Megahertz-rate NO planar laser-induced fluorescence of reacting flows using tunable 5th harmonic Nd:YAG with A-X and B-X excitation

This article presents, to the best of our knowledge, the first demonstration of burst-mode nitric oxide (NO) planar laser-induced fluorescence measurement of a reacting environment via Nd:YAG 5th harmonic excitation. A tunable pulse-burst laser generated 1 ms bursts of resonant 213 nm radiation at 200 kHz, 500 kHz, and 1 MHz repetition rates with 16 mJ, 3.5 mJ, and 0.35 mJ average pulse energies, respectively. Nitric oxide, naturally formed in the vicinity of an oxy-acetylene flame, was targeted via three excitation schemes: <i>γ</i>(1, 0), <i>γ</i>(1, 0) + <i>β</i>(5, 1), and <i>β</i>(4, 1). The generated fluorescence was collected in two separate spectral windows to demonstrate the selectivity of the electronic excitation and explore new filtering approaches for high background luminosity environments: 240 nm-260 nm and 275 nm-375 nm. Overall, this new method of UV-light generation for NO excitation performed very well with a substantial amount of collected signal even with modest energy levels accessible at a 1 MHz rate. The primary advantage of utilizing the 5th harmonic to access NO resonances was its simplicity and robustness through direct burst mode laser up-conversion and simplified master oscillator, power amplified (MOPA) frequency control, obviating the need for an optical parametric oscillator.