CO2-laser gas discharges in narrow gaps

Abstract

We have studied RF discharges as excitation mechanisms for distributed feedback (DFB) CO2 lasers. For CO2 laser plasmas the reduced electric fieldE/N has to be in a well-defined range. The reduced electric fieldsE/N of gas discharges in the narrow gaps with widths of the order of 100 μm required for DFB are considerably above this range. In order to study the feasibility of these RF-excited discharges for DFB CO2 lasers we have measured the electron temperatureT e in their plasmas. From helium-line-intensity ratios we have deduced a lower limit of the electron temperatureT e of 4eV. The observed high intensities of bands of singly ionized nitrogen indicate an even higher electron temperature, but an efficient pumping of the upper laser level is not possible with an electron temperature above 2.5 eV. We have estimated the electron densityn e and the current densityj e from ratios of the intesities of forbidden and allowed helium lines. The high current densityj e is in the range of abnormal glow discharges. In the gas discharges between narrow gaps the electron oscillation amplitudex e is large than the electrode separationd. In order to replace the resulting high electron losses a high electron temperatureT e is necessary to sustain the gas discharge. Because of this high electron temperatureT e an efficient pumping of the upper laser level is not possible.