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In product development of hammer drills simulation models and physical replacement models are used for validation purposes. These models require a consideration of the interaction of the drill bit with concrete to be accurate. Current simulation models focus on either crack formation, rock fracture mechanisms or on penetration rate predictions. Physical replacement models currently aim to evaluate vibration emissions on hammer drill users. A model of the drill bit and concrete interaction based on realistic hammer drilling including impact and drilling direction is not yet available for hammer drill product validation. This contribution introduces the coefficient of restitution as a modelling approach for the interaction between drill bit and concrete. Furthermore, a new measurement system and a study varying feed force, concrete and drilling depth for correlations with the coefficient of restitution are presented.

Walking rehabilitation processes include many repetitions of the same physical movements in order to replicate, as close as possible, the normal gait trajectories, and kinematics of all leg joints. In these conventional therapies, the therapist's ability to discover patient's limitations-and gradually reduce them-is key to the success of the therapy. Lower-limb robotic exoskeletons have strong deficiencies in this respect as compared to an experienced therapist. Most of the currently available robotic solutions are not able to properly adapt their trajectories to the biomechanical limitations of the patient. With this in mind, much research and development is still required in order to improve artificial human-like walking patterns sufficiently for valuable clinical use. The work herein reported develops and presents a method to acquire and saliently analyze subject-specific gait data while the subject dons a passive lower-limb exoskeleton. Furthermore, the method can generate adjustable, yet subject-specific, kinematic gait trajectories useful in programming controllers for future robotic rehabilitation protocols. A human-user study with ten healthy subjects provides the experimental setup to validate the proposed method. The experimental protocol consists in capturing kinematic data while subjects walk, with the donned H2 lower-limb exoskeleton, across three experimental conditions: walking with three different pre-determined step lengths marked on a lane. The captured ankle trajectories in the sagittal plane were found by normalizing all trials of each test from one heel strike to the next heel strike independent of the specific gait features of each individual. Prior literature suggests analyzing gait in phases. A preliminary data analysis, however, suggests that there exist six key events of the gait cycle, events that can adequately characterize gait for the purposes required of robotic rehabilitation including gait analysis and reference trajectory generation. Defining the ankle as an end effector and the hip as the origin of the coordinate frame and basing the linear regression calculations only on the six key events, i.e., Heel Strike, Toe Off, Pre-Swing, Initial Swing, Mid-Swing, and Terminal Swing, it is possible to generate a new calculated ankle trajectory with an arbitrary step length. The Leave-One-Out Cross Validation algorithm was used to estimate the fitting error of the calculated trajectory vs. the characteristic captured trajectory per subject, showing a fidelity average value of 95.2, 96.1, and 97.2%, respectively, for each step-length trial including all subjects. This research presents method to capture ankle trajectories from subjects and generate human-like ankle trajectories that could be scaled and computed on-line, could be adjusted to different gait scenarios, and could be used not only to generate reference trajectories for gait controllers, but also as an accurate and salient benchmark to test the human likeness of gait trajectories employed by existing robotic exoskeletal devices., The research was partially funded by CONACyT Doctoral program, Tecnolgico de Monterrey/Robotics Lab and research chair program for distinguished professors in Biomechatronics.

Secure communication is a key requirement for vehicular networks. However, high mobility and density bring several challenges to the roaming mechanism needed to handover the control of vehicles as they move along the roads. Authentication and key management must take place on the way, with strict timing and safety requirements that are seldom matched by the traditional Internet solutions. Those solutions are usually based on Public Key Infrastructures, use ordinary Internet protocols, and are implemented atop general-purpose operating systems that pose additional security threats. In this paper, we propose an extension of the Trustful Space-Time Protocol to implement a low-latency, secure roaming mechanism capable of handing over trust along roadside gateways using a token-based authentication mechanism that fits within a range of realistic scenarios. We modeled such realistic scenarios in the OMNeT++ simulator, using the Castalia Framework, and demonstrated that our protocol is capable to hand vehicles over among gateways of a large, highly utilized road with a roaming latency less than 14 ms.

