In order to contribute to the development of a reliable criterion for beam quality characterization, a review of the commonly used criteria is done. Several meaningful experimental works using oscillator-amplifier configurations for high power industrial CO2 lasers, are analyzed. The influence of some forms of inhomogeneity on the beam quality, in an oscillator-amplifier configuration, has been also investigated through numerical methods.

Characterization of Fabry-Perot resonators and their transmitted intensity at the focus of an optical system which collects the output beam emerging from the cavity is analysed in terms of mirror surface irregularities and of fluctuations of the refractive index of the medium filling the resonator. It is pointed out that an amplitude averaging method should be employed in order to correctly describe the system behavior, instead of the intensity convolution procedure commonly used in the literature. Explicit expressions (easy to compute) are derived for the amplitude at the focal point of the optical system from the standpoint of the geometric-optics approximation.

A new laser gas flow design for transversely rf-excited CO2-lasers has been presented at the GCL VII Conference. Due to the imperfect electrode shape the electrical input power was limited by the occurrance of filaments in the discharge. The prensent paper deals with improvements of the rf-electrodes. Metals with high electrical conductivity and low permeability i.e. diamagnetic materials as copper, aluminum, and brass are proper electrode materials. Furthermore, the shape of the electrodes has main influence on the distribution of the current of the glow discharge. The use of small contoured electrodes enhances ignition of the discharge, however, the maximum input power for arc free operation is low Electrodes with a larger outer diameter enable a streamer free discharge at higher input powers. Consequently the electrodes were optimized concerning length, diameter, and alignment to ensure the use of the maximum electrical power available from the generator. The electrode temperature is determined by heat conduction from the discharge tube. Electrodes which are separated from the tube need no cooling. Air cooling with a blower improves the performance of the system.

The excitation of CO2-N2-He gas mixtures by means of microwaves (V = 2.45 GHz) leading to laser action is posstblb. In order to avoid hot boundary layers near the discharge wall a proper distribution of the gas flow and the microwave field must be chosen. Additionally an ignition setup is necessary to achieve a homogeneous glow in the discharge tube. In a first part of this paper ignition and permanent maintenance of a microwave discharge as well as the kinetic processes leading to laser action are studied on a theoretical base. The theoretical data are compared with experimental ones obtained from an ignition experiment and CO2-laser devices of 0.3 or 1 kW cw output power, respectively. Examples for continuous and pulsed laser operation are given.

Since solid state laser, especially Nd:YAG laser, find more and more applications in the medical and industrial field, methods to improve the efficiencies of these systems are under discussion again. Among these, cavities and flow-tubes made of so called "fluorescence-converting" materials face some special attraction. They can be used without a major redesign or reconstruction of the laser head, just replacing the corresponding parts. In our work the performance of Samarium doped glass is evaluated, which has absorption bands in the UV spectral region and fluorescence bands in the visible region, close by or at the absorption bands of Nd:YAG laser crystals. In this way unused or even harmfull UV radiation could be transferred and should intensify the pumping. By measuring the relevant spectra of flashlamps and arclamps with and without the Samarium doped filter glass in a laser cavity we try to clarify this process. We examine the thermal lensing of the different laser rods (Nd:YAG, Cr:Nd:GSGG and Er:YAG) under both conditions and determine the output power or energy for different pump powers or energies respectively.

Efficient generation of first Stokes Raman radiation can be achieved with an External Raman Oscillator (ERAMOS). In our experiments a CH4-Raman oscillator was pumped by a Nd:YAP laser. By incorporating this Raman Resonator (IRAMOS) in the pump resonator a further increase in efficiency and a lowering of the threshold can be obtained. The experimental results were discussed and compared with our theoretical calculations.

Soft or apodized apertures with smooth decreasing from centre to edges transmission profiles are used in laser physics for beam shaping. This paper gives the results of the studies of four types of these units for UV, visible and IR lasers. They are made of glasses or crystals with the use of one of the following technologies: 1)absorption induced by ionizing radiation; 2)photodestruction of colour centres or photooxidation of impurities ions; 3)additive colouration; 4)frustrated total internal reflection. The special feature of such apertures is their high optical damage resistance under the irradiation of single--pulse laser radiation. They are approximately 3-50 mm in diameter but the methods of making them give the possibility to create near-Gaussian and flat-top beams with dimensions less than 1 mm and larger than 200 mm. The results of using them in high-power single-pulse lasers are presented herein. Damage thresholds of these apertures in such types of lasers have been defined.

