Kinetic Modeling of the High-Power Carbon Monoxide Laser

Abstract

A model for the kinetics of the cooled direct-discharge-excited carbon monoxide laser is presented. The kinetic mechanism responsible for creating the observed population inversions cannot be explained by simple one-step resonance transfer between an excited metastable and the CO molecule, in view of the many vibrational bands which lase in this system. The present paper analyzes a kinetic model of the CO laser which includes the following processes: (a) Vibration-to-vibration (V-V) energy exchange among the anharmonic vibrational states occurring in CO–CO collisions. (b) Resonance electron impact excitation of the lower CO vibrational states. (c) Radiative decay of the CO vibrational states. (d) Collisional quenching of vibrational excitation in CO–He collisions. Using a Morse anharmonic oscillator model of the CO vibrational states, kinetic equations are formulated which govern the individual vibrational state populations, subject to the preceding processes. The resulting set of nonlinear algebraic equations is solved by an interative technique for the steady-state vibrational populations. Small-signal laser gain is also predicted as a function of the following discharge conditions: (1) electron temperature, (2) electron concentration, (3) heavy species translational temperature, (4) CO partial pressure, and (5) He partial pressure. Comparison is made with recent experimentally obtained small-signal gain data for the CO laser, as well as with other experimental results for CO lasers. It is shown that experimental results are consistent with an inversion created by electron impact excitation of the lower CO vibrational levels, followed by rapid redistribution of energy among the higher CO vibrational states via off-resonant vibration-vibration energy exchange. The present kinetic model successfully interprets the variation of gain with vibrational state, the observed strong temperature dependence of the gain, and the influence of He diluent in the discharge. The possibilities for using this pumping mechanism to obtain cw lasing with other diatomic species and in various laser configurations are also discussed.