Ultrafast strong laser filamentation and applications in high-temperature combustion field
2017
The propagation of ultrafast strong laser in optical media such
as ambient air can induce a dynamical balance between Kerr self-focusing
and defocusing of plasma, resulting in a unique nonlinear phenomenon—laser
filamentation, which have shown promising applications in areas such
as atmospheric sensing, THz generation, and weather control. In this
article, we present an overview of ultrafast strong laser filamentation
in combustion fields and its potential application in combustion diagnostics,
which is of particular significance in rationalizing the physical
and chemical nature of combustion systems such as auto engines for
efficient combustion of fuels with low-pollution products. We first
introduce the dynamical processes and propagation properties of ultrafast
strong laser filamentation in combustion fields, in which the determination
of the two fundamental physical parameters, critical power and clamping
intensity, for femtosecond laser filamentation in combustion flames
is presented. Although these values in combustion flames are found
to be smaller than in air, the intensity clamped in flame filaments
are enough to induce multiple photon excitation and ionization of
combustion intermediates such as CN, C 2 and CH free radicals,
and atomic C and H to fluoresce. The fluorescence signals are sensitive
to the position of interaction of the filament with the flame that
indeed reflects the concentration distributions of the species to
be sensed. We then present the underlying mechanisms of ultrafast
strong laser filamentation induced nonlinear spectroscopy, in which
the femtosecond filament-induced flame fluorescence emissions are
found to mainly originate from the interaction of femtosecond laser
pulses with the combustion intermediates existing in the combustion
environment, but not from the fragmentation of parent fuel molecules.
In particular, due to the high nonlinear properties of femtosecond
laser filamentation in combustion flames, filament- induced nonlinear
flame fluorescence technique could be applied for simultaneous identification
of multiple combustion intermediates in different fuel flames by comparing
the signal ratios between the intermediates, providing the possibility
of this technique in application to various combustion conditions
that strongly depend on the fuel species. Interestingly, with the
femtosecond laser filament excitation, it is demonstrated that in
flames, some combustion species can be populated inverted, generating
lasing actions by observing the backward fluorescence signals as a
function of filament length, which provides a way to overcome the
quenching of specific species in combustion diagnostics, and thus
to improve signal to noise ratio in the combustion monitoring. It
is also demonstrated that filament-induced fluorescence of flames
can provide real-time monitoring of combustion intermediates by using
a single shot femtosecond laser pulse, which is very important in
combustion diagnostics because of the turbulent nature of combustion
flames. We finally give an overview of the current research status
and progress of ultrafast laser filamentation in application to the
diagnostics of high-temperature combustion fields, and discussed both
current challenges and a future perspective. Since high-power femtosecond
laser system with high repetition rate of up to 10 kHz is available,
the abovementioned results using femtosecond laser filamentation in
flames reveal the possibility for high-speed monitoring of multiple
combustion intermediates by means of femtosecond laser filament excitation.
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