Femtosecond Laser Electronic Excitation Tagging (FLEET) Fundamental Pulse Energy and Spectral Response

We report testing Femtosecond Laser Electronic Excitation Tagging (FLEET), a nonseeded ultrafast laser diagnostic technique, under a variety of different conditions. A higher power, kHz repetition rate laser system was utilized with significant improvement in data collection over the previous lower power, 10 Hz system. The effects of laser pulse energy on the emission signal were investigated, with the result of a linear dependence from a pulse energy of 1.5mJ to 6mJ. Intensity clamping was observed at a peak intensity of 1.3 × 10 W/cm, leading to an increasing emission beam waist that shifted closer to the focusing lens. Operating at a shorter wavelength led to significant signal enhancement, but the conversion efficiency and signal dependence on laser power served to negate this gain. FLEET was shown to produce good signal levels in low pressure environments of air and pure nitrogen. Spectra of the signals showed an expanded emission range spanning 520nm to 780nm that included most of the vibrational transitions associated with the first positive system of nitrogen. The data presented provide great value when optimizing a FLEET system for use in a variety of ground testing facilities and for fundamental laser physics analysis.