The growth directions of 55 Sn whiskers, i.e. the crystallographic orientation parallel to the whisker-growth axes, were determined using (i) a focused ion beam microscope for the determination of the physical growth angles of the whiskers with respect to a specimen (reference) coordinate system and (ii) an electron backscatter detector in a scanning electron microscope for the determination of the crystallographic orientation of the whiskers. The Sn whiskers were found to grow preferentially along low-index directions of the β-Sn crystal structure. The experimental findings of this study (and most of the results presented in the literature as well) were explained by applying, in a modified way, the Hartman–Perdok concept of periodic bond chains, i.e. chains of strong bonds running uninterruptedly through the structure, to the Sn whisker-growth phenomenon.

The fast separation of a mixture of 29 compounds by using comprehensive two-dimensional gas chromatography is reported. Capillary column sets with shorter lengths and smaller inner diameter in both the first and second dimensions have been tested, for both fast chiral and achiral separations. Fast chiral separations, which included enantiomer separations of limonene, linalool, citronellol, and alpha-isomethylionone, were achieved within 23 min, which corresponds to approximately 2-fold faster than analyses under conditions previously considered as normal. Fast achiral separations, which do not have the restriction of requiring a minimum quality of chiral resolution, were obtained within 5 min, which is markedly faster than separations on the normal column set under conditions more commonly employed. The achiral fast GC x GC method used a 5 m x 0.1 mm i.d. first dimension column, interfaced to a 0.3 m x 0.05 mm i.d. second column, with temperature program rate of 35 degrees C.min-1; a modulation period of 1 s was employed. Peak widths at baseline on the first column were a little over 1 s, while modulated peak widths at half-height recorded with a flame ionization detector operating at 200 Hz were approximately 30 ms. The benefits and limitations of GC x GC for fast chiral and achiral separations are reported and discussed.

Shadowgraphic and interferometric images of vapor/plasma plumes generated by pulses of a ns-Nd:YAG laser reveal the distinctive behavior of the shock wave expansion for each of three processing wavelengths (1064 nm, 532 nm, and 355 nm), indicating that different mechanisms dominate the interaction between the optical fields and the material vapor/plasma. Electron density distributions within the plasma plumes calculated from interferometrically obtained refractive index distributions allow us to describe the regions of absorption and the absorption mechanisms using a simple model of inverse bremsstrahlung and photoionization, which allows us to estimate absorption coefficients.

During a cardio-pulmonary bypass, as well as post-operatively, high levels of endotoxin, interleukin-6 (Il-6) and C-reactive protein (CRP) were measured in 30 patients. A significant increase in endotoxin plasma level occurred during surgery, culminating in a peak during reperfusion. Plasma levels of endotoxin continued to be slightly raised until the fifth day after surgery, whereas those of Il-6 rose at the time the operation came to an end and were at their highest 6 h later. CRP levels were also high, post-operatively, and were markedly raised on day 2. A definite, statistically significant correlation between the plasma levels of endotoxin and Il-6 during the operation was established, leading us to conclude that the endotoxin liberated during cardiac surgery acts as the main trigger in the release of Il-6 and thus induces the post-operative acute phase reaction. There was no evidence of a correlation between CRP and endotoxin or Il-6 plasma levels.

This paper presents the experiment design, results, and analysis of a human user study that tests and validates the minimum hand jerk (MHJ) model for a human forearm reaching movement task when manipulating a multi-mass object. This work validates and extends prior work that demonstrated the MHJ criteria, a mathematical approach to human movement modeling, more accurately represents movements with multi-mass objects than the alternate optimally smooth transport (OST) model. To validate the prior work, we developed a visual and haptic virtual environment with a five-mass system with friction connected by springs and viscous dampers. The point to point reaching task we implemented required participants to move their hand with the set of masses to a target position, thereby generating movement profiles for analysis. Our experimental design uniquely extends the application of the MHJ criteria to forearm pronation movements and our results show that the MHJ model holds. Our extension to forearm movements and the more general MHJ criteria are economic models of human movements applicable to fields such as computer animation and virtual environments.

A hybrid Nd:YAG laser beam welding technique has been investigated in welding of aluminum alloy. Connecting the filler wire with one pole of a DC power supply and the workpiece with the other, a closed electric circuit is built. By this means, the wire is resistively preheated, so that less energy from the laser beam is dissipated in melting the filler metal. On the other hand, the current flowing in the weld pool generates a magnetic field and electromagnetic forces which affect the fluid flow of the weld pool and then the welding process. It is found that the weld depth increases and the weld seam becomes slender when a certain current is supplied. To clarify which effect, the thermal effect or the electromagnetic effect, dominates, welding with pure hot wire was carried out. The experimental results demonstrate that the pure hot wire addition has no obvious affect on the welding efficiency and the weld cross section, but worsens the process instability. Thus we can conclude that the influences of the current do not result from the resistive heat from the wire, but the electromagnetic effects. This new approach provides a possibility to increase the processing efficiency and flexibility, improve the process stability and weld quality, and also shape the seam cross section.

