There is a strong correlation between the development of civilization on this planet and our ability to apply different forms of energy on the problems of life. Optical energy has recently become available in a form and at intensities which can be used for an expanding number of applications. This article examines the opportunities and current developments of this new form of industrial energy. It identifies some of the innovative applications which are being developed and discusses how engineers are adapting to the challenge. Underlining these surging developments are the growing problems associated with the lack of engineers trained in the use of optical energy.

May 1967 marks the 30th anniversary of gas assisted laser cutting. This paper looks back at these first experiments, made using a 300 W pulsed slow flow CO₂ laser, and a pressure chamber to combine the focused laser beam with an oxygen gas jet. The results of this early work and the conclusions to the original publication on the subject will be discussed in terms of how laser cutting has developed over 30 years into an advanced and profitable manufacturing technique.

The present work reports on the analytical model describing the HAZ geometry near the cut edges resulting from laser cutting of steels induced by the heat flow during the moving of the molten layer in the kerf. This model is based on the molten layer thickness, in macroscopic state, which depends on mean melt velocity and mass balance between mass loss and gain. The energy balance which describes the equilibrium set up in the molten layer is presented and analyzed. Results yield average value for the temperature of molten layer surface and mean melt viscosity. This model indicates that there is a strong dependence between the geometry of the HAZ and the molten layer features. Investigation of the influence of experimental parameters is also performed. Calculated results are compared with experimental measurements.

In the past a variety of scientific investigations has proven, that during laser beam cutting optical emissions from the process correlate with properties of the cut kerf and thus can be used to characterize the quality of the machined parts. Laser beam cutting has been well established in many fields of industrial use and has reached a high grade of productivity and flexibility. Due to this, modern on-line techniques for process monitoring in laser beam cutting have to be as fast and flexible as today's laser beam cutting machines. The objective of the work being presented is to provide useful support for the machine personnel and to enable automatic quality monitoring even in case of a small number of units to be cut. The paper introduces some general considerations on the demands of process diagnostics in laser beam cutting and presents `hardware'-tools which have been developed during an industry-university-partnership. The equipment can be adapted to all kind of cutting machines and measures the optical emission from the process within the cutting head. The crucial thing of the work is still the adequate application of analyzing and visualization techniques to extract significant information from the measured signals. Here, this is done by relating the information derived on- line from the process to the geometry of the cut contour. It makes visual quality inspection much easier and more effective: sections of the processed parts with minor cut quality are indicated to the personnel as scrap or at least as to be checked visually. A first approach of integrating such a system into a laser beam cutting machine is presented and discussed.

Laser cutting has been investigated for a number of aluminum-synthetic laminates, newly developed materials for the aeronautic and automotive industry. The materials consist of alternating aluminum and synthetic layers. It is shown that these materials can be cut at rates comparable to those of homogeneous aluminum alloys. The cuts show little dross attachment. Also some damage on the synthetic layers has to be accepted. These results initiated a modeling exercise, which resulted in a numerical simulation code. The applied cutting model is based on describing the material in several horizontal layers, each with its own specific thermophysical and optical properties. The separate layers are coupled by known mass, energy and force balanced equations.

The application of laser cutting in Laminated Object Manufacturing has been investigated and applied since the early eighties. Virtually all of the work done has concentrated on normal cutting using oxygen gas assist. In assembling models from normal-cut sheets additional work is required to remove steps which are time consuming and raise production costs. This work investigates the application of slant cutting in the construction of metal laminated models. This paper presents the results of an experimental investigation into normal and slant-cutting operations using both CW and pulsed CO₂ laser modes for 1 mm mild steel. The effects of different cutting gases and cutting gas pressures have been investigated including the use of oxygen, air and nitrogen. High pressure Laval nozzles were used for air and nitrogen gas assist cutting with stand-off up to 15 mm. For normal-cutting, the experiments revealed that pulsed laser cutting with oxygen gas assist provides optimum cutting with a wide range of speeds and gas pressures. It also results in a stable kerf width with 0.78 micrometers standard deviation which produces high dimensional accuracy. Thus laser cutting would be suitable for complex part profiling and layered manufacturing applications. The use of oxygen in slant cutting produces higher kerf widths and edge burning for samples angled from the horizontal above 20 degree(s). The quality of slant cutting was improved with air and nitrogen assist gas. Slant angles up to 50 degree(s) were cut using CO₂ laser with 12 bar air and 20 bar N₂ at the nozzle.

Laser cut initiation in full material usually results in a start hole with bad quality. Starting strategies have been developed to avoid these crater-like holes. Start areas were obtained showing almost the same quality as the rest of the cut. As a result of the experiments and literature research the following starting strategy has been applied: drilling in a pulsed laser mode followed by a power ramp in c.w. laser mode simultaneous with the acceleration of the workpiece. The chosen strategy has been implemented in the laser controller. For mild and stainless steel of various thicknesses start settings have been determined. The best results were found at relatively low acceleration (1 m/s²). The power ramp during acceleration has been proven to be more advantageous for thin (1 mm) mild and stainless steel than for thickness sheets. A slightly varying kerf width over the first millimeters of the cut has been observed. This can be explained by varying cutting front velocity and temperature caused by the workpiece acceleration.

During the last few years technical innovations in the field of laser beam sources have led to almost a complete equalization of the process impeding properties of copper for laser beam cutting. The present work was performed to outline the potential of laser beam gas cutting of copper for generating quality cuts under practice-orientated conditions. With regard to the importance of cutting speed on both cut quality and economic efficiency of processing the focus of interest was put on the influence of cutting speed on application relevant features. For this purpose a series of speed varying cutting experiments on copper sheets of 0.5 to 15 mm thickness was carried out. The tests were arranged to two commercially available cw-CO₂-laser beam cutting machines and maximum power ratings of 3 kW and 18 kW. Cutting speed exercises an influence on all features examined. The most susceptible features are deviation from right angle and height of dross. Deviation from right angle can be improved for 100% to several 100%. Dross height can be reduced for between 40 and 80%. On the other hand the features kerf width and surface roughness are less susceptible to cutting speed. For the examined range of cutting speeds and sheet thicknesses width of kerf varies between 5 and 15%, surface roughness between 10 and 30%. Sheets in the range of 1 to 10 mm thickness are possible to be cut with an accuracy better than 50 micrometers , sheets thicker than 10 mm with an accuracy of 150 to 200 micrometers . With inclination angles of 0.5 to 2 degree(s) kerfs can said to be parallel sided. For sheets of thicknesses below 1 mm roughness values of about 25 micrometers are achievable. Sheets of thicknesses of between 1 and 5 mm show roughnesses of approximately 50 micrometers . For sheets of thicknesses above 10 mm roughnesses in the region of 100 micrometers can be obtained.

For testing the validity of this proposed method many alloys have been studied. So different (austenitic, ferritic and martensitic) stainless steels have been welded by laser technology. Diverse joint geometries as butt, lap and cruciform ones, have been realized. As laser sources 500 W c.w. VALFIVRE/2 kWc.w. B.O.C./1.5 kW c.w. ROFIN SINAR RS 1500/6 kW c.w. TRUMPF TLF 3000 Turbo and 15 kW c.w. AVCO- EVERETT have been employed for experiments by using two covering gas (He or N₂), many working speeds, a 0.5 to 13 kW laser power level range. By the application of this method it has been possible to confirm the idea to leave out of account the knowledge of the thermo-physical parameters of the alloy to be welded. The Melting Area values plotted welding speed values have shown the usual hyperbolic trends. The Welding EFficiency value always plotted versus welding speed range have shown a trend with a maximum one corresponding to the `critical welding speed'. The same Welding Efficiency values plotted versus the laser power levels have shown range values from 20 to 50 mm³/kJ for 0.5 to 2 kW and from 30 to 55 mm³/kJ for 3 to 14 kW power level ranges. Finally a schematic layout for using this model has been carried out.

