Long Distance Single-Ended Imaging of Femtosecond Filaments

Femtosecond filaments with various pulse energies were imaged at multiple distances using an S20 intensifier and a CCD. Imaging at a 1 kHz rate was demonstrated out to an imaging F-number of F/105 with a 3.5 mJ pulse. The signal to noise ratio ($\frac{\mu}{\sigma}$) was found to decrease linearly with distance and potentially be a good predictor of performance. Return signal was found to roughly decrease proportionally to the square of distance, potentially indicating a weak intensity dependence on focusing strength. This is not what was expected and warrants further investigation.