Kinetic modelling of fundamental (00̊1) and sequence (00̊2) band CO₂ lasers

Abstract

The vibrational kinetics of the CO2 laser system are studied experimentally and theoretically. A sequence (00°2) band/ fundamental (00°1) band gain ratioing technique is used to measure the CO2 asymmetric stretch mode temperature (T3) in low-pressure cw laser discharges; the relationship of discharge current to electron density is determined by X-band microwave cavity resonance. The experimental measurements are compared to theory using a comprehensive computer model of CO2 laser kinetics, based on the vibrational temperature approximation. It is demonstrated that the observed saturation of vibrational temperature with increasing discharge current is caused by the de-activation of excited molecules by electron superelastic collisions, at a rate predicted by the principle of detailed balance. Superelastic collisions crucially determine the attainable vibrational temperatures, and limit T3 to values below the optimum for 00°1 or 00°2 band laser action. The associated laser gain limitations are investigated, and it is shown that superelastic collisions inflict efficiency losses on pulsed TE CO2 lasers even at moderate input energies. The operating characteristics of CO2 sequence band lasers are also examined. A comparison of oscillator performance with corresponding small-signal gain measurements indicates a sequence band saturation intensity which, is higher than that of the fundamental band. This observation is supported by model computations, which predict that the. extractable 00°2 band laser power (alphao Is) is typically 60% of that available on the 00°1 band.