The effects of focusing power on TEA CO2 laser-induced gas breakdown and the consequent pulse shaping effects

Abstract

Transversely Excited Atmospheric (TEA) CO2 laser pulses were used in order to generate an optical breakdown in a variety of mono- and polyatomic molecules using different focusing powers. The dependence of the spark kernel geometry and the transmitted pulse shapes on the focusing power as well as the pressure, molecular weight, and ionization energy of the gases was investigated in detail. Partial removal of the transmitted pulse tail in the 0.05–2.6 μs range together with shortened spikes in the 10–60 ns range has been observed by applying a 2.5 cm focal length lens for all the gases. At higher focal lengths, this effect is only incompletely observed for He gas. Spatial-temporal analyses of the laser beams and the relevant plasma plumes indicate that this behavior is due to the drop in the plasma density below the critical level, before the laser pulse tail is completed.

Comparative study of spark pin-array and printed circuit board surface plasma pre-ionization systems for transversely excited atmospheric CO2 laser performance

“…Further, a photon drag (THORLABS PM100D) connected to a storage oscilloscope (Tektronix, TDS 2022B) have been used to trace the temporal shape of the laser pulses, typically shown in figure 4. As a rule, when TEA CO 2 lasers operate with non-negligible amounts of N 2 in their gas mixtures, the output pulse shapes contain a sharp frontal spike with ∼100 ns time duration after which, a low amplitude, several µs long tail appears [19]. This posterior tail arises from the low-rate collisional pumping of the CO 2 molecules by the remaining excited N 2 molecules via vibration-vibrational relaxation processes.…”