A picosecond actively mode-locked C; YAG:Nd-laser with the axial interval 20 ns (larger than typical) and high efficiency was developed. It is possible to use such laser in time resolved laser spectroscopy and picosecond technology.

This communication describes the results of some experimental investigations with neodymium glass lasers using wavefront conjugation phenomenon. Amplifying channel with real loads and model of multichannel amplifier with wavefront conjugation are studied. Also perspectives and details of wavefront conjugation application in inertial confinement fusion systems are discussed.

The investigations with the purpose to increase amplitude and frequency stability of a solid-state lasers, which may be applied in laser spectroscopy, metrology, communication, information processing, are reported. The spectral distribution of natural amplitude noise of cw solid-state lasers was considered. The effect of noise suppression at frequency of relaxation oscillation in laser with intracavity frequency doubling was analyzed as well as sub-Poissonian photon statistics. In the experiments with cw YAG:Nd lasers the amplitude noise close to natural limit was achieved over a wide range of fluctuation frequency. It was shown that the mentioned effect of noise suppression results the amplitude noise less then 0.001% over the range 10-200 kHz. The molecular cesium spectroscopic reference standard for long- term frequency stabilization in 1 μm region is described. The sub-Doppler spectrum of Cs2 saturation absorption is represented over the YAG:Nd laser tuning range near the wavelength 1.06415 μm. This spectrum produces the optical scale of reference frequencies, which looks favorable for various heterodyne applications. Frequency stabilization of YAG:Nd laser onto Cs2 absorption lines is reported. The Allan variance measurements provided the highest frequency stability of 6x10-11 relative units.

Experimental study of waves damping in a high repetition rate excimer laser is undertaken. Excitation of laser active medium in a subsonic loop is achieved by means of a classical discharge, through transfer capacitors. The discharge stability is controlled by a wire ion plasma (w.i.p.) X-rays gun. The strong acoustic waves induced by the active medium excitation may lead to a decrease, at high PRF, of the energy per pulse. First results of the influence of a damping of induced density perturbations between two successive pulses are presented.

Increasing interest has been shown in scaling excimer lasers to high average power either with a high energy per pulse or with a high repetition rate since their applications are very promising and attractive in several fields, such as materials processing, photochemistry, and so on. In order to cover a broad range of applications, the work at the ENEA Frascati laboratories has mainly been devoted to developing two different discharge-excited laser sources, one operating at - 10 J/pulse and a repetition rate of - 100 Hz, and the other at 1 J/pulse and a repetition rate of - 1 kHz. As an intermediate step, two x-ray preionized XeCl laser discharge devices were studied: one device has a discharge volume of 10 liters (10x10x100 cm3) (Laser:L) with a longi-tudinal gas flow mode in the discharge region, while the other operates in a transverse gas flow mode with a discharge volume of 0.45 liters (3x3x50 cm3) (Laser:S)*. Some of the experimental results obtained with the two devices are described in this report.

A dye laser gain analysis reveals that dye lasers can be upscaled to high energies, provided improved laser dyes are employed. Superior laser dyes were recently discovered in the quasi-aromatic families of syn-dioxabimanes and pyrromethene-BF2 complexes. The representative new dye ,u-bis-(carboethoxy) methylene-syn-(methylene, methyl) bimane lased at 507 nm with 50% greater efficiency than was obtained with Coumarin 30. A similar determination showed 4,4-difluoro-1,3,5,7,8-pentamethy1-4-bora-3a,4a-diaza-s-indacene to lase at 546 nm with about 300% greater efficiency than was obtained from Coumarin 545. These quasi-aromatic systems are readily available and offer desirable properties of stability (photo, thermal, and chemical), solubility, and diminished triplet-triplet absorption in the lasing spectral region.

Laser effect around 630 nm is observed in a sulforhodamine 640 doped iiica gel pumped by a Q switch doubled Nd:YAG laser. A conversion efficiency of 20 % is obtained. The laser wavelength can be tuned over 40 nm.