The use of electromagnetic body forces in laser beam welding of aluminum alloys is a new method to shape the geometry and to enhance the quality of the weld seams. In this new approach, electromagnetic volume forces are utilized by applying magnetic fields and electric currents of various origins. Acting in the liquid metal, they directly affect the flow field and can lead to favourable conditions for the melt dynamics and energy coupling. Numerous welds with full and partial penetration using both CO2 and Nd:YAG lasers demonstrate that this method directly influences the seam geometry and top-bead topography as well as the penetration depth and the evolution of pores and cracks. In the case of full penetration, it is also possible to lift or to lower the weld pool. The method, therefore, can be used to shape the geometry and to enhance the quality of the weld seam. Depending on the orientation of an external magnetic field, significant impacts are achieved in CO2 welding, even without an external current: the shape of the cross-sectional area can be increased of up to 50% and also the seam width is changed. Whereas for such conditions with Nd:YAG lasers no significant effect could be observed, it turned out that, when an external electric current is applied, similar effects are present with both wavelengths. In further investigations, the effect of electromagnetic body forces resulting from the interaction of an external current and its self-induced magnetic field was studied. Hereby, the current was fed into the workpiece via a tungsten electrode or a filler wire. The resulting phenomena are the same independent from wavelength and means of current feed.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This contribution examines the basic concepts and results of two laser ablation models based on commercially available hydrodynamical codes. In both cases the different material phases are described continuously by a single numerical algorithm. The first approach uses a finite-element model for the simultaneous description of solid and melt. It is thereby particularly suited for the description of melt formation and ejection. The results indicate a slow acceleration of the melt during the laser pulse up to velocities of some 10m/s followed by a rather steady-going ejection which is finally cut off by the resolidification. Although it was possible to examine this expulsion process, the model showed considerable numerical stability problems for higher intensities and the ultrasonic vapor expansion cannot be included. To overcome these shortages another model is proposed which is based on an equation of state for the target material in combination with a special pressure-based solver. Besides the continuous description of the material states, it also includes a continuous treatment of the beam propagation and energy coupling by solving Maxwell's equations. Although the work on this model is still going on, some of its basic prospects and limitations can already be discussed.

The main goal of welding aluminium with double focus technique is to improve the quality of the welding result, that means to reduce porosity and to avoid melt ejection. The idea behind this approach, namely utilizing two or more individual focal spots on the workpiece instead of just one, is to shape the keyhole geometry such that the tendency of a keyhole collapse is considerably reduced. With one spot or with a small distance between the two laser spots, the weld bead is irregular and a high number of process pores can be observed. With a larger focal distance the bead is becoming stable and the porosity is reduced dramatically. At the same time, however, the welding depth is decreasing compared to the situation with a small focal distance. The best quality was achieved at the focal distance 0.6 mm. In this case, pores which are larger in diameter than 0.6 mm were avoided completely and the number of smaller pores is strongly reduced. After a further separation of the keyhole the number of pores increases again. Whereas the process quality can be improved by appropriate arrangement of the spots some reduction of the process efficiency might occur.

From process considerations and economical reasons, an ideal welding laser should provide the necessary power at short wavelength and the highest possible beam quality at a high overall efficiency. The concept of a diode-pumped Yb:YAG thin disc laser, in principle, fulfills these requirements at the same time. Its characteristic features are discussed and results of fundamental theoretical and experimental investigations are presented. Performance data of first industrial devices underline the advantages that can be expected from this design: a power of 1.5 kW is delivered through a 0.15 mm fiber at an optical-to-electrical efficiency of 27%. Some preliminary results of welding studies with up to four of such devices demonstrate its great potential for thin metal sheet welding and many other applications where the achievement of well-defined slender weld seams is essential.

