High Rate Nitrogen Backward Lasing in Air for Localized Non-Intrusive Diagnostics

This study investigates a dual-pulse femtosecond laser scheme for generating backward stimulated emission in air, with the goal of enabling high signal-to-noise ratio (SNR) and high repetition rate non-intrusive methods for flow diagnostics. Building upon prior work in cavity-less air lasing, atomic nitrogen is targeted as the gain medium, produced via laser-induced dissociation of atmospheric N2 by a 1030 nm infrared (IR) pre-pulse. A subsequent ultraviolet (UV) pulse, centered at 206 nm, then excites the atomic nitrogen to the 3p 4S◦ 3/2 level via resonant two-photon absorption at 206.65 nm, commonly used in nitrogen TALIF diagnostics. The presence of a local gain region results in stimulated emission at 744 nm. The study presents characterization of the backward lasing signal using a CMOS camera, and explores the dependence of lasing signal intensity on pulse timing and energy. The results demonstrate successful generation of backward lasing in ambient air at 5 kHz, with opportunities to extend to higher rate measurements upon optimizing the energy split between the IR and UV pulses.