Critical power and clamping intensity inside a filament in a flame.

We report on measurements of both the critical power for self-focusing of a Ti: Sapphire 800 nm femtosecond laser and the peak intensity clamped inside a single filament in an ethanol-air flame on an alcohol burner array. By observing the shift of focal position of femtosecond laser pulses, we determine the critical power in the flame to be 2.2 ± 0.3 GW, which is 4-5 times smaller than the usually quoted one in air. The clamped laser intensity inside the filament is measured to be roughly half of that in air. Our results provide insights into the understanding of femtosecond laser filamentation in flames for practical application of combustion diagnostics.