Due to its fast periodic voltage variation the rf-discharge has a lower demand on electrical field strength for achieving a self sustained discharge as compared to a dc-discharge. As a consequence higher values of electrical power density can be deposited in the gas at enhanced uniformity. Low electrode losses and reduced gas degradation increase the efficiency of this technique. Experimental performance data of transverse and axial flow lasers are presented which demonstrate the large potential of rf-excitation for cw and modulated operation.
The limiting factor of the energy density obtainable in high power dc-discharges for CO2- lasers is determined mainly by the eventual arising of instabilities. It was found that these instabilities are strongly influenced by the gas flow conditions. Theoretical and experimental results leading to significant improvements of the discharge behaviour are discussed.
In the last few years it has become common knowledge that the RF excitation of high power CO2 lasers has some advantages over DC excitation with respect to their discharge physics. To introduce these advantages into a laser system to make it an interesting product for a wealth of applications, a lot of different technical aspects have to be considered. One of the most important parameters for the technical embodiment of such a RF excited laser is the excitation frequency because it determines the type of RF generator, the impedance matching system, the type of energy coupling into the discharge, and last but not least the discharge physics themselves. In this work these problems will be examined in some detail with respect to the basic processes as well as to the technical realization. The experimental verification of the mechanisms one could expect theoretically was carried out, using a transverse flow CO2 laser with different excitation frequencies in the range of 20 to 150 kHz (in the following called AC) and at two frequencies in the RF range: 13.5 MHz and 27 MHz. An extrapolation up to the microwave range (2.45 GHz) was tried, based on earlier theoretical and experimental work done by the author.
New fast axial flow CO2-laser systems excited by rf-discharge with outputs of 1000W and 1500W, respectively are presented. The power can be pulsed up to 10 kHz and average power is changeable continously between zero and maximum. The very stable behaviour of the laser beam permits constant working conditions which is shown in connection with an 5-axis machine.
A new high performance industrial 2.5 kW CO2, laser is presented. The microprocessor controlled laser operation and the laser beam diagnostic unit allow the user to accomplish, in a very simple way, all the operations concerning laser working and source maintenance.
Previous computer simulations of high mean power, high pulse repetition frequency (p.r.f.) lasers have predicted the characteristics of the first output pulse only. This pulse, however, is not representative of subsequent pulses as the simulation is initiated using conditions based on thermodynamic equilibrium. Using a modified kinetic model which incorporates plasma temperature variation, optical cavity characteristics and a transverse gas flow, the simulation was extended to include the second and hence subsequent output power pulses. A substantial difference was found between the first and second pulse profiles and also their temporal relationship with the electrical pumping pulse. This extended model gives closer agreement with experimental measurements made under continuously running conditions. The modelling is essential as it identifies the control variables which can be used to optimize beam characteristics for material processing applications.
The state of the art of high power CO2 lasers for materials processing is reviewed. The main characteristics of the active medium, of the optical cavity, of the fluidodynamic circuit and their effects on the beam quality, overall efficiency, sizes and weights are discussed for the various configurations. An outline of the areas where improvements can be expected is also given.
After a short introduction of the different types of uniform field electrodes, the main topic of the paper describes the usual preionization techniques with emphasis on the corona preionization. It is shown that the dielectric used in the corona preionizing board determines the amount of energy that can be deposited in the gas prior to arc formation and the laser output energy. We found Macor to be superior to all other dielectrics in this regard. A new surface discharge corona preionizer is then introduced. The preionizing board has a slit aperture in the middle of it, so that a transverse gas flow can be installed. When a transverse gas circulation was used, repetition rates up to 100 Hz could be achieved.
The semiconductive preionization technique involves the use of a semiconductive plate to produce a row of spark discharges which illuminates the laser gas. It is shown that the transverse resistance of the preionizer determines the arc free input energy, the laser output energy and the efficiency of the system.
The energy extraction from a TEA CO2 laser amplifier is optimized by using both multiline and multiple-pass amplification techniques. Special care was taken to keep the beam profile Gaussian. Multiple-pass amplification appears to be the best way to a significant increase in energy extraction efficiency. A maximum amount of energy of 9.7 J/ℓ was extracted at an efficiency of 4.3%, both values with respect to the volume of the beam inside the amplifier.
Active mode-locking of transversely excited (atmospheric) carbondioxyde (TE(A)-CO2) lasers usually does not result in pulses as short as predicted by theory. A qualitative experimental study of the mode-locking process shows that only within a small range of parameters the full advantage of the pulse compression is being used.
Excimer lasers have become one of the most important types of lasers for applications in materials technology, chemical processing, atmospheric diagnostics and research in the chemical and biological sciences. This paper attempts a brief summary of the present state of development and future perspectives for this class of lasers. The discussion is organized in three sections according to the following list of topics:
I) Laser physics and spectroscopy
- Survey of emission wavelengths and types of transitions
- Laser kinetics
- Gain, amplification, energy extraction, short pulses
- Frequency variation to longer and shorter wavelengths
- New systems
II) Performance and output
- Pumping schemes and prei-onization
- Chemicals and materials problems
- Excimer laser performance data: Present and future
III) Selected applications
- Basic research
- Remote sensing
- Materials working
- Chemical processing
- Treatment of biosystems
A cryogenically cooled 2 cm aperture sealed-off UV or X-ray preionized TEA CO laser was succesfully operated. It was found that X-ray preionization performed better than UV preionization. 17.5 J/ℓ output energy was extracted from a 2x2x40 cm3 discharge volume. The maximum efficiency was 12.6%.