Laser welding and cutting processes have a large fall out in industrial manufacturing and high and medium power CO₂ lasers have been successfully introduced in production lines. The present research has been concentrated on the development of a non-intrusive sensor for the monitoring of the quality of the laser machined steel pieces. The sensor consists of three electro-optic modules able to detect infrared, visible, and ultraviolet radiation coming from the interaction zone. During the welding process the three signals have been simultaneously stored. The statistical analysis of these signals have been correlated with the results of the radiographic tests. During the cutting process only the infrared radiation have been detected for the process monitoring.

High reliability and fault tolerance of workcells for laser welding often require the use of adequate seam tracking sensor systems. Moreover flawless path processing demands vibration-free path tracking. This is not always guaranteed since the system sensor-robot is prone to vibrations under unfavorable circumstances. To determine the reasons for the vibrations critical contours are welded while the path of the robot is recorded simultaneously. From these results possibilities to compensate the vibrations are derived. Furthermore the detection of a faulty seam position is desired. This includes missing or wrongly positioned parts as well as a gradual change in the seam positioning, due to wear of tools or clamping devices, for example. By the implementation of a computer based system to compare the actual path to a reference path deviations in the position of the seam as well as the current gap height can be detected, amounted, visualized and recorded. The data may be used to detect a gradual change of the position of the seam, to avoid collisions or to recognize and single out faulty parts. By combining both means a possibility to improve the process security in laser material processing was developed.

The 45 kW CO₂ laser system of Institut de Soudure was used to evaluate and explore the possibilities offered by the high power laser beams for welding different materials in various thickness and in different welding positions. Stainless steels, low carbon steels, aluminum and titanium alloys were studied. Butt joints in 10 to 35 mm thick plates were achieved and evaluated by radiographic, metallurgical and mechanical tests. Gaps and alignment tolerances were determined with and without filler wire in order to obtain acceptable welds concerning the weld geometry, the aspect on front and end root sides. The main problem raised by heavy section welding concerns weld porosity in the weld which increases drastically with the thickness of the weld. Indications are given on their origin and the way to proceed in order to better control them. Lastly some large parts, recently welded on the system, are presented and discussed before drawing some conclusions on the prospects of very high power laser welding.

High average laser output power was realized by combining the beams of three Nd:YAG lasers of the 2 kW class, each being coupled into an individual 600 micrometers fiber. The end faces are imaged onto a 1 mm fiber, which then transmits 6 kW cw power to the processing station. With this laser system, butt and edge joints with up to 5 mm thick sheets of AlMgSil and AlMgMn4,5 were performed. Velocities of about 3 m/min in combination with filling wire and helium shielding gas yielded satisfying results. The sheets of 1.5 mm thickness were best welded with 8 m/min and a O 1 mm filler wire delivery speed of 5 - 7 m/min. One of the drawbacks of the system is the deterioration of beam quality due to the tilt angle between the incoming beams, so that a 1 mm fiber has to be used. With a modular oscillator-amplifier system with fiber link it should be possible to use fibers with smaller core diameter.

One of the major problems by laser beam welding of aluminum alloys is still the appearance of seam imperfections. This problem, mainly due to weld instabilities can be reduced by the processing method. Nevertheless the origin of these welding process instabilities is not fully understood. The presented paper shows that the oscillation of the keyhole and the melt pool are in resonance dependent on the melting and vaporizing process. Due to the intensity of this resonance surface ripple irregularities, notches and blow- holes appear. Therefore it can be claimed, that resonant oscillations of the keyhole and the melt pool are one reason for the appearance of imperfections like blow-holes. The resonances were generated by artificial laser beam power modulations. Using this method imperfections like blow-holes were reproducibly produced.

Solid-state lasers with high beam quality are now available in the power range up to 4 kW. Since the beam of this laser type can be transmitted via flexible optical fiber for processing complex automotive structures, this laser has increasingly become an interesting alternative to the CO₂ lasers now used. The solid-state laser displays a greater basic absorption of its beam wavelengths by technically relevant materials than the CO₂ laser and thus a theoretically better coupling characteristic. However, in view of the relatively poor beam quality of the solid-state laser, the question as to whether greater processing efficiency can result still remains open. Welding experiments using transferable processing parameters were carried out to obtain reliable data for a direct comparison of both laser types. The most powerful Nd:YAG laser (HAAS HL 3006D) now available on the market as well as two CO₂ laser manufactured by TRUMPF with different beam quality and power were used for the experiment. Whereas the CO₂ laser shows advantages at low welding speeds and high welding depths, the solid-state laser can compensate for the distinctly poorer beam quality in the case of higher welding speeds that are typical for sheet-metal working. In this area, the Nd:YAG laser even showed distinct advantages when using large focal diameters. A drastic influence of the beam quality on processing efficiency was demonstrated for the CO₂ laser using identical processing parameters apart from the beam quality.

The new concept of a coaxial high power CO₂ laser offers the possibility of welding at high power with an excellent beam quality. The first applications show great progress in welding quality compared to competitive systems. In further experiments it is provided that the beam of this coaxial high power CO₂ laser allows welding of different materials, especially aluminum, copper and brass with good quality and properties. The results are presented with pictures of the cross section and the properties of the weldseam.

This development is in the framework of the activity carried out to establish welding techniques applicable in radioactive environment to the vacuum vessel of the ITER fusion reactor. An automatic welding equipment will consist of three functional blocks: carrier, end-effector and beam delivery system. A trolley is driven along the walls of the component to be welded in order to position a welding head close to the operating zone. Accurate approach to the final position is demanded to an intelligent end-effector capable of on-line processing and monitoring. To be integrated on- board of the carrier but not directly belonging to the operating head is the power beam delivery system. In the case of CO₂ laser beams a special active beam delivery system based on units capable of automatic alignment and of mutual tracking is required. The work described in this paper demonstrates the feasibility and effectiveness of such a system based on self controlled units capable of deflecting the beam from a fixed station to a moving operating head, thus realizing a sort of transmitting/receiving device applied to a free propagating beam. One of the units, the receiver, set on the carrier, is automatically aligned with a twin static unit, the transmitter, which is fed directly by the laser source. Each unit is a two degrees of freedom rotatory head, capable of covering nearly the whole solid angle. This system is equipped with sensors, suitable to provide high precision alignment of the power beam into the final focusing head with adequate stability. The experimental set-up tested makes use of a power level of 6 kW. The distance transmitter-receiver is in a range 2 - 10 m, and trials are performed on 5 - 10 AISI 316 L stainless steel at a welding speed up to 2 m/min. Results are consistent with expectations. As the welding head is moved, the alignment error detected by the self-tracking units is immediately recovered within small tolerances. Sharp corners are matched with higher accuracy than required by heavy section welding. This system for beam delivery is completely innovative and of relevant interest for application to components of large dimension, approached from inside in a standard configuration not different from that used with conventional techniques.

Compared to traditional arc welding practice, laser welding offers significant potential advantages for structural fabrication, including reduced distortion, higher welding speeds, reduced costs for consumables, more efficient structural design and greater reproducibility and consistency. As robust, high power CO₂ lasers that can make single pass butt joints in steel greater than 10 mm thick are now commercially available, laser welding is now being actively considered for a new range of structural applications. However, the use of laser welding is a radical departure from present practices and before its introduction to production its technical suitability and cost effectiveness must both be clearly demonstrated. The paper considers the issues to be resolved before laser welding can be used with confidence in these areas. Weld quality issues are addressed and joint properties are reviewed, bearing in mind that many conventional tests such as Charpy and all weld metal tensile tests are not suitable for the narrow welds resulting from laser welding.

This paper reports the results obtained by employing materials such as austenitic stainless steel (AISI 304) sheets, with different face thicknesses and core geometry's, usually some trapezoidal ones. A ROFIN SINAR C.W.CO₂ laser has been used as a fast axial flow source and a 1500 W max power level to weld the many bases of the corrugated cores to the external faces has been utilized. Four different constructional solutions, for the preparation of some new modular structural elements, with one or two beads for each welded base, have been experimented. Each modular structure has sizes 300 mm wide, 700 mm length and 60 mm height. After studying many different mechanical clampings, the best one has been realized. So, eight different constructional solutions and different operative sequences of the welds have been made. All this for evaluating the effect of the weld sequence on the distortion of the panel and for obtaining different localization of the residual stresses on the structural element.