Double focus welding is a proven technique to reduce or even to avoid blowholes and porosity in the welding process of aluminum. The application of two focal spots arranged close to each other offers advantages concerning process stability and quality, which is a result of a more stable keyhole geometry respectively of modified energy coupling and fluid dynamic conditions. The opportunity of designing a task-adapted intensity distribution opens possibilities for the bridging of gaps and the shaping of the weld seam geometry which means an increase of flexibility. This contribution will discuss the joining process and will show the connection between spot distance and intensity distribution on one hand and the welding result -- depth, efficiency and quality -- on the other hand.

A new method to shape the geometry and to enhance the quality of weld seams is presented in this paper. Hereby, electromagnetic volume forces are produced by utilizing various origins of the magnetic field and the electric current. Acting in the liquid metal they directly affect the flow field and lead to favorable conditions for the melt dynamics and energy coupling. Experimentally conducted full and partial penetration welds in aluminum alloys with CO 2 and Nd:YAG lasers demonstrate that this method can be used to influence the seam geometry and top-bead topography, as well as the penetration depth and the evolution of process pores. In the case of full penetration, it is also possible to lift the weld pool against gravitational force.

The presence of melt during the laser drilling process always signifies a balance between an efficient material removal in molten form and a reduction of quality due to recast on the hole walls and near the crater entrance. Earlier investigations have demonstrated that by reducing the laser pulse duration the amount of produced melt can be decreased and hence, the precision increased. Nevertheless, they also demonstrate that melt can never be avoided completely. Therefore, to achieve an optimum balance between efficiency and quality by a preferably complete expulsion of melt the physical fundamentals of its generation and ejection have to be understood. By applying several different analytical and numerical models ranging from simple estimations to multi-dimensional simulations, the authors will outline the peculiarities of the melt formation and dynamics during the drilling with short and ultra-short laser pulses. Since these calculations demonstrate the importance of the consideration of melt acceleration and geometric aspects, special interest will be taken in these matters. While the evaporation stops soon after the laser pulse, the melt ejection may continue until the complete solidification of the material. For a better understanding and verification, the results of the models will be compared to experimental data.

Laser beam welding of aluminum alloys is expected to offer both the technical and economical advantages. In most cases welding wire addition is necessary from the viewpoints to suppress hot cracks, improve mechanical properties of the weld as well as to reduce demands on the edge preparation, fit-up tolerance, and beam alignment. In practice, the filler wire could be added to the weld pool either in the leading or trailing direction. In this experimental work reported, the influence of the wire addition direction on the weld efficiency and process stability was investigated by using a 5kW CO2 laser to weld aluminum alloy 6009 with a plate thickness of 3mm. Beam-on-plate welds were made either in the autogeneous mode or with filler wire AISi12. Illuminated by a double frequency Nd:YAG laser, the weld pool dynamics and the wire melting process were observed applying a high speed camera. The experimental results demonstrate that welding with filler wire in the trailing direction is more efficient and stable than in the leading direction. High speed camera photographs show that the filler wire is mainly melted through weld pool heating and plasma heating in the former case and through direct laser irradiation and plasma heating in the later case. The weld pool is of vibration when welding in the autogeneous mode or with filler wire in the leading direction. However, the weld pool is much calmer when the wire is added in the trailing direction to the weld pool.

Stable phase-locking of a VCSEL array of N equals 16 emitters with the master laser on the same chip is demonstrated. To accomplish the injection-locking a very small portion of the master radiation is seeded frontally into the 16 slave lasers. All VCSELs are driven by one voltage source with small individual series resistors to compensate for frequency differences. The beams are collimated by a microlens array and are commonly focused. A factor 14 increase of the peak power density of the superimposed beams compared to the unlocked operation is achieved (overall system coherence 85%) without any phase control. The coherent operation is stable for hours without any sophisticated voltage or temperature control. By slightly varying the current of each emitter within the locking range a maximum phase shift of (pi) can be achieved for each emitter. In this way residual phase differences between the individual beams can be compensated. The fraction of the master power necessary to lock one slave laser is below 10-3. Therefore, scaling to very large arrays is possible. Apart from simply increasing the peak power density of the chip, a promising perspective for data transmission applications is the GHz-modulation of the system coherence. The locking of the array can be switched by a very small (2%) modulation of the master current thereby switching the peak power density by a factor of nearly N.