The research program aims at scaling excimer lasers of high pulse energy (>2J) in output power. A XeC1 laser operated with paralleled waterline capacitors and a hollow cathode type X-ray gun has yielded 225 W at a pulse energy of > 2,25 J and a repetiton rate of 100 Hz. A short laser of 15 cm gain length was designed as a flexible tool for quick modification of components and for having more reserve in electrical power and gas throughput for scaling. So far with the short gain length and XeC1 0,56 J pulse energy were achieved. Applications of high pulse energy lasers focus on material processing. A Siemens XP 2020 excimer laser was used for structured removal of different coating/substrate systems of metals, ceramics and polymers, e. g. metal coating on ceramics, ceramic protection on steel, superconducting ceramic films on ceramics, polyimide on copper and metals sputtered on copper.

Because of its specific features, shortest pulses with highest pulse power, excimer laser radiation seems to be suitable for processing ceramics. It reveals high surface quality and enables a 2- or 3-dimensional structuring in the field of micro machining. This paper deals with the interaction process between ceramic materials and excimer laser radiation. The effect of different beam parameters on the removal result and the laser affected zone respectively and further capabiltities of material processing with UV-radiation for various industrial applications are discussed.

Preliminary studies into the machining of certain aerospace materials using excimer laser radiation have been performed. Burn-through experiments were conducted using KrF (248nm) laser radiation in the fluence and PRF ranges 0-6J/cm2 and 1-50Hz respectively. In particular, woven Kevlar (in various forms) and carbon fibre composite structures have been machined with excellent edge quality and no visible degradation of the surrounding bulk material (SEM photographs are presented). Fluence thresholds for material removal exist and occur in the range 0.1-0.5J/cm2 for the above materials. Maximum observed removal rates have been measured as ~1-1.2μm/pulse and ~0.4μm/pulse for Kevlar and carbon fibre composites respectively at a fluence of ~6J/cm2. Complemetary studies for Kevlar are also reported using XeCl (308nm) laser radiation in the fluence and PRF range 0- 1.5J/cm2 and 50-500Hz. Machining rates are comparable to those at 248nm. Material removal rates per pulse at both wavelengths appear independent of PRF values. Both pre-cured and cured composites exhibit good matrix stability at the machined walls and there is no visual evidence of curing occurring during excimer laser irradiation. Sooty (probably carbon rich) debris was observed around the irradiated zone, the extent of which increased with increasing fluence. From extrapolation of etch rate data, industrial cutting rates are anticipated.

Thin films of platinum, molybdenum and titanium silicide were obtained by multipulse excimer laser irradiation of thin platinum, molybdenum and titanium films deposited on silicon single crystals. With this technique, by a proper control of the pulse fluence, it is possible to react a well defined metal thickness. A thin metal film can be left unreacted for successive metallization procedures. By multi-pulse excimer laser irradiation of refractory metal film in nitrogen or ammonia, the contemporaneous synthesis of a silicide layer at the metal/silicon interface and of a nitride film at the sample surface was also obtained. The nitride film acts very well as an interdiffusion barrier in metallization schemes.

The excimer laser ablation of a chresol-novolak based photoresist is reported. The process is caracterised at low gas pressure by laser beam transmission, the emission from fluorescing radicals, and analysing material which is deposited on the wafer surface. The results suggest that a supersonic ejection of the ablated molecules occurs in addition to chemical reaction which take place in the gas phase at reduced kinetic energies. Some conclusions about the ejected particle condensation and the corresponding accumulation of debris are drawn. These results allow improved understanding of how the ambient gas effects the ejected chresol-novolak molecules produced by excimer laser ablation.

In this paper only the problems regarding to the input-coupling and transmission properties of the fibre are discussed. For a given Excimer laser system (Technolas MAX 10) the laser beam was prepared to transmit pulse energy densities up to 25 J/cm2 to fibres of 200 -600 μm core diamter. Coupling of Excimer radiation into Q/Q fibres is limited by destruction of the front surface or the core of the fibre (Fig. 1). This limit is of the order of 40 - 50 J/cm2 for 308 nm and 50 nsec pulse length. To reach this limit it is necessary that the laser beam fulfills the following three demands: 1. The laser beam should have a diameter slightly less than the fibre core diameter and he should have flat intensity profile. 2. The beam divergence (θB) should about the same as the fibre aperture (θF). 3. The point 1 and 2 can be fulfilled, if the beam parameter product d θB(d = waist radius) is equal or less than the parameter product of the fibre.