An ideal material for automotive applications would combine the following properties: high corrosion resistance, high strength, high stiffness and not at least a low material price. Today a single material is not able to meet all these requirements. Therefore, in the future different materials will be placed where they meet the requirements best. The result of this consideration is a car body with many different alloys and metals, which have to be joined to one another. BIAS is working on the development of laser based joining technologies for different material combinations, especially for thin sheets used in automotive applications. One result of the research is a joining technology for an aluminum-steel-joint. Using a Nd:YAG laser the problem of brittle intermetallic phases between these materials was overcome. Using suitable temperature-time cycles, elected by a FEM-simulation, the thickness of intermetallic phases was kept below 10 micrometers . This technology was also applied to coated steels, which were joined with different aluminum alloys. Further it is demonstrated that titanium alloys, e.g. used for racing cars, can also be joined with aluminum alloys.

A remote maintenance technology to process cooling pipes internally for the ITER (International Thermonuclear Experimental Reactor) has been developed. The fundamental welding properties of austenite stainless steel obtained by using high power YAG laser indicates the tendency of porosity increase as the welding depth increases. It is also found out that the occurrence of porosity was prevented when N₂ gas was used as a shielding gas and that the material strength of weld joint is kept unchanged. Furthermore, a processing equipment that can access the cooling pipes internally has been developed. The main body of this processing equipment can be inserted into a cooling pipe and consists of an focusing optics, clamping and driving mechanism, and an optical fiber. Operational experiments of this processing equipment is carried out and the basic verification experiments such as welding and cutting tests were conducted. As a result, a good quality of pipe welding joint was obtained without any defects.

Nano processing is a cornerstone technology for several future oriented applications, as for example in the fields of high density data storage, nano matching of X-ray fresnel optics, micro electronics, micro robotics and biomolecularic. However, as well known, processing in the nanometer range with laser radiation is not possible by lenses or mirrors due to diffraction limitation. Recent research work has shown, that `focusing' of laser radiation down to a few nanometer can be obtained by using lasers in combination with nearfield technology (e.g. SNOM--Scanning Nearfield Optical Microscopy, SPM--Scanning Probe Microscopy). Lateral externally illumination of SPM probe tips with laser radiation can cause tremendous intensity enhancement in the nearfield underneath the tip. A brief theoretical consideration of this effect and results of calculations based on boundary element method is done. This kind of field energy concentration we named FOLANT-technique (FOcusing of LAserradiation in the Nearfield of a Tip). The interaction area with nanometer scale can be applied for material processing even down to atomic dimensions. Using FOLANT-technique hillocks, pits and grooves with lateral dimensions down to 10 nm have been obtained on conductive substrates as well as on dielectric materials (for example polycarbonate). Our experiments have shown, that FOLANT- technique is a promising tool for various applications in nanometer material processing.

Decreasing film thicknesses and sizes of microstructures require an ultraprecise removal of the material and a reduction of the heat-affected zone. For these applications picosecond laser pulses seem to offer new challenges. Because of the short pulse length higher power densities can be reached and rapid heating can possibly lead to an earlier evaporation of the material and to a reduction of the molten zone which resolidifies after the end of the laser pulse at the edges of the processed area. The removal of different materials was investigated using laser pulses with a pulse length of 40 ps produced by a diode-pumped mode-locked Nd:YAG-laser in combination with a regenerative amplifier. The laser radiation was focused to a 7 micrometers spot diameter, yielding power densities up to 5 X 10¹² W/cm². Pump and probe investigations were used to study the interaction of intense ultrashort laser beams with matter. By this technique ultrashort processes with time resolution determined by the pulse length of pump and probe pulses can be photographed. The measurements allow a detailed characterization of the material removal including melting, vaporization and fast resolidification and the feedback of the surrounding atmosphere to the processed microstructures. The single-shot removal threshold fluence and the removal rate per pulse for 40 ps laser pulses and a wavelength of 1064 nm were determined for Si₃N₄-ceramics. Different materials like metals, semiconductors and ceramics were microstructured by picosecond laser radiation yielding structural dimensions smaller than 20 micrometers .

The applications of conventional infrared lasers running cw or quasi-sw for drilling, cutting and shaping are limited in the precision achievable due to the long interaction time which leads to heat affected zones. The necessity to use a gas jet to blow the molten material out of the cut kerf will damage fragile workpieces like thin foils. Short laser pulses of sufficient intensity remove the material directly by evaporation and minimize the amount of heat transferred into the solid. Classical infrared laser sources generate a shielding air plasma within some ns at power densities above some 10⁷W/cm². The optical breakdown threshold value in air can be shifted to higher intensities by using visible light as well as reducing the focal diameter. An alternative way is to shorten the pulse duration to less than 10 ps that a plasma is generated only after the pulse. Thus, the material removal process begins after the deposition of the pulse energy into the material. But such short pulses will generate a pressure wave due to the sudden thermal expansion and can damage or destroy microscopic components. For industrial production the productivity is a further aspect. Hence, a certain mean power is required in order to obtain the desired production rate. Considering the above aspects, copper vapor lasers (CVLs) with ns pulse duration are well suited for precision machining of metals and ceramics. Processing with CVLs is an advantage in that its wavelength is highly absorbed by metallic targets and the probability for the optical breakdown in air is low. CVLs in an oscillator-amplifier-setup incorporate diffraction limited beam quality and high average power. The present paper outlines the potential of the CVL for the industrial use regarding high processing speed and precision. Under these aspects the limiting mechanisms on the material removal process and the necessary processing strategies for scaling up the productivity are shown. The relevant laser parameters for increasing the working speed and the relationship to the achievable precision are given. The design aspects of a copper vapor laser system with high mean output power and repetition rate are outlined. To conclude, several typical machining tasks, e.g. cutting of green foils, drilling of scimmer holes for thermal analysis are presented.

Laser beam removal with Nd:YAG-lasers is a manufacturing method to produce micro-structures with the precision of a few micrometers. Because many materials can be structured with laser radiation, especially hard metals and ceramics, it is a very flexible technology for the production of tools for micro-embossing and micro-stamping. The combination of the laser and stamping technology offers the possibility to produce high quantities of micro-parts at moderate costs. A system for laser beam removal, existing of a handling with a CAD/CAM coupling and a diode pumped solid state laser was developed and in several investigations the quality of these micro-tools was tested.

Laser beam bending was invented more than one decade ago, but just a few applications have been found and were realized in manufacturing processes. With regard to the minimization of new products and increasing demands concerning tolerances there was a fall back on laser beam bending in recent time. However, several basic investigations for the understanding of the bending mechanism and to find the limits of the process have been carried out before. Still there is a lack of continuous and subsequent investigations which discover general knowledge to allow and simplify the step from experimental state into manufacturing line. In this context first questions are the influence of the geometry, the heat input position and the pre-treatment of the material. Smallest bending angles but largest torsion angles have been found for the laser beam treatment with scans across the whole width at the smallest specimen. When the bending angles increase with the width, the torsion angles decrease. Concerning the heat input position largest bending angles can be achieved with a laser beam manipulation (pulse or scan) in the middle of a metal strip. To obtain greatest bending angles several scans across the specimen should be performed one upon the other. Bending angles of more than 90 degree(s) can be produced by this method. A pre-treatment of the material like cold forming or annealing causes different bending angles. Best results concerning large bending angles can be demonstrated for cold formed and not annealed specimen due to the residual stress in the material supporting the bending process. Also a subsequent annealing process causes an additional bending of the specimen for single laser scans but can be avoided by performing a second scan.

In the industrial production of electrical, optical, and micro-mechanical components, progress in miniaturization requires improved adjusting techniques. Sub-micrometer accuracy adjustment must be obtained within seconds, and the accuracy should be stable over many years. All methods that are presently applied for manipulation in sub-micron dimensions are cumbersome, time-consuming, and tedious, and require expensive equipment. A novel method, laser adjustment, is being explored in which permanent deformation of thin metal sheets are obtained by using thermo-mechanical stresses that occur when the sheets are locally heated using short, intense laser pulses. Manipulation along several degrees of freedom can be realized by both out-of-plane and in-plane laser adjustment or a combination thereof. Within the Brite-Euram project AMULET this new automated micro- manufacturing technology for mass production is developed in order to assemble components where tolerance conditions and accessibility are beyond human capability.

