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Effect of cavity transients and rotational relaxation on the performance of pulsed HF chemical lasers: a theoretical investigation.

Two rate-equation models of a pulsed HF laser are presented. Calculations with these models reveal the time evolution of the gain and intensity of lasing on all lines in vibrational-rotational bands studied and the time histories of concentrations of the chemical species. The present formulation permits an accurate assessment of the effect of vibrational relaxation and rotational relaxation. The model predictions are compared with earlier models to illustrate their unique features. Multiline lasing on vibrational-rotational bands is predicted; however, the general trends of J shifting are similar to those of earlier models. It is found that lasing does not begin when the medium gain reaches threshold, but intensity increases sharply after the gain is far above threshold. The gain subsequently drops rapidly and oscillates near the threshold value. The effect of rotational relaxation on laser performance is shown for multiline laser operation, and the enhancing effect of cascading is clearly revealed. The present models are found to predict output and performance consistent with experiment.