In order to outline the possibilities of industrial applications of CO-lasers an over-view on different types, operating continuously in the power range of some ten watts up to several kW is given. Two lasers having the potential to be scaled up to a power level of several kW are discussed in more detail. The first one, operated with precooled gas, has reached an industrial stage of development. The second one, a laser with gasdynamic cooling, has demonstrated operation over several hours with a closed gas cycle. A simplified model is used to calculate and compare system efficiencies and operation costs of both lasers.
Industrial applications of high power lasers means material processing. Two lasers are mainly involved in this field, the CO2-and the Nd-Laser. Solid state lasers make about one third of the turnover in material processing laser-systems, and among them more than 90% Nd-lasers. A broad variety of cw and pulsed solid state lasers is on the market as summarized in table 1. Due to higher efficiency and improved crystals solid state lasers of output powers up to 600 Watts are now available. In the labs 1 kW were realized, 2 kW are possible and multikilowatt systems are envisaged in the EUREKA solid state laser program. Those systems may compete CO2-laser for several reasons, compiled in table 2, if beam quality can be improved. Beam quality is beside others the most important parameter in material processing and a crucial disadvantage of high power systems at the moment.
Optical lightguides (fibers) allow to guide light in a flexible manner to a working area. Such lightguides are however enherently very narrow structures, which leads to extremely high intensities in the guides even for moderate powers. The limiting processes for undisturbed transmission are described and for a variety of parameters and situations the corresponding numerical values are given, as far as available. The high intensity processes recently led to several interesting new device-applications.
A review of present active (or adaptive) optics technology is provided. By active optics wavefront aberrations in optical systems are measured and corrected in real time. Particular emphasis is given here on active optics applications to laser systems. The sources and properties of beam aberrations are described and basic concepts for their correction are discussed. Different types of active optics systems are compared, characterized by typical wavefront correction devices and wavefront sensors. Characteristic technical features of active mirrors developed as wavefront correctors for a high power CO2-laser system are presented. Finally, performance and limitations of active optics are examined.
For materials processing with lasers, beam delivery systems are necessary for directing the radiation from the laser head to the working point on the workpiece. The more new fields of application are assumed by the laser, the greater the need for beam delivery systems which have been appropriately designed to meet the requirements of the task to be performed. Depending on the task on hand the appropriate design may be a fixed pipe with a focussing lens at its end or a six-axis articulated arm. This paper will describe the design principles and their optical and mechanical properties. The discussion of the advantages and disadvantages may be of some help in choosing an adequate delivery system.
The intensity of radiation which an optical component can withstand without damage is a crucial parameter in evaluating the role of the component in high-power laser appliations. In many cases, this damage threshold is the limiting factor in the design of high power beam trains. In this paper, our current understanding of the interactions that lead to laser induced damage are reviewed. In particular, those features that distinguish thin-film damage mechanisms from bulk damage mechnanisms are emphasized. The scaling laws predicted by current models of laser-induced damage are discussed along with a comparison of their predictions and experimental data. Speculation is then presented on those properties of materials in thin film form that make them more susceptible to laser induced damage than when in bulk form.
Substrate material preparation, surface finishing and coating techniques as well as optical evaluation methodology were developed to design and manufacture metal reflective optics and transmissive optics made of ZnSe and KC1 substrate. These optics were used successfully for high power cw CO2 laser of up to 20 KW. On the basis of these technologies, the triple layer AR coating for KRS-5 infrared fibers was developed. Maximum transmitted power of a cw CO2 laser beam through the fiber was found to exceed 90 W. The operating life exceeding 1000 hours in 40 W continuous irradiation test was achieved with the AR coated 1500 mm long KRS-5 fiber with 0.5 mm diameter.
A recent laser-damage testing round-robin experiment is reviewed. New developments in exposure methodology, damage detection, and calibration are described. Prospects for a follow-on round-robin to develop "standard" damage test procedures are discussed.