Excimerlaser induced ablation of polymers is often accompanied by a modification of the surface morphology at the irradiated regions. Only very recently models have been proposed for this effect, the origin being searched in different absorption properties of crystalline and amorphous material. A study of the authors on synthetic fibers irradiated by the uv-laser is in contrast with that assumption and gave evidence of a thermal contribution to laser-polymer-interaction. A strong influence of internal or external stress fields is found. On this basis a synergetic understanding of the surface structures is proposed. This model regards blocked stress fields, which are frozen in internally or applied externally, as the driving forces of self-organizing material convection, namely Marangoni-convection. These stress fields are thought to be released by the high temperatures and temperature gradients induced by the laser pulse.

As part of the Eureka Eurolaser initiative, Lumonics built two experimental high power pulsed YAG lasers in 1987/8 which were used for supra-kilowatt processing studies. These two lasers were capable of 1.35 and 2.3 kW average power and 20 kW and 35 kW peak power, respectively. The high intensity beams from these experimental lasers have demonstrated novel processing performance. The characteristics when welding thick section material have been particularly interesting - penetration to 25 mm has been achieved. This paper reviews the work in this area and compares the results obtained by Lumonics with these lasers welding stainless steel with that reported for other power beam sources at high power. These include; continuous wave CO2 and YAG lasers and e-beam sources. It is shown that processing with a pulsed YAG laser beam provides a number of key advantages over the other power types.

Processes like plasma development and quenching modulation of the evaporation rate, pressure variation, energy coupling or melt transport and their interdependence determine the welding process. The investigation of these processes with optical and acoustical methods is presented. When measuring the radiation of the luminous phenomenon in different wavelenght ranges, the transition from metal vapour to laser-induced plasma and vice versa as well as plasma density fluctuations in the keyhole are shown. This result is supported by measuring the pressure variations in the enviroment with a microphone. The case of full penetration welding is particularly investigated. In this case the luminous phenomenon below the workpiece and the CO2 radiation, transmitted through the keyhole is measured additionally. All these investigations result in a presented model. The model shows that the variation of the metal vapour density mainly determines the above-mentioned processes like energy coupling, pressure variations and so on, and therefore has a great influence on laser beam welding.

Welding of pipes and manufacturing of plated metal sheets are two applications of a laser beam in which the distribution of the absorbed intensity differs considerably from the irradiation. Laser beam propagation between two adjoining metal surfaces is investigated experimentally and the results are compared with theoretical calculations. The radiation transfer equation is solved for the redistributed laser intensity at the adjoining metal surfaces. The influence of beam radius, polarization state and transversal disadjustment are discussed.

Laser welding experiments of thick metal sheets with a thickness of up to 20 mm at high power levels up to 20 kW beam power are reported. Due to the high intensity of the laser beam exceeding 107 Wcm-2, the effect of plasma suppression by helium gas shielding was investigated. Furthermore, the welding parameters were determined for a laser beam power range between 10 and 20 kW. The first results indicate that an extrapolation of the laser welding parameters which already had been observed when welding at lower power levels < 10 kW, is not applicable for the 20 kW range. Moreover, the influence of the welding speed on the porosity of the welding seam in low carbon steels of 20 mm sample thickness was slightly different from results of thin sheet welding with a thickness of less than 10 mm.

The industrial laser market is continuously changing in its applications and technologies. For firms dealing in laser technologies, this means risk management strategies to overcome market and technological variability, diversity and complexity. How is one to measure these risks ? What are the implications for industrial strategies ? An analytical approach will give some answers to these questions and give light as to what has happened on the industrial laser market in the 1980's and what may be expected for the 1990's.

