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Industry has growing interest in lasers with high output power of high beam quality. Therefore, laser developers have to concentrate their efforts on the optimization of every component involved in the beam generation, e.g. the rf-power incoupling and the gas flow generation. At present CO2 lasers of very high output power are developed by increasing the dimensions of the whole system. Inevitably, a more or less pronounced reduction of beam quality and efficiency will occur, if scaling laws are not considered. In order to reach the goal of a well designed system with high performance data the knowledge of homogeneity and stability of the discharge as a function of the various parameters involved is essential. The evolution of filaments for example increases with the rf-input power. This filamentation, however, is a complex mechanism that also depends on gas temperature, gas turbulence and contamination as well as on the excitation frequency of rf-discharges. In this work, diagnostic methods for determining the degree of filamentation are presented. The results achieved are used to determine the frequency-dependency and scaling laws of rf-excited CO2 discharges. The range of parameters favorable for a stable discharge is generally not the same compared to the parameters yielding high values of small signal gain and saturation intensity. The optimal discharge geometry (length and diameter) depends on various quantities. The values for mass flow and of rf-input power are given by the gas circulating system and the rf- generator. The maximum rf-input power density depends on the limit of stable operation, which itself is a function of gas pressure and velocity and various other parameters. Taking into account the gain saturation of the medium, the optimal gas pressure itself depends on the input power. Measurement data of small signal gain and saturation intensity spatially resolved parallel to the gas flow direction as well as electro-optical efficiency will be given. Based on these measurements, scaling laws are derived and will be presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

For the development of compact high-power CO2 lasers high input power densities in discharge tubes with sufficiently large cross sections are required. To achieve a uniform distribution of power deposition into the laser gas flowing in tubes with typically circular cross sections an appropriate electrode shaping is necessary. For this reason, experimental and theoretical investigations have been performed in a wide range of input power, starting from values typical for todays CO2 lasers up to very high levels. Different electrode shapes have been used for optimized stability and homogeneity of the discharge in order to minimize the distortion of the intra-cavity wavefront. The occurrence of filaments limits the range of a stable discharge operation. The homogeneity of the discharge has been assessed by means of spatially resolved small signal gain measurements. Further, the homogeneity of the discharge as a prerequisite for minimal phase distortion has been investigated using an interferometer. A discharge with 17 kW input power at 13.56 MHz in a single tube element is presented showing a flat profile of the small signal gain and a phase distortion of 0.21 micrometers .© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fast axial flow rf-excited discharges ((nu) rf equals 13.5 MHz) in tubes with rectangular cross sections and plane electrodes have been investigated. Probing the tubes on-axis the dependence of small signal gain as well as saturation intensity on the input power density is identical for all the different cross sections. Under equal conditions the small signal gain profiles revealed significant decrease in the vicinity of the side walls with increasing tube dimensions. This is attributed to the change of discharge behavior with increasing interelectrode discharge length. It was found that the critical pressure for the onset of filamentation is reciprocal to this dimension. With this geometrical dependency the laser power of axial flow rf-excited discharges is not volumetric scalable in a straightforward way.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Numerous industrial applications require high power lasers with at the same time high beam quality. Therefore, among other requirements of the laser design and operation particular attention has to be paid on the optical properties of the laser-active medium.