Approximate method for calculations of unstable telescopic resonators

A simple method is proposed for the design calculations on unstable telescopic resonators for continuous-flow lasers. The method uses radiation transfer equations in the geometric-optics approximation and is based on averaging all the parameters in the direction of the resonator axis. A CO₂ gasdynamic laser is used as an example to demonstrate the high accuracy with which the output power and the intensity at the mirrors are predicted. It is noted that the frequently used isothermal approximation is unsuitable in regimes close to quenching of the laser action.