In the past few years, laser technology in the field of sheet metal machining achieved the highest growth rate compared with similar applications. This can mainly be traced back to the fact, that apart from short processing times and machining with absence of interaction, the laser represents a contactless, force-free, and mostly non-wearing tool with the facility of adaptation to multiple machining tasks. This flexibility in production and the facility of reactionless beam guidance in accordance with optical laws open up new applications /1/. With the use of the laser in industrial production the economy of laser cutting depends mainly on the selection of a suitable plant concept. The plant concept is generally based on the interaction of laser beam guidance and machine kinematics. In the future it will be important to the decision maker to find methods for selecting the most appropriate laser cutting system for particular product spectra. A specific sheet metal component can be machined on various laser cutting plants. These laser plants vary with respect to their kinematic layout. The following variations are possible: - A movable laser machining optic with stationary workpiece; - Movable workpiece and stationary laser machining optic; - Combined movement of laser machining optic and workpiece.

The laser is a non-contacting, forceless tool which is nearly free from wear. In regard to its tasks and its potential for process automation due to ease of control of the laserparameters, laser processing seems to be predestinated for flexible manufacturing. By the use of free moveable, robot guided laser beam guiding systems, the laser as a tool, offers a new potential for widely varied tasks. In an overview on existing concepts for laser beam guiding systems, the possibilities for laser beam handling are discussed and system concepts of coupling laser beam guiding systems to a robot are presented. With the help of a pilot laser workstation, realised at the IPA, the different components of a flexible laser workstation are specified. Basing on this the principal explanations of robot guided laser beam guiding systems are introduced. Different kinematic diagrams show the influence of the combination of laser beam guiding system and robot kinematic due to the workenvelope of the entire system. Measurements of vibrations, analysing the frequency spectrum and the acceleration force on different characteristic points of the laser beam guiding system are carried. out. Further more the vibrations have been brought in comparison with the beam pointing stability and all these measurements have been undertaken by the consideration of the corresponding cutting results.

A simple model for reactive gas laser cutting is presented. A thin metallic film under laser irradiation in an oxidizing atmosphere is considered. The stability of the diffusion-controlled growth of an oxide layer with external cooling of the system is investigated in terms of nonlinear point-system dynamics. Depending on the material and input parameters the system can exhibit limit cycles. Instabilities of this kind can be sources for striations on the cutting surface.

The process of laser fusion cutting depends on physical quantities which are not adjustable directly, but established by the process itself. Examples are the slope of the cutting front, the melt-film thickness, the surface temperature and the cutting kerf width. Especially, up to now, the cutting kerf was approximated to have the same extent as the laser beam itself. Our present model makes allowance for the distribution of the laser intensity, the global momentum balance determining the melt-flow, and the heat-conduction equation in connection with the boundary condition for the absorbed energy density flow. The resulting three-dimensional cutting front geometry is calculated and the physical mechanism determining the cutting kerf width is discussed.

Guided by experimental investigations laser cutting of glass fiber reinforced reactive injection moulded (RRIM)-polyurethanes which are used e.g. in car industry for bumpers, spoilers, and further components is described. A Comparison with other cutting techniques as there are water jet cutting, milling, punching, sawing, cutting with conventional knife and with ultrasonic excited knife is given. Parameters which mainly influence cutting results e.g. laser power, cutting speed, gas nature and pressure will be discussed. The problematic nature in characterising micro and macro geometry of laser cut edges of fiber reinforced plastic (FRP) is explained. The topography of cut edges is described and several characteristic values are introduced to specify the obtained working quality. The surface roughness of laser cut edges is measured by both, an optical and a mechanical sensor and their reliabilities are compared.

An investigation has been made aiming to produce a composite surface layer of high hardness consisting of Stellite alloy 6 and SiC particles by using a 2 kW CW CO2 laser. Using a pneumatic powder feeding system, a mixture of Stellite alloy 6 and SiC was blown into the melt pool produced by the laser on the mild steel (En 3b) substrate. A few tracks were also produced by blowing only Stellite powder, for comparative studies. The structure, chemical composition and hardness of the two sets of clads are discussed. Stellite blown clads were found to be low in dilution with a negligible amount of porosity; hardness levels in the range 540-580 Hv were obtained. The laser produced Stellite-SiC composites showed higher dilutions with a small amount of porosity; higher hardness levels (up to 1390 Hv) were obtained as compared with Stellite clads.

