Abstract

By aiming to establish single-ended standoff combustion diagnostics, bidirectional lasing emissions of atomic hydrogen at 656 nm wavelength have been generated via two-photon resonant excitation by focusing 205 nm femtosecond laser pulses into a premixed ${\mathrm{CH}}_4/{\mathrm{O}}_2$ flame. The forward lasing strength is approximately one order of magnitude stronger than that of the backward one, due to the geometry of traveling wave excitation over a 2-mm-long pencil-shaped gain volume and the short gain lifetime of 3.5 ps. The gain coefficient of hydrogen lasing was determined to approximate 52/cm. As for the underlying physics of hydrogen lasing, amplified spontaneous emission (ASE) occurs simultaneously with four-wave mixing (FWM), and ASE dominates in the forward direction, whereas the backward lasing is virtually only ASE.

© 2019 Optical Society of America

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