Aluminum alloys are getting increasingly interesting not only for the classical application in the aircraft industry, but also in rail and road vehicles and the aggregate manufacturing as well as in many other fields of the metal processing industry. The laser is rarely found as a tool for joining aluminum in series production, up to now. Among others, this is due to the fact that only few instructions for the design of an appropriate joining geometry are available in the literature. In addition, neither hints concerning the laser's suitability for industrial application are provided nor essential issues for series production by lasers are to be found, such as e.g. acceptable tolerances regarding the joining geometry. In this paper, exemplary solutions to the problems mentioned above are presented, resulting from the application of a 3 kW fiber-guided Nd:YAG-Laser for tasks in the automotive industry. Taking the example of an overlap joint geometry on the one hand, and the connection of two extrusions forming a 'T-joint-geometry' on the other hand, there will be shown which tolerance fields exist and in which way and up to which extent a gap can be bridged, not only in a gravity position. In addition, results will be presented, demonstrating that the energy coupling as well as the melt pool dynamics can be influenced by varying the wire position or using a second wire in an adequate position. Concerning an overlap joint, gaps of more than the doubled size can be bridged with this technique.

Mask based structuring in excimer laser applications is costly if mask with a small ratio of the hole pattern area compared to the whole illuminated area are used. In this case, the major part of the laser beam is reflected or absorbed at the mask. The herein presented system shapes the reflected part of the beam and guides it onto the mask again. This principle has been realized for up to eight irradiations of the mask. In order to achieve high quality ablation result both the beam divergence as well as the homogeneity play an important role. Therefore, a special homogenizer was developed which converts the excimer laser beam efficiency into a flat-top profile with negligible influence on the beam quality. The achieved optical resolution keeps within 2 micrometers and the homogeneity is sufficient to achieve nearly the same structure quality over the entire image field. Providing that the transmitting areas of the mask do not exceed a few per cent of the total irradiated area. An efficiency enhancement by more than a factor of five was achieved for eight mask irradiation passes compared to a single irradiation.

Laser beam welding of aluminum alloys is becoming of increasing interest with respect to light-weight structures. Compared to welding of steel, the process is rendered difficult by the specific material properties. On the basis of a fundamental understanding of the relevant physical mechanisms, guidelines for a successful conduction of the welding process and quality assurance are presented. Factors relevant for deep penetration threshold, process efficiency, process stability and for achieving high seam quality are discussed. In particular, by the combination of two individual focal spots along or normal to the welding direction a process-adapted distribution of the laser power yields substantial improvements. With respect to quality assurance a new method presented here uses the relationship between the geometry of the keyhole and the emitted laser light to control welding depth and to monitor seam defects.

Most high power applications either production or defense oriented require good beam quality. A state-of-the-art review shows that current solid state lasers exhibit a clear deficiency as compared to well developed gas lasers. This may be overcome by the new concept of a diode pumped thin disc laser combining the advantages of low optical degradation of the laser medium and good output power scalability. This solid state laser benefits further from the shorter wavelength and better coupling efficiency for a wide range of materials. Yb:YAG is chosen as the preferred laser material because it primarily has a high quantum efficiency, guarantees good energy storage for pulsed operation, and exhibits a broad absorption line at 941 nm for easy and effective pumping with InGaAs diodes. Performance data of the thin disc laser are shown for output powers up tp 350 W CW with optical and slope efficiencies of 49% and 56%, respectively. Scalability of the concept is demonstrated by using a multiple disc design with a zig-zag stable resonator. Beam quality parameter, M2, has also been determined for al operation conditions. Best values of M2 equals 1.22 have been obtained for 100 W output power at 39% optical efficiency. In addition, passive mode locking and Q-switched operation are demonstrated. Finally, performance in alternative laser active materials (Nd:YAG, Nd:YVO4, Tm:YAG) is also presented.

For optimized laser active media with respect to high efficiency the optical quality, the discharge stability and the small signal gain have to be on a high level at simultaneously high power densities. This goal was achieved by combining a defined swirl flow with helical electrodes and a so-called ignition improver. Besides of the regular gas components CO 2 , N 2 and He the gas mixture contains Ar and C 4 H 10 . The benefit of using optimized laser active media was demonstrated with a straight unstable resonator superimposed to two discharge tubes. At an efficiency of 18% and a beam power of 5 kW the laser system produces about the same beam propagation factor as calculated for the undisturbed gain medium. The beam propagation factor was significantly increased by an on-axis reflaxicon mirror system within the external beam path.