An investigation has been made of laser cladding by feeding a mixture of fine powder of Zr02-8wt% Y203 and 316L austenitic stainless steel into a laser generated melt pool on a 0.2wt% carbon steel substrate. The effects of the powder feed rate, scanning speed and laser power are discussed. Microstructural studies and compositional analysis of single tracks have been made using optical microscopy and scanning electron microscopy with dispersive X-ray spectrometry (EDS). X-ray diffraction was used to investigate the phases present after laser cladding. The clad layer contained distinct regions of stainless steel and of ceramic. The austenitic stainless steel regions were free of cracks, while the yttria partially stabilized zirconia (YPSZ) regions showed some cracking particularly along the interface between the stainless steel and the ceramic. The stainless steel regions showed a high hardness, associated with a cellular/dendritic structure, corresponding to cooling rates in the range of 10 to 10K/s during solidification.

The heat treatment and surface hardening of metals is a process that is of considerable interest to the metal working community. The use of lasers to accomplish this heat treatment permits one to produce results not possible with other methods. However, due to the somewhat complex nature of this process it can be confusing and it is sometimes difficult to understand exactly how to proceed: What type of beam shaping to use, what type of coatings to apply, what power levels to use, what laser, etc. In this paper , an examination of four distinct methods of beam shaping are presented. They are: 1. Focusing lens only 2. Beam scanner 3. Beam integrating mirror 4. Kaleidoscope (also known as the light pipe) Coating such as graphite paint, Manganese phosphate and black paint are used. The performance of these devices together with these various coatings, is then examined. Also factors such as the following are considered: 1. Power loss in each device 2. Energy uniformity that each produces 3. Penetrations possible 4. Limitations of each 5. Ease of installation and operation 6. Suitability of each for certain applications. Concerning the coatings, such things as: 1. Types 2. Coating thickness are studied. Each device was used in actual heat treatment with the data then examined and presented here with the hope being that specific merits and drawbacks would become evident.

The influence of CO2 laser radiation (power density 103-105 W/cm2) on the surface of quartz and silica--contained minerals such as nepheline, microcline, etc. in oxidizing, neutral and reducing gaseous media has been investigated. It was found from spectroscopic and chemical analysis, that emitting of silicon-oxygen groupments mainly occured with laser radiation absorption in samples studied. This phenomenon can be explained by selective excitation of Si 0 valent vibrations in solids up to complete destruction of the bonds. The obtained results clearly explained the earlier discovered phenomenon of irreversible "burning out" (clear up) of line in IR absorbtion spectra of solids on the frequency of the laser influence by CO2 laser radiation. On the surface of melted quartz in the medium enriched by hydrogen a laser-stimulated reducing reaction that resulted in appearing of Si H bonds containing combination has been obtained.

A simple method for noncontact measuring acoustic emission generated during laser welding of steel X5CrNi18 9 and pure aluminium is presented. Laser beam quality is kept constant to insure that AE signal is just affected by the weld process. It will be shown that measuring in noncontact manner depends on formation of plasma, otherwise AE is generated due to backscattered laser radiation, which heat the fixture of the AE sensor.

The paper describes the propagation of CO2 laser beam through a flexible hollow metallic waveguide. Input power of 1 kW has been considered, making such a guide useful for material processing. The guide has been theoretically studied and experimentally tested in various bending and twisting configurations. A transmission of up to 95 % has been obtained for near kW incident power.

Results of spectral diagnostics of the plasma produced by intense laser radiation during the welding process of steel and aluminium with CO2 lasers are presented. The experiments were carried out in an intensity range I = 106 ÷ 107 W/cm and with He, N2, Co2, Ar as shielding gases. The electron plasma temperature T and density n were measured under various processing and shielding gas conditions.It is shown that the time-averaged plasma temperature correlates with the welding results such as e.g. a welding depth obtained by varation of the welding speed and process gas. In this way it is demonstrated that spectral characteristics of the plasma can be used in the monitoring of the plasma and in the optimalization of the welding process. Furthermore, it is shown that spectroscopic results help to explain the influence of the process gas. Examples of cooling, heating and shielding effects caused by process gases are shown and discussed in view of the efficiency of the welding process.