The coaxial construction of a laser has proved to be a very interesting solution to achieve a very high laser power with comparatively small efforts. Optics for coaxial resonators have to be changed as well as the optics for the beam guidance. This new kind of laser is already being used to weld thick steal as well as various difficult materials. The fact that only two optical elements are used inside the resonator and another two optical elements outside makes the whole process very stable.

Many applications of solid state lasers in research, material processing and especially in micromachining and medicine require high brightness. Resonators for solid state lasers with phaseconjugate mirrors based on stimulated Brillouin scattering (SBS) allow significant improvement of the beam quality up to theoretical diffraction limit at high average powers. We could demonstrate operation of a flashlamp-pumped Nd:YAG laser with SBS mirror with an average output power of more than 30 W and TEM_oo-mode. The beam diameter in the laser rod of 8 mm diameter was larger than 5 mm.

Adaptive mirrors are used in laser material processing machines in order to control the propagation of the raw beam through the beam-guiding system as well as the geometry of the focused beam. As modern machine concepts for laser material processing tend to operate at increasing working speed, fast and lightweight adaptive mirrors with excellent optical properties and a large range of radius variation are needed. In this paper, a new approach for a fast, compact and lightweight adaptive mirror is discussed. In the approach the deformation of a circular plate under pressure is used. The pressure is generated by a small piezoelectric actuator which presses against a small water layer behind the mirror plate, thereby deforming it. The deformation of the mirror surface is almost parabolic across a large area of the overall surface, which results in a high optical quality even at large deformations. Based on this concept, an adaptive mirror was designed that can be operated at a bandwidth up to 400 Hz at maximum deformation (which results in a range of radius-variation from -20 m to +20 m) and about 2 kHz at smaller deformations. Furthermore, extensive investigations have been performed using FEM analysis of the adaptive mirrors in order to understand the influence of mirror thickness, diameter and operating pressure on the deformation and the resulting radii of curvature. The result is an analytical method which allows to calculate the mirror dimensions in order to achieve a desired characteristic of deformation for given beam diameters.

The applications of laser scanning systems in areas such as laser welding, cutting and marking continue to increase with the improvement in scanning mechanisms such as galvanometer scanners. These continue to challenge the optical designer to produce high performance lenses which can fulfill the need for a smaller focused spot size over larger flat field scan areas. CO₂ lasers offer the advantages of high power (kW) but their long wavelength of 10.6 microns compared to 1.06 microns of Nd-YAG means that CO₂ has 10 times the focussed spot size for the same aperture. Increasing the aperture of CO₂ scanning systems will therefore help to increase the application potential of these systems. This paper presents a review of the optical design factors which determine the performance of a CO₂ scanning system. These factors have been used to produce several lens designs which are capable of scanning a focussed spot size of 100 microns or less over areas up to 100 mm square.

The potential of non-rotationally symmetric optical elements (NOEs) is demonstrated with respect to mask projection techniques with UV excimer laser radiation. NOEs are diamond turned mirrors with an almost arbitrary surface shape. The operation principle of the NOEs presented here is the concept of beam integration including beamlet shaping. Efficient and innovative processing is enabled by generating complex beam profiles adapted to the mask apertures demonstrated for the generation an annular shaped beam profile.

Laser material processing by the mean of Diffractive Optical Elements (DOEs) is a promising field, that has not yet entered the industrial world, except for hybrid on-axis spherical or aspherical lenses fabricated by diamond turning. Laser cutting, engraving, welding, and heat treatment are several applications among the potential of numeric-type beam shaping DOEs, designed and optimized by iterative algorithms and fabricated by microlithographic techniques. However, these DOEs have to be etched within materials which should withstand very high energy distributions (either from YAG, CO₂ or Excimer lasers). Typical physical DOE configurations in transmission mode are etched ZnSe, ZnS, Ge substrates or even Quartz substrates, and in reflection mode, Au or Al coated etched substrates, blank etched Quartz substrates (YAG), or even SiC etched substrates.

Customization of products is an important factor in selling products. This will lead to an increasing number of types in the production. To control this, a flexible way of decoration should be integrated in the production. Without a flexible and high yield mass customization method, the integration of customization in a mass production cannot be achieved. Laser decoration is one of the possible techniques that is able to meet the demands for flexibility and high yield. There are already some methods available, but the combination of quality with price is, at the moment, an obstacle. Laser decoration is the only technique which is already integrated into a mass production environment and is able to join decorative marking with high yield and low costs. Color flexibility is, however, still a major challenge for laser decoration. In order to expand towards a more mature technique and to explain the necessary developments, we would like to focus on the process. The process is a combination of laser technology, material technology and software (decoration, engraving, marking, tunable lasers, multi color, process control).

Hydrodynamical bearings for X-ray tubes made of molybdenum offer several advantages over ball bearings, one being the reduced noise, another the extended life time. A process has been developed to texture a molybdenum surface with a laser beam without generating blur. In order to avoid melt which may cause burr the material is evaporate via its oxides. A two step oxidation followed by the sublimation has been identified as governing process. The flat bottom of the groove is explained by the limited diffusion velocity of the oxygen and the reaction products. Incorporating a process control an accuracy of +/- 3 micrometers at groove depth of 20 micrometers was realized.

A theoretical model for laser cleaning of microparticles from solid surface was established by taking adhesion force and cleaning force into account. The threshold fluence can be obtained from this model and verified by the experimental results. It was found that laser irradiation from the reverse side of transparent substrate is more effective to remove particles than that from the front side. Laser irradiation with shorter wavelength can result in higher cleaning efficiency and lower threshold fluence for removal of particles from solid surfaces. Keywords: Laser cleaning, microparticles, Van der Waals force, cleaning force, cleaning threshold

During electronic device fabrication it is necessary to remove the oxides from copper surfaces prior to soldering in order to improve the surface wetability and achieve a good quality solder joint. The usual method of achieving this is by using acids in a flux. The work reported here explores the possibility of removing these oxides by laser cleaning using the harmonics of a Q-switched Nd:YAG laser, a technique which could be incorporated into a industrial laser soldering process. The effect of Q-switched Nd:YAG radiation (5 - 10 ns pulses), at 1064 nm, 532 nm and 266 nm, on the oxidized surface of a copper alloy foil is studied with increasing fluence. In order to successfully compare the effect of increasing fluence at the three wavelengths each area treated was only subjected to one laser pulse. The laser treated surfaces were characterized using optical microscopy, SEM, and surface analysis performed by static secondary ion mass spectrometry (SSIMS). SSIMS and SNMS (secondary neutral mass spectrometry) with mechanical depth profilometry were used to characterize the oxide layer. The reflectivity of the oxidized plates for the three wavelengths was ascertained using a reflectivity spectrometer. Successful cleaning was achieved at all wavelengths, above certain threshold values which defined the lower end of the process operating window for single pulse operation. The threshold for the cleaning process decreased with laser wavelength. Surface melting was evident at the lowest fluences examined for all the wavelengths (< .5 J/cm²). This value is well below the lower end of the process windows of all wavelengths. Microscopic `explosive' features were found at the onset of copper oxide removal possibly resulting from ionization or a plasma induced shock waves. There was some possible evidence of mechanical effects at 1064 nm and 532 nm. Large amounts of sputtered debris was found around the 266 nm craters. A SSIMS analysis was performed on the 532 nm spots. The SSIMS plots of the ratio of the CuO+/Cu+ counts versus laser fluence decrease sharply at the cleaning threshold to less than half the value for an untreated plate. SSIMS analysis below this point indicates a laser material interaction involving further oxidation of the copper oxides (predominantly Cu₂O) to black copper oxide (CuO), rather than material removal. This is also evident from the change in color of the copper surface. Continued increasing of the fluence eventually led to craters with irregular surfaces and large borders which would be unacceptable from the point of view of surface damage at 1064 nm and 532 nm. This defined the upper end of the process window. At 266 nm, the laser spots produced at the highest fluences used did not differ as significantly from those within the process window below this value.

