Effect of gas thermal lens in laser resonator on beam mode of high-power CO 2 laser

For gas laser, working gases in resonator have two interaction with laser. One is to generate excited radiation, the other is to affect the propagation of the laser beam. For the latter, gas thermal lens has great effect on laser beam in the resonator for high-power CO2 laser because of long resonator and high laser power in resonator. In this paper, a physical model for gas thermal lens is proposed. As well as the influence of thermal lens upon the propagation of the laser beam in the resonator according to this model is analyzed. It is found that gas thermal lens will transform the laser beam acting as a negative lens which is at the middle of the resonator since the symmetry of the resonator. This negative lens will transform the beam waist position and radius to deviate the idea beam waist position and radius which are determined by the passive resonator. And this deviation causes the mismatch of the transformed laser beam with the resonator. Further more this mismatch results in the energy couple of TEM00 beams to the higher ones to form the mix-mode laser beam and decreasing laser beam quality. In the paper the effects of the laser power, on laser beam modes are analyzed. And as an example, the laser output mode of a high-power CO2 laser with radio-frequency excited and fast-axis-gas-flow is calculated from the above theory. Meanwhile, the output mode of this type of laser is measured by high-power laser beam analyzer - laserscope UFF100. And the calculation is accord with the measurement. So for high-power C02 laser, the gas thermal lens is an important reason which influence the output mode ofthe laser beam.