Output power and intensity distribution of a transverse-flow CO2 laser

We have measured the intensity distribution of a transverse-flow CO2 laser along the flow direction and the dependence of its output power on the cavity width. The experiments were performed with a transverse-flow CO2:N2 laser chemically excited by the reaction of nitric oxide with atomic nitrogen. The intensity distribution is found to be of a multimode structure, fairly symmetrical about the optical axis. The overall output power for a given flow velocity and gas mixture is found to increase with increasing cavity width, up to a maximum value, and then it diminishes with any further increase of the cavity width. These results do not agree with those expected from a simple geometric-ray model. On the ohter hand, our results conform qualitatively with those expected from a coupled multimode approach. The main differences between the two approaches are discussed, and simple arguments are presented to illustrate the limitations of these two approaches.