Influence of the electric field frequency on the performance of a RF excited CO 2 waveguide laser

Abstract

Abstract. An analysis is presented of the effect of the RF frequency on the active media of CO2 waveguide lasers. It is found that the characteristics are improved with increasing RF frequency because the space charge sheath width decreases with increasing excitation frequency. We also found that the sheath width decreases with the discharge current; this fact was never discussed before. The higher the exciting frequency the higher is the maximum input power of the discharge in the stable low current mode. It is attractive to extend the input power while keeping the discharge in this mode. Finally, a stabilizing excitation technique is described for the inherent unstable region of the discharge.It is well known that the best performance of CO 2 waveguide lasers is obtained with RF excitation [1][2][3][4][5][6]. The choice of the used excitation frequency was mainly determined by practical arguments like the available power supplies. Its impact on the laser performance has not been studied thoroughly. So far this problem received little attention. There are two main points of view. There is the opinion that a change of frequency influences only the properties of the quasi-neutral plasma of the active medium [-2,7]. The other one is that the frequency changes the thickness of the charge sheaths near the electrodes so that the energy deposition into the quasineutral plasma and consequently the laser output are influenced [8,9]. The theoretical background for these considerations was discussed either phenomenologically or qualitatively.In a previous paper we presented the laser characteristics of the CO2 waveguide system with RF discharge [5]. The present paper, based on the previous model, will analyse the mechanism of laser excitation as it depends on various parameters, especially on the RF field frequency. Initial assumptions and system descriptionThe system that will be analysed theoretically is our experimental device [4,5] having a waveguide discharge volume of 2.25 × 2.25 x 370 mm 3. The used gas mixture is CO2-Nz-He (1:1:8) at a pressure of 100 Tort. The RF voltage is by means of parallel shunt inductors homogeneously applied to the two parallel electrodes. The field distribution in the discharge is calculated by solving the Poisson equation with the appropriate boundary conditions determined by the oscillating voltage over the electrodes. Because of the homogeneity of the RF discharge the problem is treated one-dimensionally with the coordinate x perpendicular to the electrode surfaces. The Boltzmann equation for the electron energy distribution is then solved by using the relevant cross-sections for the electronic excitations, vibrational excitations, dissociation of the CO2 molecule and the ionisations of N2 and CO2. The discharge heat is removed through both electrodes and through the ceramic side walls of the cavity. The considered frequencies of the electric field are in the range from 62 to 233 MHz. The relaxation frequency of the electron drift velocity is assumed to be larger than the frequency of ...

RF discharge in CO2 laser mixtures at moderate pressures

“…Analytical models describe an RF discharge only qualitatively [10]. In a number of papers, the characteristics of an RF discharge were calculated numerically (see, e.g., [5,11,12] and review [9]). However, no comparative studies of the results of numerical simulations and available experimental data have been made.…”