Many materials as substrates and surface products have been tested. Typically ferrous (Carbon Steels and Stainless Steels) and non ferrous (Al and Cu metals and its alloys) ones have been employed. Some epoxy, polyurethane, polyester and acrylic paints in different thickness and color have been tested. Many types of the surface rust and oxide on different bulk material have been undertaken to test. Similarly some different types of oils and greases, usually used in industry against the oxidation, have been studied. Anyway many types of dirt, grit, calcareous one and so on, present on industrial components, have been laser cleaned without using solvents, acid baths and other ones. Different types of laser sources have been employed: an axial fast flow, 1.5 KW CO₂ c.w. and pulsed laser source, emitting a 10.6 micrometers beam; a portable CO₂ laser, c.w. (1 to 25 W) and pulsed (1 to 100 Hz and 400 ms max pulse duration) source, emitting a 10.6 micrometers beam with a multi-articulated seven mirrors guiding device and focussing head; a portable Nd-YAG laser, Q-switched and normal-mode source. 1st harmonic 1.06 micrometers (6 ns pulse duration), 2nd harmonic 532 nm (120 microsecond(s) duration pulse- 1J max per-pulse) wavelengths, multi-articulated seven mirrors beam guiding device, 20 Hz repetition rate. This lets shots with 600 mJ max energy per pulse and 100 MW peak power per-pulse with a very low beam divergence, 0.5 mrad at full angle; a transverse fast flow 2.5 kW CO₂ laser.

Laser-Induced Breakdown Spectroscopy (LIBS) is a remote, online, in situ technique used for the quantitative analysis of elemental constituents in matrices such as steels, non ferrous metals, polymers and soils. A typical industrial application already established is the sorting of non ferrous metals for the purpose of recycling. A new device, the Laser Identification and Marking System introduced here, uses a combination of material identification by means of LIBS and instantly marking the workpiece using the same Nd:YAG laser. This method was developed since the application required a strongly decreased probability of mixing up of different steel qualities in comparison to conventional methods. At the same time a decisive disadvantage of LIBS, the insufficient detection limits for several elements, can be lowered by using repetitive bursts of multiple laser pulses.

In general mask based laser material processing is a process which suffers from a low energy efficiency, because the majority of the laser light is absorbed in or reflected by the mask. We have developed a device called an energy enhancer which is capable of improving the energy efficiency by a factor of 2 - 4 for a typical TEA-CO₂ system for mask based laser marking. A simple ray-tracing model has been built in order to design and optimize the energy enhancer. Thus we present experimental results as well as simulations and show fine accordance between the two. Important system parameters like component reflectivity and alignment sensitivity are investigated in order to evaluate the possibility of making commercial use of the device. The obtainable image quality and how this is influenced by the focusing and imaging system is discussed in some detail.

Surface treatment with restricted thermal effects can be achieved using pulsed excimer lasers. Therefore these tools are appropriate to thermally sensitive materials such as polymer. The present work deals with the possibility of using the excimer laser to achieve a prebonding treatment on the PEEK and to study the influence of the wavelength on the efficiency of the treatment in terms of bonded joint strength. Regarding the mechanical test results, it appears that the adhesive properties of the polymer are mainly dependent on the laser wavelength. Without any pretreatment, the bonded joint strength does not exceed 5 J/m² and the failure occurs at the interface between the PEEK and the adhesive. After a 193 nm irradiation, below or above the ablation threshold, a cohesive failure in the adhesive at 700 J/m² is obtained whereas at 248 nm the laser treatment leads to an adhesive failure at the PEEK-adhesive interface at 330 J/m². In order to understand why the wavelength have such a strong influence on the adhesive properties, the modifications induced on the surface after treatment are investigated.

The growth and morphology of nitrided layers formed during the solid phase nitriding of pure titanium by CO₂ laser were investigated. In the case of a laser treatment carried out under isothermal conditions, it was shown that CO₂ photons irradiation of the substrate does not produce any specific assisted nitride growth: nitriding kinetics, nitride composition and structure were similar to those obtained after nitriding using a classical heating system. From the nitriding kinetics, nitrogen diffusion coefficients were determined using an analytical solution of Fick's equation. This allows to plot the evolution of the nitrogen concentration with respect to depth and to compare calculated profiles to those determined experimentally by Nuclear Reaction Microanalysis.

Chemical transformations on stainless steel surfaces (304 AISI) are obtained by melting with an excimer laser. The very high quenching rate allows to create a thin continuous metastable gradient in composition and structure by opposition to other melting techniques which promote multiphase layer. The mechanical properties of the bulk are maintained and the external layer is chemically modified for a better resistance to environmental aggressivity. Molybdenum is used for corrosion protection and ruthenium for cathodic and anodic properties. The composition is analyzed on cross-sections obtained by ultramicrotomy. A continuous gradient approximately 500 nm (nanometer) of Mo or Ru with decreasing grain size (< 0,3 micrometer compared to 10 - 20 micrometers for 304 AISI) is observed. Mo and Ru surface alloys show an important beneficial effect from cathodic and transpassive potential range.

This paper presents an exploratory investigation on verifying the feasibility of using a laser surface alloying technique to produce designs in the surface of coinage blanks. The specific aim of the work concerns the production of design features in coins that are difficult to produce by other techniques and which hence act as a barrier to forgery and features which permit automatic recognition in vending machines, particularly as a means of establishing the authenticity of the coins. Coins in many countries today are commonly manufactured from metal composites, where one substrate metal or alloy is coated with another by a process of electrodeposition or by mechanical bonding. The technique here described entails the use of a high power CO₂ laser to bring about localized melting of the two layers. Visible distinction between alloyed and unalloyed regions or difference in other physical properties such as conductivity or magnetic properties can be obtained. The work also involved a fundamental study of the influence of the thermal properties of the materials on the CO2 laser alloying process. It was found that the thermal properties such as thermal conductivity of the substrate materials and the difference of the melting points between the coating layer and the substrate materials played an important role in the process. Laser control variables required for localized alloying for different substrate and coatings types were determined. The influence of both thermal properties and laser control variables on alloy type and alloy depth were investigated. Initial work on coin validation showed promising results of an automatic recognition of laser treated coins.

Usually the P.M. alloys are heat treated like case hardening, gas nitriding or plasma nitriding for a better wear resistance of the product surface. There is an additional method for gaining better tribological properties and this is the surface hardening (or remelting or alloying) of the P.M. alloy by laser treatment on a localized part of the product without heating the whole sample. This work gives a cured experimentation about the proper sintering powder alloys for laser surface processing from the point of view of wear, fatigue life and surface quality. As concerns the materials three different basic alloy groups with graduated carbon contents were prepared. Regarding these sintered powder alloys one group holds Fe, Mo and C and other group holds Fe, Ni, Mo and C and the last one holds Fe, Ni, Cu, Mo and C contents. Obviously each group has a different surface hardness, different porosity distribution, different density and diverse metallurgical structures (pearlite or ferrite-pearlite, etc.). ON the sample surfaces a colloidal graphite coating, in different thicknesses, has been sprayed to increase laser energy surface absorption. On some other samples a Mo coating, in different thicknesses, has been produced (on the bulk alloy) by diverse deposition techniques (D.C. Sputtering, P.V.D. and Flame Spraying). Only a few samples have a Mo coating and also an absorber coating, that is a bulk material- Mo and a colloidal graphite coating. All these sintered alloys have been tested by laser technology; so that, many laser working parameters (covering gas, work-speed, focussed and defocussed spot, rastered and integrated beam spots, square and rectangular beam shapes and so on) have been experimented for two different processes at constant laser power and at constant surface temperature (by using a temperature surface sensor and a closed controlled link). For all experiments a transverse fast axial flow CO₂ 2.5 kW c.w. laser source has been employed.

