A brief survey is given of the trends in the development of high power gas lasers for industrial applications. The properties of laser media, problems of beam quality, and short wavelength performance of high power gas lasers are considered.

Over the last 17 years more than 60 lasers have been developed at Battelle-Frankfurt. Part of the corresponding work is described below. In particular we report on the development of various CO₂ TEA lasers and continuous high power CO₂ lasers. Preionisation is effected in the TEA lasers by UV light generated in a corona discharge. The TEA lasers of the last generation are particularly compact, reliable and efficient. They operate with pulse frequencies of up to 30 Hz and a mean power of up to 100 W. After the development of a continuous 1.2 kW high power CO2 laser of conventional design, the first gas transport laser was constructed in 1971. Knowledge and experience gained with this laser were the basis for various patent applications. The new developments in this area are based on these patents. At present two different gas transport lasers with 1 kW and 2 kW output power are being completed and tested. After selection of the most favourable principle a 5 kW system will be built. The new lasers differ from the known systems by the complete integration of the various components and the use of particularly efficient blowers. Their volume and weight will be lower by a factor of 3 or more.

Traditionally Ruby and Nd+3 YAG are the common Materials for high energy and high power pulsed lasers. The different materials properties demand quite different details in system developments. Both kinds of laser materials are mostly used at the fundamental wavelengths of the laser active material. New developments resulted in the creation of Alexandrite, which laser material allows to generate laser systems with tunable wavelengths of the output beam. This new material is promising for future applications in laser material processing.

A new boo Watt CO2 -Laser is described. It is a fast axial flow type with a once folded resonator. The laser output power is continuously variable and can also be pulsed up to 2.5 kHz.

Despite the high degree of satisfaction expressed by most laser users due to the good quality of produced parts and attained cost reduction, occasionally 01 jections are voiced regarding reliability and convenient operation of the lasers. One of the reasons may lie in the fact that these machines were not developed by engineers who are familiar with rigid production requirements, but by physicists who place more emphasis on the scientific side of laser technology and laboratory atmosphere. Acknowledgement of this, coupled with five years of experience in the production and operation of CO2-hic-h power lasers within the 1-5 kW performance range, led to the develop-ment of a new laser generation that fulfills the basic requirements necessary for day to day pro luction. The result of this was a compact, robust, reliable and easy-to-operate unit. An explanation of the main features follows:

Based on the iterative solution of a generalized Kirchhoff-Fresnel integral equation we have developed a computer model for realistic simulation of arbitrary linear or folded resonators. With known parameters of the active medium (small signal gain, saturation intensity, volume) we can determine the optimal parameters for the resonator (e.g. out-put mirror transmission, radius of curvature of mirrors, diameter and place of diaphragms, length of resonator) to get highest output power with a certain mode pattern. The model is tested for linear as well as folded resonators.

The CO2 high power laser is increasingly used in material processing. This application of the laser has to meet some requirements: at one hand the laser is a tool free of wastage, but at the other hand is to guarantee that the properties of that tool are constant in time. Therefore power, geometry and mode of the beam have to be stable over long intervalls, even if the laser is used in rough industrial environment. Otherwise laser material processing would not be competitive. The beam quality is affected by all components of the laser - by the CO2 plasma and its IR - amplification, by the resonator which at last generates the beam by optical feedback, and also by the electric power supply whose effects on the plasma may be measured at the laser beam. A transversal flow laser has been developed at the Technical University of Vienna in cooperation with VOest-Alpine AG, Linz (Austria). This laser produces 1 kW of beam power with unfolded resonator. It was subject to investigations presented in this paper.

The results of experimental studies of the electron temperature in a high power gas transport CO₂ laser discharge are presented . Gas mixtures CO₂ : N2 : He of 0 : 0 : 1 , 1 : 2 . 3 : 1 5, 1 : 1 .2 : 5 .8 , 0 : 1 : 0 are investigated by probe measurements.

High energy lasers are used for industrial measurements in connection with additional instrumentations. The most advanced system for this purposes is the Image Derotator. This system in combination with high energy laser systems is a powerful engeneering and scientific tool in the field of holographic interferometry and speckle photography. Traditional measurements complete the application range of the Image Derotator.

High power CO2-laser optics have been investigated with respect to their thermal runaway and thermal distortion characteristics. It is shown that for highest power densities GaAs is the best choice as a transmitting optic while Copper and Molybdenum are best suitable for high reflective optics.

When using CO2- and YAG-lasers in industry and laboratories it is of great interest to know the intensity profile of the beam, the mode of the laser. On the basis of the idea of the rotating wire - the principle was developed by Dr. Steen, Imperial College, London - an apparatus was developed for the employment in industry and laboratories.

