Femtosecond Filament-Induced Nonlinear Spectroscopy for Combustion Sensing
Combustion diagnostics is of particular importance in combustion science for raising the combustion efficiency with low-pollution products as well as for monitoring the nanoparticle growth by combustion synthesis. In this chapter, we discuss the feasibility in the detection and diagnosis of combustion intermediates using femtosecond filament-induced nonlinear spectroscopy (FINS) by referring to our recent studies and determine the critical power for self-focusing of a Ti: Sapphire 800-nm, 35-fs laser pulse in alcohol burner flames in ambient air. We find that the laser filamentation in the flame produces a clamped intensity in the range of 2 × 1013–3 × 1013 W/cm2 and the flame filament can induce optical emissions from multiple combustion intermediates such as free radicals CH, CN, NH, OH, and C2, as well as atomic C and H. We find that the emission intensities vary sensitively dependent on the fuel species as well as on the positions of the filament within the flame and that the filament can induce nonlinear frequency conversion via harmonic generation, which can be efficiently scattered by soot nanoparticles formed inside the flames. Our results show that FINS has a high potential applicability in remote and in situ sensing of spatial distributions of combustion intermediates in flames.
This work was supported in part by the National Natural Science Foundation of China (61427816, 61625501), the National Basic Research Program of China (2014CB921302) and JSPS KAKENHI grant (JP15H05696). SLC acknowledges the support of Laval University, Quebec City, Canada.
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