Biomedical Engineering Reference
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Fig. 5.9
A comparison of supercontinuum with other broadband sources
the pulse out into a SC spectrum. The supercontinuum combines high brightness
with broad spectral coverage—a combination offered by no other technology.
Figure 5.9 shows the comparison of supercontinuum with other broadband sources.
The generation of supercontinuum is due to a plethora of nonlinear optical effects
which generate new frequencies, which covers a broadband. Mathematically, the
nonlinear effects can be described on the molecular scale by the following total
polarization:
C .3/ E 3 C /; (5.35)
where P is the total polarization of the molecule, m is the permanent dipole
moment, E represents the electric field, .1/ is the linear polarization, and .2/
and .3/ are the first and second hyperpolarizability coefficients. Here, the most
noticeable terms are the second and third hyperpolarizabilities, and the related
phenomena are called second harmonic generation (SHG) and third-order non-
linear effects, including self-phase modulation (SPM), four-wave mixing (FWM),
stimulated Raman scattering (SRS), and others. Broadband light sources can be
generated through efficient nonlinear effects. In order to enhance nonlinear effects,
two conditions must be satisfied: extremely high power and long walking distance
for keeping such high power. Ultrafast lasers such as femtosecond lasers can
produce very high peak power. In recent years, renewed interest in supercontinuum
generation has been closely related to the remarkable progress in the development of
photonic crystal fibers (PCF) as well as the development of robust, well-engineered
single-box ultrafast lasers. Coupling an ultrafast laser pulse directly out of an
D m C " 0 . .1/ E C .2/ E 2
P
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