A novel technique for CVD synthesis of materials, that does not demand a vacuum chamber and provides high deposition rates, have been developed. It is based on CW CO2-laser maintenance of a stationary optical discharge in a gas stream, exhausting over a substrate into the air. Nano- and polycrystalline diamond films were deposited on tungsten substrates from atmospherics pressure Xe(Ar):H2:CH4 gas mixtures at flow rates 2 1/min. 2,5 kW CO2-laser focused beam produced plasma. Deposition area was about 1 cm2 and growth rates up to 30-50 micrometers /hour. Peculiarities and advantages of laser plasmatrons are discussed.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The EUREKA-project CHOCLAB (Characterization of Optical Components and Laser Beams), which was initiated in 1995, aims to investigate and develop instruments and standard measurement procedures for laser beam and laser optics characterization. From the start of the project, one of the main work items in the field of laser beam characterization has been the determination of the beam propagation parameters (beam width, divergence angle and beam propagation factor). The most common measurement devices for these purposes are 2d-matrix-cameras (i.e. CCD-cameras), 1d or 2d scanning devices (i.e. scanning pinholes and/or moving knife- edges)--both of them requiring further evaluation of the measured power density distribution to obtain laser beam parameters--and variable transmission/reflectivity optical elements, which allow the direct measurement of beam properties. The advantages and drawbacks of the different types of measurement devices, as well as measures to reduce systematical and statistical errors will be discussed.

To increase the flexibility of the generative LAPS-J (laser aided power solidification -- powder jet) process, a special focusing device has been developed. The beam of a Nd:YAG laser is delivered into the production machine via a step-index glass fiber. To utilize the resulting top-hat intensity distribution, the end facet of the fiber is imaged on the workpiece by a special optical system, consisting of four lenses. It allows the variation of the scale of imaging between 4:1 and 1:1. By the computer controlled movement of two motors it is possible to change the width of the generated tracks in this range during laser processing. Integrated in a turning center, the LAPS-J proces allows new and complex applications, e.g. in the fields of cladding, rapid prototyping or repairing of metal parts. With an additional process control, the quality and accuracy of generated metallic parts can be considerably increased.

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.

We outline results obtained from Schlieren and dye laser resonance absorption imaging of the plume ejected from an aluminum target in a nitrogen atmosphere of 1 bar and 100 mbar by a KrF excimer laser (lambda equals 248 nm, FWHM equals 30 ns). The results show that for relatively low and high laser fluences (14 J/cm2 and 36 J/cm2), the plume closely follows the shock wave which is generated by the ablated material pushing against the surrounding gas. Calculations of the evolution of the ambient gas and ablated material show that the temperature and electron density vary greatly depending on the laser fluence and the external gas pressure. We report maximum plasma temperatures of 39888 K and electron densities of 4.2 multiplied by 1026 m-3 for a laser fluence of 36 J/cm2. These results indicate that inverse bremsstrahlung may play a very significant role in how the laser pulse energy is distributed in the plume for high laser fluences.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A numerical beam propagation model has been developed to calculate the absorption and defocusing of a carbon-dioxide laser beam by the laser induced plasma plume during deep penetration welding. By directly solving the paraxial wave equation with a finite difference scheme, the model allows us to calculate the laser beam propagation through the plasma plume and to determine the 'effective' intensity distribution in the focal plane. In combination with a previously published model that takes into account Fresnel absorption at the keyhole walls and plasma absorption inside the keyhole, now a calculation of the welding depth as function of the process parameter becomes possible. In particular, the propagation model clarifies the role of plasma absorption above the workpiece. The calculations identify the defocusing of the laser beam by the plasma plume as the main mechanism that degrades the laser deep welding process. Absorption by the plasma is shown to be of little importance. Additionally, the propagation model elucidates the process stabilizing potential of shielding gas mixtures, especially of helium and argon mixed in a ratio of 3:1; a result also of phenomenological investigations that are used frequently in practice.

The design of compact and efficient laser sources having high beam-quality and high beam-power is an objective of gas laser research. Therefore, it is necessary to realize highest input power densities while producing only minimized distortion of the optical quality of the laseractive fluid. A non-concentric helical configuration of electrodes in combination with a particular swirl flow allowing for homogeneous deposition of 28 kW of electrical power in a single tube element and operating at the choking limit, while producing only 350 nm of maximum phase distortion, is shown in theory and experiment.

