Reduction of the pumping efficiency in CO2 lasers at high discharge energy

It has frequently been observed that, beyond a certain point, the gain obtained in TE CO2 lasers fails to increase with increasing discharge input energy. This saturation effect could be caused by problems of a secondary nature such as the onset of discharge deterioration, or the effects of increasing gas temperature. On the other hand, an intrinsic property of the discharge may be responsible for this limitation on the pumping efficiency. In this paper, evidence is presented in favor of the latter possibility. Measurements of small−signal gain have been carried out in He: CO2 and He: N2: CO2 gas mixtures at moderately low pressure in a transversely excited resistor−pin amplifier. It is shown that this system permits the clear separation of the region of discharge deterioration from the onset of gain saturation. Furthermore, auxiliary measurements are undertaken which permit corrections to be made for the effects of increasing gas temperature. The net result is that the discharge pumping effectiveness at high energies falls to values drastically below those obtained at low discharge energies. This saturation effect occurs in a manner consistent with the existence in the discharge of a deexcitation process for the upper laser level. If such a deexcitation process indeed accounts for these observations the cross section must be an order of magnitude higher than the corresponding excitation process.