Supercontinuum Generation by Nonlinear Optics - Optical Signal Processing by Silicon Photonics - page 69

no resonant FWM or four-photon coupling for the irst time. This experiment
concluded that the generated lights spectral width is 10 or more times wider than any-
thing investigated previously. The authors do not highlight this characteristic in their
publication particularly. The phrase “super continuum” was not even used in their
paper; sometime later (Manassah et al. 1984) introduced it [ 7 ]. Bondarenko et al.
(1970) reported results which were not far different from them [ 8 ]. Interestingly, at that
time, nonlinear spectral broadening of laser light was not completely new, Stoicheff
(1963) had already observed it [ 9 ].
Spectral broadening is the phenomena that had also been observed in CS2 [ 4 ]
and interpreted correctly in terms of the SPM nonlinear process. In the meantime,
the SC generation phenomena was referred to as anomalous frequency broadening,
super broadening [ 8 ], or white light continuum.
The SC phenomenon is also reported as wide frequency range white light
sources [ 10 ]. SC production is a highly complex process in bulk material, involv-
ing a complex coupling between temporal and spatial effects. On the other hand,
a temporal dynamic process is involved in SC generation in optical fibers, with
the characteristics of transverse mode which can only be determined by linear
waveguide properties. As a matter of fact, it is suggested that a further inspiration
in studying the SC generation in PCFs helps in clarifying the nature of temporal
nonlinear propagation effects if improvement is required in understanding of more
complex spatiotemporal (bulk) case [ 1 ]. Femtosecond pulses of low intensity are
used to observe supercontinuum generation [ 11 ].
In 2004, a new way was demonstrated by the generation of spectral broadening
in highly nonlinear photonic crystal fibers by the contribution of SPM and FWM
[ 12 ]. Later on in 2005, work was done on sub-wavelength diameter waveguides
that reduced the threshold power for generating SC [ 13 ].
In 2006, spectral broadening in nonlinear fibers occur over range from 900 to
1,400 nm by using nanosecond noise pulses from Ytterbium-doped fiber amplifier
(YDFA) [ 14 ]. In 2007, spectral broadening of more than 600 nm was investigated
by launching noise like pulses of energy 10 nJ into high nonlinear fiber [15]. 15 ].
In 2010, SC generation was taken up over wavelength range of 1,100-
1,800 nm. This is achieved by pumping the pulses with the peak power of 66 W,
centered at wavelength of 1,550 nm into arsenic sulfide fiber, uniformity in SC is
also reported [ 16 ].
6.3 SC Generation in Photonic Crystal Fibers
Usual optical fibers consist of two concentric glass cylinders typically having dif-
ferent refractive indices. In case of the refractive index of the outer cladding lower
than the inner core, the assistance takes place through total internal reflection at
the boundary of core and cladding. Conventionally, most of the fibers have the
core-cladding refractive index difference smaller (0.1 %), and a lot of the propaga-
tion characteristics are acquiescent for analysis [ 17 ].