Diamond-like carbon (DLC) films were deposited using the excimer laser assisted physical vapor deposition at room temperature. The films deposited at high vacuum (10^-5 mbar) revealed more diamond-like character than under other atmospheres of argon and hydrogen. DLC- films can be deposited with a thickness more than 1 micrometers with the help of either an additional Ti-buffer layer or an in-situ laser treatment during the deposition. The adhesion of the films was qualitatively determined by using the indentation and bending tests. Additionally, the adhesion was found to be dependent on the power densities for the target ablation (I_T) and for the in-situ laser treatment (I_S), as well as, on the applied buffer layer. The roughness was found to be proportional to the film thickness at various surface morphologies of the substrate. The friction coefficient of DLC-films against steel (100Cr6) was found to be approximately 0.1 and the wear loss of the films was dependent on the properties of substrate material.

In order to improve the understanding of the mechanisms of laser surface treatment and to optimize industrial applications, a general model of laser surface treatment has been accomplished which enables calculation of the thermodynamic phenomena and analysis of the process. With particular emphasis on laser cladding and alloying, powder heating has been investigated in detail, which can be divided into heating during travelling through the laser beam and subsequent heating by the melt pool. While the time scale of the former heating mechanism is of the order of 1 ms, pool heating takes place within 10 microsecond(s) , hence almost immediately. While the pool temperature determines whether powder will be melted or not, in principle any powder, even WC, can be melted when passing the beam, as long as the calculated threshold intensity range is exceeded. Laser cladded tracks prove to be determined by the melt pool shape which is calculated analytically for non-gaussian beams, as well. While for low powder feeding rates cladding is limited by the mass balance, for substantial powder delivery the energy balance turns out to be the limiting criterion, while dilution diminishes. Increased degree of overlap between two tracks decreases the roughness of the cladded surface.

Laser surface treatment, more specifically laser - (re)melting, -alloying and -dispersing, are techniques for improving wear, fatigue and erosion resistance of mechanical parts, using high power lasers. Analytical models which decrease these processes in a simplified way can be helpful for (a) reducing the time required to find optimum process parameters, (b) understanding major relations between process parameters and process results, and (c) the design of a real time process control system. In this paper a (quasi-stationary) analytical process model is presented, which relates the depth of the melt pool (in case of laser - remelting, -alloying and -dispersing), to the laser power and the relative velocity of the laser beam to the sample (feed rate). The model accounts for the latent heat of fusion and the energy produced in (or taken from) the melt pool, by exothermic (endothermic) reactions within the melt pool. The model shows a linear dependence of the melt pool depth on laser power and an inverse dependence on the square root of the relative beam velocity. The model was validated by experiments on Ti6Al4V alloyed with nitrogen, which is an exothermic reaction yielding TiN, and experiments on AISI304 alloyed with pre-placed chromium. Good correspondence between the experimental results and the model predictions indicates that the simplifying assumptions made in developing the model are justified within the range of application of the model.

The blown powder laser cladding process has recently been greatly enhanced by the development of a coaxial powder feed system. This system is described in this paper together with some of the new applications arising from it. The powder feeder depends on various gas flow streams. The effect of these streams on the focus of the powder stream is discussed from experiments using image analysis. The thermal behavior of the powder particles as they fall in a laser heated stream is also analyzed theoretically and experimentally. The effect of the powder focus and preheat on the catchment efficient of the resulting clads is mathematically modelled and compared to the experimental results for a stainless steel clad on a mild steel substrate.

In the present work, the results of laser cladding by galvano scanning and modulated beam using the CO2 and Nd-YAG lasers are presented. The results are compared with the stationary beam. It is realized that the scanning frequency as well as the modulation effects were found to have significant effects on solidification and microstructural development process. The results indicate that the dendrite structure shall be fine at higher frequency or low duty modulation condition, compared to the coarse structure at lower frequency or high duty modulation condition. A shallow bead width as wide as 10 mm was obtained in a single pass. Macro- and micro- crossection, EPMA and microhardness profile have been employed to characterize the cladding. The results are correlated with the cladding quality. Microstructure, hardness, dilution etc. are discussed as a function of the processing parameter. Because of the high solidification rate a fine dendrite structure is developed in the cladding zone. A few of the actual results and EPMA analysis of the laser cladding of stellite 6 on 12 Cr-Ni turbine blade as shown in the figure are presented.

Thermal fatigue consists in repeatedly cycling the temperature of a specimen under test without any other constraint and stopping the test when predefined damage aspects. The result is a lifetime in terms of number of cycles. The parameters of the thermal cycle are the following: minimum and maximum temperature, time of heating, of cooling and time at high or at low temperature. When the temperature jump is very big and fast, phenomena of thermal shock can be induced. Among the numerous techniques used to perform these tests, the laser thermal fatigue cycling is very effective when fast heating of small and localized zones is required. That's the case of test performed to compare new and repaired blades of turbogas machines or components of combustion chambers of energy power plants. In order to perform these tests a thermal fatigue system, based on 1 kW Nd-YAG laser as source of heating, has been developed. The diameter of the heated zone of the specimen irradiated by the laser is in the range 0.5 - 20 mm. The temperatures can be chosen between 200 degree(s)C and 1500 degree(s)C and the piece can be maintained at high and/or low temperature from 0 s to 300 s. Temperature are measured by two sensors: a pyrometer for the high range (550 - 1500 degree(s)C) and a contactless thermocouple for the low range (200 - 550 degree(s)C). Two different gases can be blown on the specimen in the irradiated spot or in sample backside to speed up cooling phase. A PC-based control unit with a specially developed software performs PID control of the temperature cycle by fast laser power modulation. A high resolution vision system of suitable magnification is connected to the control unit to detect surface damages on the specimen, allowing real time monitoring of the tested zone as well as recording and reviewing the images of the sample during the test. Preliminary thermal fatigue tests on flat specimens of INCONEL 738 and HAYNES 230 are presented. IN738 samples, laser cladded by powder of the same material to simulate the refurbishing of a damaged turbine blade after long-term operation, are compared to the parents. Lifetimes are decreasing when high temperature of the cycle is increased and shorter lifetimes of repaired pieces have been found. Laser and TIG welding on HY230 specimens are compared to the parent. Parent and repaired samples have no evidence of cracks after 1500 thermal cycles between 650 and 1000 degree(s)C.

Laser Shock Processing (LSP) consists on focusing a high energy pulsed laser beam on metals to create shock waves and thereby, generate compressive stresses. These stress are similar to those of conventional mechanical treatments like short peening. Nevertheless, at LSP the affected depths are greater and the surfaces keep their roughness and hardness. The present study compare the effects of LSP on the surface mechanical properties of two stainless steels: an austenitic (AISI 316L) and a martensitic (Z12 CNDV 12.02). The surface effects are characterized in terms of microstructure, hardening and residual stress levels (measured by X-ray diffraction technique). The effects of LSP on the pitting corrosion resistance of the martensitic stainless steel in a NaCl 0.01 M + Na₂SO₄ 0.01 M solution are presented. Electrochemical tests were carried out by using open circuit and polarization techniques, to determine electrochemical parameters (free and pitting potentials, passive current densities). Laser-induced work-hardening effects were shown to be more important in the case of 316 L for which they strongly depend on the impacts repetition and the laser power density. Significant modifications on localized corrosion properties were noticed for each treatment condition (8 GW/cm² - 20 ns pulses and 40 GW/cm² - 2,3 ns pulses) i.e. pitting potentials were not modified but free potentials were shifted to anodic values and passive current densities reduced.

