We report on an observation of bi-directional 656 nm lasing action of atomichydrogen in premixed CH4/air flame induced by resonant femtosecond 205 nmtwo-photon excitation. In particular, the backward-propagating lasing pulse issystematically characterized in the spectral, spatial and temporal domains forthe sake of single-ended diagnostic. Its picosecond-scale duration enablesspatially resolved concentration measurements of hydrogen atoms in millimeterrange, which is successfully demonstrated using two narrow welding flames.
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