Lasing action induced by femtosecond laser filamentation in ethanol flame for combustion diagnosis

We experimentally demonstrate the generation of lasing action in the laminar ethanol/air flame on an alcohol burner array using femtosecond laser filament excitation. By probing the backward emissions of combustion species, it is found that as the interaction length of the filament in the flame increases, the signal's intensity at the 388 nm band for the B2Σ-X2Σ transition of CN increases exponentially, but that at the 474 nm band for A3Πg-X′3Πu transition of C₂ increases linearly. The exponential behavior of the CN emissions is ascribed to amplified spontaneous emission, which opens up a way to overcome the quenching effect of specific species in combustion diagnosis.