The main objective of this work was first to set-up the basic principles of LSP, then to characterize and control the laser-induced surface stress loadings and lastly, through different `material' applications, to evaluate its potential as competitive surface treatment. In a first part, all the general aspects about LSP are presented, from the physical shock wave generation mode in water confined regime to the mechanical modifications conventionally induced in metals like plastic strain and compressive stresses. In a second part we focused on an experimental characterization of the process to highlight the influence of several process aspects on the stress wave generation: with the use of a velocimeter system (VISAR) for measuring back free velocities behind thin targets, different parameters were investigated like coating effects, laser spot size effects or plastic flow limit determination at very high strain rate (10⁶ s^-1) for most of the materials investigated. THe next part of the paper concerns the mechanical changes induced within targets by LSP in water confinement regime. Experimental results are shown for residual stress fields, surface roughness or work-hardening level increases focusing on 55C1 steel. Based upon these results, general trends are drawn concerning the mechanical changes of surfaces with LSP as a function of shock parameters. Lastly, recent fatigue results obtained on 55C1 investigated under different shock conditions displayed some 30% increases on the bending fatigue limits at R equals 0.1. As a conclusion, the recent developments of LSP are discussed, mainly dealing with the use of new high cadency excimer lasers for shock processing which seem to provide the most convenient configuration for industrial applications.

Laser Shock Processing in water confinement regime was investigated for an incident 25 - 30 ns/40 J/(lambda) equals 1.064 micrometers laser beam. The experimental measurement of the transmission of the parasitic plasma breakdown at the surface of water confining has been performed with continuous Argon laser probe. Above 6 GW/cm², the critical electronic density for the incident laser wavelength is achieved in the plasma by cascade ionization. Above 10 GW/cm², it has been shown that the power density transmitted through the plasma is limited at 10 GW/cm². The laser pulse transmitted through the breakdown plasma corresponds to the part of incident laser pulse preceding the transmission cut-off. Results are in good agreement with those obtained previously.

Recent improvements in the performance of high-power diode lasers and beam shaping techniques are driving developments of diode laser systems for direct industrial material processing. The paper summarizes principle concepts, physical limits and activities at the ILT in the field of diode lasers systems and their direct applications.

Basis of the developments discussed in the presentation are 10 mm GaAs diode laser bars mounted on copper micro channel heat sinks. Optimizing the micro channel heat sinks leads to decreased thermal resistance and decreased pressure drop. In the presentation the steps to ten times reduced pressure drop and optical power output of the diode lasers of over 100 Watts will be described.

Laser material processing requires intensities of up to 10⁹ W/cm² at power levels ranging from several watts to tens of kilowatts. Single laser diodes emit a highly divergent beam at much lower power. To make them available for material processing, radiation of many diode lasers has to be gathered and combined into one beam with reasonable beam quality. A number of different design approaches have been investigated worldwide by various research groups. In this paper, two commercially available high power diode laser systems are presented and assessed. By means of a comparison of beam characteristics, suitable applications in material processing are identified for both systems. Suggestions of possible industrial applications carried out at the Zentrum Fertigungstechnik Stuttgart are presented and discussed.

CO2 and Nd:YAG high power lasers have become established as machining tools in industrial manufacturing over the last few years. The most important advantages compared to conventional processing techniques lie in the absence of forces introduced by the laser into the workpiece and in the simple arid highly accurate control in terms ofpositioning and timing making the laser a universally applicable, wear-free and extremely flexible tool /1,2/. The laser can be utilised costeffectively in numerous manufacturing processes but there are also further applications for the laser which produce excellent results from a technical point of view, but are not justified in terms of cost. The extensive use of lasers, particularly in small companies and workshops, is hindered by two main reasons: the complexity and size ofthe laser source and plant and the high investment costs /3/. A new generation of lasers, the high power diode lasers (HDL), combines high performance with a compact design, making the laser a cheap and easy to use tool with many applications /3,4,5,6/. In the diode laser, the laser beam is generated by a microelectronic diode which transforms electrical energy directly into laser energy. Diode lasers with low power outputs have, for some time, been making their mark in our everyday lives: they are used in CD players, laser printers and scanners at cash tills. Modern telecommunications would be impossible without these lasers which enable information to be transmitted in the form oflight impulses through optical fibres. They can also be found in compact precision measurement instrumentation - range fmders, interferometers and pollutant analysis devices /3,6/. In the field of material processing, the first applications ofthe laser, such as for soldering, inscribing, surface hardening and plastic or heat conduction welding, will exceed the limits ofthe relatively low performance output currently available. The diode laser has a shorter wavelength than the CO2 and Nd:YAG lasers making it more favourable in terms ofthe absorption behaviour ofthe laser beam - an advantage that will soon have a significant effect on the range of its applications.

A Diomed 60W-cw high power diode laser (HPDL) has been used for the marking and engraving of various building materials, including; marble, granite, clay tiles, ceramic tiles, roof tiles, ordinary Portland cement (OPC) and clay bricks. Morphological and microstructural characteristics have been investigated. The basic mechanism of marking/engraving and the characteristics of the beam absorption are discussed. The effects of material texture, color and laser processing parameters are reported. The work shows that engraving depths of over 2 mm (0.75 mm for a single pass) can be achieved on marble substrates by thermal disintegration of CaCO₃ into loose CaO powder and CO₂ gas. Uniform amorphous glazed lines (1 - 3 mm line width) of a color different from the untreated materials can be generated on clay tiles, ceramic tiles, roof tiles, clay bricks and OPC by solidification phase formation after laser melting of these materials. Effects of atmospheric conditions, for instance using O₂ and Ar gas shrouds, have been examined, with different colored marks being observed when different shroud gases are used. To demonstrate the practical worth of the process a UMIST crest has been marked on a ceramic tile using the system. Laser beam reflectivity is found to depend not only on material composition but also its color. Reflectivity has been found to range between 12% to 18% for the various construction materials used in the experiment, except for marble (grey) which showed over 27% reflectivity. Since the HPDL is a portable device, on-site application of these processing techniques can be realized, which would be either impossible or difficult when using other types of lasers.

For industrial applications of high power laser diodes it is of interest to know the focal region of the laser beam for efficient fiber coupling and material processing. Similar questions also occur with laser diodes for the optical communication industry. For efficient fiber coupling the beam shall be round and its waist situated at the fiber entrance. With an optical near field microscope we investigated the near field at the facettes of the laser diode and documented the change when leaving the laser diode in micrometer steps. It is a non paraxial laser beam. The beam radii can be evaluated from the intensity profiles and so the caustic curve of the laser beam in x- and y- direction, which in the ideal case yields a hyperbolic propagation law.

An efficient high power cw Nd:YAG slab laser, partially end pumped by diode laser stacks is described in this paper. With conventional stable resonator, the optical efficiency amounted to 44% and the slope efficiency 55%. The output beam with this resonator was rectangular and the beam quality asymmetric in two orthogonal planes. The symmetry of the beam quality was obtained by a step mirror beam shaping technique. By using a hybrid resonator, near diffraction limited beam quality in the unstable direction was obtained with an efficiency of 46%.

We have demonstrated efficient TEM_oo operation of the Nd:YVO₄ rod laser, one-end-pumped by a fiber-coupled diode laser. 11.6-W linearly polarized laser output with an optical efficiency of 54% in a TEM_oo mode has been achieved.

Room temperature operation of a Nd:YVO₄ thin disk laser with excellent beam quality is reported. Pumping by a fiber coupled diode laser with 10 W at the fiber end yields a continuous wave output power of 4.4 W at 1064 nm. The corresponding total optical efficiency is 44% and the average slope efficiency is 46%. For higher pump powers, a maximum output power of 13.7 W was achieved in a near diffraction limited beam.

A technique for coupling the radiation of a high-power diode laser bar into one multimode fiber with high efficiency, easy alignment requirements and low manufacturing costs is demonstrated using a single fiber with 400 micrometers core diameter. The principal item of the fiber-coupling system is a pair of micro step-mirrors--a novel design for beam shaping. The overall efficiency from diode-laser to fiber is 71% with 20 W cw laser power through the fiber. Polarization and wavelength multiplexing renders the system scaleable to higher output power which makes it highly suitable for material processing and pumping of lasers.

Nonlinear optical devices, such as harmonic generators, provide a means of extending the frequency range of available laser sources. In this paper we report on a novel concept for scaling the output power of frequency doubled Nd:YAG lasers. Conceptually, the laser consists of multi oscillators, which are placed in a common resonator. The frequency doubler is an etalon shaped nonlinear crystal with dichroitic coatings. The second harmonic generated in different oscillators is coaxially superpositioned.