Brillouin scattering was used to determine the relaxation time of the vibrational degrees of freedom of the molecules in liquid carbontetrachloride at the pressure of 1 bar and at a constant density of 1.6225 g.cm-3 from +5 to +71 C. The temperature dependence of the mean free path between intermolecular collisions was calculated from these relaxation times. It was found that the experimental results contradict the predictions of liquid models based on hard sphere molecules.

Lasers offer new and extended possibilities for material processing. As the laser beam is a very precise working tool of high flexibility, free from wear-out, there are already various applications such as drilling, cutting, welding, glazing, annealing, hardening, and others. To utilize the advantages of laser processing it is necessary to understand the physical background of laser material processing as there is a strong coupling between laser and target by the working process, severely influencing the processing results, as well as the performance of the laser tool itself. So any laser tool has to be matched carefully to the special application, in order to achieve the results desired.

Laser material processing is accompanied by a laser induced plasma in front of the target surface as soon as the laser radiation exceeds a certain critical intensity. For cw CO2-laser machining of metal targets the threshold for plasma onset is about 106 W/cm2. Critical condition for plasma generation at this intensity level is to reach evaporation temperature at the target's surface. At intensity levels exceeding 106 W/cm2 the laser light is interacting with the laser induced plasma and then the plasma in turn interacts with the target. The absorptivity is no longer constant, but increases with increasing intensity of the incident radiation, so that the total amount of power coupled to the target is increasing. This holds up to intensity levels of 2'10 Wicm2. Then the plasma begins to withdraw from the target surface, thus interrupting plasma-target interaction so that the laser power is no longer coupled into the target completely. The results of laser welding (welding depth) in the intensity level of 106 W/cm2 are governed by the product of incident intensity times focus radius, so that welding results are a measure to determine focus radius and laser intensity.

High power CO2-laser systems with radiation output from 0.5 to 10 kW are ready now to be incorporated in a production line. They offer new possibilities of material processing like deep-penetration welding, transformation hardening of thin surface layers, hard facing, surface glazing, etc. At the present time, besides applications in microtechniques like spot welding, drilling and laser trimming, primarily laser cutting is used in industry. In order to achieve a universal cutting tool of high productivity, research should not only concentrate on the processing machine consisting of gas assistant components, CNC systems, beam guidance, and the laser itself, but should also take the material properties and the physical mechanism of the cutting process into consideration. On the other hand, it is important to find new ways of engineering design and manufacturing processes adapted to the specific requirements and advantages of laser cutting. As a thermal cutting method used mainly for thin sheets up to 6 mm thickness, laser gas cutting competes with the mechanical cutting methods. It is characterized by a high cutting speed, a small kerf width, a narrow heat affected zone, and a high quality cutting edge. Thus, no distortion of the workpiece occurs, and, in most cases, no finishing operations are required. In connection with a CNC table, complex two or three dimensional cuts of high accuracy can be done. Moreover, laser gas cutting can be automated easily. At the Institute of Welding and Materials Technology, we are interested in the behaviour of the material during the cutting process, in a determination of the cutting quality attainable by present laser systems, and in the mechanical properties of laser-cut elements. We are also concerned with the comparison of the various cutting methods and the classification of their fields of application. With regard to these problems, besides others, a physical understanding of the interaction between high density laser radiation and matter is required. For our purpose, however, a cutting model should be as simple as possible, able - to give an estimation of the cutting speed, - to show basically how the quality of the cutting surface depends on the process para-meters, - and to predict the metallurgical impact on the workpiece. In this paper, some primary steps are briefly given. A more detailed description of our in-vestigations will be published later.

During laser cutting of relatively thick workpieces the erosion takes place at a nearly vertical plane at the momentary end of the cut. That plane is covered by a thin molten layer, that is heated by absorbed laser radiation and by reaction. The removal of material from that layer is carried out by evaporation and by ejection of molten material due to the friction between the melt and the reactive gas flow. A computer simulation of that model yields a more detailed understanding of laser cutting and agrees well with experimental investigations.

The influence of the laser beam stability on the cutting quality is discussed. A simple method for recording central parts of the beam contour is given. The role of slag transportation on hand of practical results and the problems with drilling starting holes are mentioned. Moreover some interesting observations accompanying laser cutting are described.

Laser or electron beams are used as tools for welding, cutting, drilling melting, tempering or vaporizing in many machining tasks. These beams have fundamentally different physical natures. They are produced differently, and behave fundamentally differently when penetrating material. The differences between electron and laser devices that occur in machining, the advantages and disadvantages of these technologies in comparison with one another, will be discussed in this article. Using a more detailed description of electron beam technology, comparisons between laser and electron beam technology will be discussed.

Today I would like to give you an outline of laser technology and its use in VOEST-ALPINE. These activities take place in the company's Finished Products Division.

Different techniques of laser surface alloying are discussed. Surface quality, micro-structure and resultant properties are reported for laser gas alloying and laser cladding.

Cu based alloys have been laser surface melted. Absorption behaviour, microstructure and resultant properties are reported.