The role of laser beam parameters and material properties in establishing a stable deep welding process is investigated, experimentally and theoretically. Particular emphasis is placed on the mechanisms involved in the energy coupling. It is shown that high quality welds are achieved by a proper choice of parameters and a sophisticated process conduction.

The properties of the laser active medium were investigated using a drift chamber, and the effect of gas additives on the laser power is evaluated. The transport coefficient drift velocity is measured in carbon-dioxide-laser gas mixtures. The dependence of the drift velocity on the applied electric field is very similar for the different gas compositions in the positive column of the discharge, and a semi-empirical formula is given. The influence of gas additives is investigated (1) a selective energy reduction due to elastic impacts with Ar, (2) striking voltage reduction by adding C 4 H 10 to the laser gas.

The acceptance of laser material processing in industry is often detracted by relatively high costs. A generally adopted approach to overcome this drawback is the search for laser sources with higher efficiency and optical components with improved performance. A quite different and even more successful measure, however, is the purposive choice of beam properties for the particular task. As the interaction phenomena at the workpiece decisively depend upon them, quality and speed of the progress can be considerably increased. On the basis of several examples, the role of properties such as wavelength, polarization and 'beam quality' are discussed and the potential enhance the process efficiency by their proper use is demonstrated.

Laser diodes with diffraction-limited beam quality offer high power densities of the order of 107 - 108 W/cm2, but are limited in output power to some watts. Scaling to higher powers has to be realized by superposition of a number of laser diodes. Coherent superposition allows us to further increase the power density in the far field. This is realized by injection locking of three slave laser diodes (Toshiba TOLD) 9140, 20 mW, 690 nm) by one master laser diode (TOLD 9140) and superpositioning of the three slaves lasers by a lens array. The feedback of the slaves into the master is suppressed by two Faraday isolators. For superpositioning the light of the slaves while maintaining the high beam quality, the light of each diode is coupled into an optical single-mode fiber. Phase shifts due to mechanical or thermal disturbances of the single-mode fibers for frequencies up to 1 kHz are compensated by a single-mode optical fiber piezoceramic phase modulator and an electronic control circuit. A phase stability with a maximum phase error smaller than 6 degrees is kept over an hour. The power-density distribution in the focal plane of the focusing lens shows a peak power 2.6 times that of the incoherent superposition and a modulation corresponding to the Fourier transform of the nearfield distribution of the lens array.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new free vortex aerodynamic window, developed for industrial high power CO2-lasers, is presented. To meet the industrial requirements improvements with regard to a reduction of the total nozzle pressure were necessary. The lower pressure results in reduced turbulence and therefore in less influence on the laser beam. This was verified by interferometric measurements as well as by experiments on an industrial laser. Some results of a numerical simulation of the flow in an aerodynamic window aimed at a better understanding of the phenomena involved are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In general, the beam quality of industrial high-power CO2 lasers does not reach the maximum theoretical value, but it is dependent on the operation parameters of the particular system. This is due to the distortion of the intracavity wavefront which results from the non- homogeneity of discharge and gas flow and from the power absorption of mirrors and windows. For this reason, investigations have to be done in order to identify and then to minimize or to compensate the distorting effects caused by basic `elements' of a fast axial flow rf-excited laser. Conventional linear electrodes, improved helical electrodes, with or without a discharge-stabilizing air gap between electrodes and quartz tube, have been compared with respect to the minimum achievable phase distortions. Bending mirrors, resonator mirrors and outcoupling windows absorb a part of the beam power and of the uv-luminescence of the discharge and, therefore, become deformed. The temperature of the quartz tubes changes with laser operation time leading to a transient thermally induced lens in the laser active medium. This effect in combination with the time constants of mirrors and windows determines the warm-up period of a laser system. The detrimental transient effects can be reduced either by optimization of the basic elements of the system or by inserting optical elements which can be used for active compensation. One example is the use of an intracavity adaptive mirror offering the possibility to alter the mirror surface curvature. An experimental high-power CO2 laser has been used to demonstrate and to verify approaches to minimize phase distortions caused by the effects mentioned above.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This work investigates the possibility of ablating paint from aerospace material with a XeCl- laser. The main advantage of this type of laser is the low heat generation during the ablation process. This is important when stripping thermally sensitive materials such as polymer composites. The dependence of the ablation process on energy density, pulse frequency as well as other laser parameters are presented. The results show the influence of chemical and UV artificial aging processes on ablation depth. Further, the behavior of the time-averaged transmission of the laser beam through the plasma is described as a function of the energy density. The time-varying temperature in the substrate at the point of ablation was measured during the process. An abrupt change in the temperature variation indicates the end of point ablation. This measured temperature variation is compared with the calculated temperatures, which are derived from the 1D heat equations. Finally, first results of repaintability and ablation rates will be presented.

