Ultrafast strong laser filamentation and applications in high-temperature combustion field

Abstract

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, C2 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.