On the basis of the dynamical model for the intracavity frequency-doubled laser we demonstrate a criterion for the stable operation of laser-resonators with intracavity nonlinear elements. This allows the determination of the laser stability from the resonator parameters and material constants.

To achieve a high return on investment, laser systems must be used to their fullest capacity, avoiding power losses and downtimes. High-quality laser gases are therefore needed to run the laser. But if the quality of the gas cannot be guaranteed all the way from the cylinder to the laser cavity, the risk of impurities such as water vapor and hydrocarbons or particles entering the laser system is large. Unstable laser operation and damage to the resonator optics can result in costly repairs. The profitability of laser operations is also affected by the selection of the assist gas. High-purity oxygen and high-pressure high-purity nitrogen are frequently used to optimize the productivity of laser cutting. In contrast, different assist gases are used for laser welding depending on the wavelength of the laser radiation, the material, the energy per unit length of weld or the assist gas nozzle arrangement. Helium is often the most convenient choice for CO₂ laser welding of mild steel, resulting in optimum seam quality with respect to formability and appearance. Helium-argon mixtures can be used effectively for lower power CO₂ laser welding and for aluminum. Nitrogen mixtures may be used to stabilize the austenitic phase in duplex steels whereas hydrogen additions give a shiny bead surface in stainless steel. Argon is suitable for Nd:YAG laser welding and productivity is increased by small additions of oxygen. In addition argon- CO₂ mixtures may be used to achieve acceptable results depending on the assist gas nozzle arrangement. Consequently, high-purity gases and suitable gas distribution equipment are the basis for a satisfactory return on investment.

Laser opto-acoustic and thermal-wave techniques have been applied for investigation of metals structure modification after laser and electric-arc welding. The changes in granular structure of steels due to welding have been considered. Anisotropy of thermal-conductivity due to thermal-induced stress in the heat-affected zone of laser weld seam has been obtained.

This paper reports on some fundamental investigations of the interaction between CO₂ laser radiation and glasses. Aim of this work is the reduction of the induced thermal strain and stress to minimize the formation of microcracks during and after the process of material ablation. The following experimental results are reported: (1) The optical constants n (refractive index) and k (absorption index) of some technical and optical glasses were determined with high accuracy for the CO₂ laser wavelength of (lambda) equals 10.6 micrometers by angular-dependent measurements of the surface reflection of these glasses. (2) To optimize the deposition of energy by well-adapted pulse parameters, the ablation threshold of borosilicate glass was determined by irradiating the material with single pulses, whose intensities were enhanced in well defined small steps. (3) The threshold conditions for the formation of microcracks during and after the process of material ablation were investigated. In a first step single pulses were used only and in a second step pulse series for the generation of 3D structures were tested.

The dynamics of motion of the channel side walls has been studied in the process of narrow holes formation in glass plates (channel depth to diameter ratio exceeds 4) under the action of pulse-periodical CO₂ laser radiation at 4 (DOT) 10⁴ divided by 1 (DOT) 10⁶ W/cm² intensity. The conditions of relief formation on the channel walls and dependence of relief characteristics on radiation intensity and modulation frequency have been investigated.

Laser manipulation is a technique to confine and manipulate microscopic objects remotely using radiation pressure of a laser beam. Recently, a single-beam gradient-force optical trap, which relies solely on the radiation pressure of a tightly focused laser beam, was demonstrated in air. In this paper we report the formation of spatial patterns consisting of glass particles of d equals 5.0 micrometers in air using the technique. Once a particle was trapped, we transferred it by moving the focus of the objective lens and the microscopic stage and put it to other particles or the surface of the glass plate. We formed spatial patterns of particles by repeating this procedure. Because the attractive force between neighboring particles in the spatial patterns are much greater than the gravitational force acting on particles, observed 3D spatial patterns were very stable. The transfer of trapped particles was generally easier in air than in water because of low viscosity of air. Moreover, in this type of trap, we can improve the stability of the trap by increasing the laser power. It is expected that the present technique will be applied in various fields including microfabrication and micromachine.

This paper gives an account of the first application of a high-power diode laser system for laser bending of sheet metals. The applied diode laser has an power output of 100 Watt at a wavelength of 810 nm. Sheet metals of AlMg3, St 14 and stainless steel have been investigated. The results show that it is possible to bend sheets with a thickness of up to 2 mm. The paper shows the correlation between the obtained bend angle and the investigated parameters like path feed rate, number of irradiations, sheet material and sheet thickness. An estimate of the range of application of the diode laser for laser bending has been derived from the determined results.

Thermal lensing effects in diode pumped solid state lasers can cause severe deteriorations of beam quality and efficiency. In this paper, we suggest the exploitation of thermal lensing effects in other elements than the active laser medium itself, in order to compensate the thermal lens in the active medium. When using a tolerable fraction of the pump power, these devices should provide a self-adaptive means to correct for the thermal lens in diode end-pumped solid state lasers for all pump powers. A mirror device based on Nd-doped LG-760 has been studied in detail.

We report the successful implementation of Gires-Tournois and chirped mirrors in a diode-pumped, Kerr-lens mode-locked Cr:LiSGaF laser. The laser delivered 30 mW of 79 fs, nearly transform limited pulses at 855 nm and 90 MHz repetition rate. The mirror-dispersion controlled cavity is compared to our prism setup and pulse width limitations in diode pumped Cr:LiSGaF/Cr:LiSAF lasers are identified. Mode matching calculations of pump beam and cavity mode are presented to optimize low threshold, highly efficient fs-operation. Following this analysis a compact prismless design of roughly shoe box size is suggested, which incorporates an additional high n₂ element to enhance stability.

We developed a compact fiber-coupled high-power diode-laser unit with optical output power up to 40 W cw, coupled into a multimode fiber with 600 micrometers core diameter and NA 0.22. This diode-laser unit is suitable for pumping solid-sate or fiber lasers as well as for material processing. Essential part is a novel beam-shaping system with compact size, high flexibility and low alignment requirements, which uses a pair of micro step-mirrors. The whole unit fits into a housing of approximately 110 X 100 X 90 mm.

Monolithic linear arrays of diode lasers, also known as diode laser bars, are the basic elements for most high-power laser applications such as solid-state laser pumping or material processing. Cylindrical microlenses used as fast- axis collimators for 10-mm diode bars require very high angles of aperture (up to 100 degree FW1/e2, equivalent to a numerical aperture of approx. 0.8) to capture most of the emitted laser power. For the efficient longitudinal pumping of laser rods, or the narrow focusing of the diode laser radiation (fiber coupling, material processing), high- quality microlenses with small lens aberrations are necessary to avoid power losses and beam quality degradation. In this paper, lens design considerations are presented together with alignment tolerances and the variation of the collimated beam parameters. Measuring methods for the lens characterization are discussed. Manufacturing techniques are summarized, and the performance of microlenses is measured by the beam width and divergence of the collimated diode laser beam.

Tailored blanks are produced by welding together flat sheets of different thickness, strength or coating type which are subsequently pressed into the desired shape. By this method it is possible to 'tailor make' the part with different properties in different areas of the component. Some of the benefits of this technology are weight reduction, improved material utilization, consolidation of parts.

Frequency selected Q-switched e-beam controlled-discharge CO-laser has been researched and developed for surface heat treatment of polymeric materials (poly(ethyleneterephthalate) and nylon) having strong absorption bands near wavelength of approximately 6 micrometers . The laser generates pulses (including short ones with duration of (tau) _0.1 approximately 1 - 10 microsecond(s) ) having different spectral content within 4.9 - 6.5 micrometers spectral range. Different geometry and methods of irradiation were used to process samples with the laser radiation of different temporal, spectral and energy density characteristics. Various types of microstructure were formed on the surfaces of the samples. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.

During high-power laser welding, gas ionization occurs above the sample. The resulting plasma ignition threshold is related to ionization potential of metallic vapors from the sample, and shielding gases used in the process. In this work, we have characterized the temporal behavior of the radiation emitted by the plasma during laser welding in order to relate the observed signals to the process parameters.