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

The ablation process of materials by an excimer laser is in most cases of thermal nature. The high intensity of the laser radiation leads within a few nanoseconds to evaporation of the material which compresses the ambient gas. A strong shock wave and further discontinuities are formed. These gasdynamic processes were detected by schlieren photography and shadowgraphy. The investigations show that the propagation mechanism of the shock wave depends on the ambient gas pressure and the laser pulse energy. The measured distances are compared with the calculated values using the Sedov-Taylor blast wave theory. A very good agreement between theoretical values and experimental data is found. Further, an interpretation of the appearance of the other discontinuities and their behavior will be given. A special nozzle was constructed in order to form a 1D gas flow. The modeling of this gas flow leads to the density, pressure and velocity of the jet at any point of the gas flow. The presence of such an external gas jet parallel to the target surface shows a drastical effect on the gasdynamic processes as well as on the formation of debris.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ablation of materials in air at ambient pressure of 1 bar with a KrF excimer laser (3 J/cm2 - 47 J/cm2, FWHM 30 ns) leads to gasdynamic discontinuities in the surrounding atmosphere. A plasma plume above the target surface arises caused by its vaporization and by interaction between the compressed air and the laser beam directly. These phenomena affect the exactness of the achievable geometry as well as the efficiency of the ablation process and the debris formation in the environment of the irradiated area. In this paper time resolved measurements of the transmitted laser radiation through the absorbing plasma plume are presented. From these results conclusions can be drawn about the efficiency of the ablation process which is dependent on the energy density as well as on the amount of the energy value. The experiments have further shown that immediately after reaching the laser pulse plateau the energy coupling to the target drastically decreases. As an example the energy loss by the absorption mechanisms in the plasma plume can be up to 80% at an energy density of about 30 J/cm2. The different behavior between a polymer sample and a metal sample is discussed. The propagation mechanism of the laser induced shock wave front has been studied as well and an attempt was made to explain it by the Sedov theory. A good agreement results from the measured and the theoretically calculated shock wave distance. Thus, an evaluation of the energy content of the laser induced shock waves using the distance- time law appears reasonable and the results are presented. They confirmed the directly measured high absorption in the plasma plume. The presented results allow a monitoring of the energy coupling.

Different diagnostic techniques were used to investigate melt pool instabilities and droplet generation during the welding process with pulsed lasers. In order to find the causes and the dependencies for these effects, an experimental setup was built up that allows the measurement of three different quantities. The luminosity of the welding plasma together with the instantaneous laser power were recorded by a personal computer. High speed video pictures were taken concurrently and were temporally correlated with the other signals to understand the basic phenomena. The results of experiments, which were done for a variety of several parameters, are discussed and serve as boundary conditions for a theoretical model, which helps us to understand the complex hydrodynamic mechanisms.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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

Laser technology enables a specimen surface to be melted and alloyed without the substrate itself being heated, whereby a surface with special attributes is obtained while the properties of the substrate are unchanged. The surface of Armco iron, AISI 1045, and AISI 1095 steels were laser alloyed with TiC powder, using a CO2 laser of 2.5 kW maximum power. Optimal laser and powder feed parameters were established. Particles of TiC were injected into the molten surface layer, forming a composite material, steel + TiC. The microstructures were investigated metallographically. Some of the particles had partially melted during their passage through the laser beam and had re-solidified, forming small and fine dendrites. Surface hardness increased, mainly due to the presence of these undissolved TiC particles. A new mathematical model describing the alloying process, based on the equation of heat conductance, to the alloying process is presented.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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

The increasing availability of highest power lasers broadens the economic use of deep penetration laser welding by increasing maximum welding depth and by increasing welding speed. The problems encountered in those two areas of interest differ significantly. Whereas the maximum welding depth is strongly affected by the absorption mechanism1,2, the maximum welding speed is limited by fluid dynamic phenomena . Theoretical modelling allows to seperately investigate the different processes.

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

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.