiber.
12.5.1 Second-Harmonic Generation
Strictly speaking, second-harmonic generation (SHG) should not occur in silica fibers
because the inversion symmetry exhibited by SiO 2 glass forbids all nonlinear effects