Optics for the material processing with high power C02-lasers are highly stressed during oryation: They are loaded with beam power up to 20 kW and with intensities up to 20 kW/cm . Additionally droplets, dusts and vapor from the workpiece are deposited on the surface during the material processing so that the absorption increases. Semiconductor optics - these are lenses made from Ge, ZnSe or GaAs - will be destroyed in a short time. Transmissive optics have some more disadvantages, which are mainly accepted but have some negative influence on the processing: For the adjustment of the system usually a He-Ne-la-ser is used, emitting in the visible red. This beam is focussed in another way than a CO -laser beam because the wave length is shorter by a factor of sixteen shorter than the CO2-laser radiation. This means, that the position of a He-Ne laser beam only gives appro-ximately a proper adjustment. Furtheron, the small amount of absorbed beam power leads to a heating o the lens and therefor to a variation of focal length depending on the incident power. The same reason leads to deformations of the geometrical form of the two surfaces of the lens in an interesting but hardly predictable way. This leads to lens errors and by this to a deformation of the intensity distribution in the focal spot. Additionally, it is practically impossible to get two lenses with exactly the same focal length, so that after changing the optic the system has to be adjusted in new. Transmissive optics therefore only are a compromise, not always satisfying and should be replaced by mirror optics whenever possible. The state of the art in diamond turning 9f parabolic mirrors allows to manufacture high quality surfaces at a reasonable low price. In this paper a report is given on mirror optics and systems which were developed with the following aims:
- Small losses of laser power in the system with a high efficiency of the laser beam processing system
- Long lifetime of the mirrors under material processing conditions
- High Standard of the optical quality
- Flexibility for different applications. The requested qualities are guaranteed by the whole construction of the optics and the system. The theoretical works, the state of the art of the development and the future aspects of these laser working head systems are reported.
The argument for in-process monitoring is stated and two devices are described. One is the Laser Beam Analyser which is now maturing into a second generation with computerised output The other is the Acoustic Mirror, a totally novel analytic technique, not yet fully understood, but which nevertheless can act as a very effective process monitor.
The characteristic properties of high power laser beams are the time and spatial dependent power density distribution, the power, the polarization and the so called focusability, d ,χ which is the product of beam diameter and beam divergence. These four parameters have a large influence on penetration depth and/or processing speed during material processing as sell 23 on the quality of the processed material. In the following two measuring devices are explained to measure the above mentioned properties of a 5 kW-CO2-laser beam with the exception of the polarization degree.
A novel diagnostic system is presented to measure the two dimensional beam distribution and to calculate the contour line defining the cross sectional area. In order to describe symmetrical as well as asymmetrical beam intensity distributions we define a cross sectional area which 90 % of the beam power containes and which is bounded by a constant beam intensity. This area, or the maximum and minimum diameter of this area are practical parameters for the definition of the beam characteristics. Measurements of focused multi - kilowatt beams are demonstrated. Focus diameters and beam divergences behind the focussing optics are compared for different laser systems.
A general simple experimental technique is given which allows the determination of the parameters characterizing the free propagation of a high power laser beam. The verification has been done with a 5 kW CO2 industrial laser operating in its two possible stable configuration modes. The dependance of beam characteristics on laser power has been also investigated. The beam size was found to decrease with increased rated power while the waist moved consistently: this has been mainly attributed to the output window thermal distortion. The number of peaks in the intensity distribution along the direction of maximum diameter, was not altered, but the geometry of the best beam envelope varied with power.
The physical processes involved in laser materials processing are reviewed like the plasma generation with intensity-dependent absorption, the optical feedback involved in laser workpiece interaction and the heat conduction in the workpiece with load and losses of the heat affected zone. Specific examples of various applications demonstrate the improvement of laser processing by matching the thermophysical properties of the materials to the physical mechanisms that lasers offer economic as well as technical advantages over other processing methods.
The paper is concerned with laser target interaction processes involving new methods of improving the overall energy balance. As expected theoretically, this can be achieved with high repetition rate pulsed lasers even for initially highly reflecting materials, such as metals. Experiments were performed by using a pulsed CO2 laser at mean powers up to 2 kW and repetition rates up to 100 Hz. The rates of temperature rise of aluminium for example were thereby increased by lore than a factor of 3 as compared to cw-radiation of comparable power density. Similar improvements were found for the overall absorptivities that were increased by this method by more than an order of magnitude.
During laser processing the spatial coherence and polarization properties typically impress spatially periodic structures on the surface of many materials, which are discussed on the base of electrodynamics, spatial spectra, kinetics and growth with respect to the dependance on angle of incidence, wavelength of laser, dispersion relation with polarization of light, multiple subthreshold illumination, and surface roughness. The spontaneous appearing periodic structures are generated by a runaway growth process with non homogeneous energy deposition as a consequence of interference effects at the surface and formation of capillary waves. As the surface condition obtained by conventional preparation techniques the photoninduced periodic surface structures strongly influence the energy coupling in breakdown phenomena and laser materials processsing.
Laser cutting has reached a high degree of industrial maturity.Nevertheless to extend the present range of industrial applications, the performance and quality of laser cutting must be improved.Such improvements can only be achieved if the physical mechanism of laser cutting is deeply understood.Special attention has to be payed to dynamic effects, since they are most important for cut quality.Examples for dynamic effects are the formation of periodic striations, that cause a certain roughness or pulsed operation, that allows cutting of parts with changing curvature without overheating/18/. In the following bulk of the paper, a mathematical analysis of reactive gas assisted laser cutting including dynamic effects is presented. Although that work has partly been published before, the present paper contains an important task, that has never been published before, namely the calculation of the reactive energy gain.
Since 1972 solid state lasers are used as an industrial tool for drilling holes of precision in ruby and ceramics substances /1,2/. In achieving the standard of spark erosive removal of material the laser is superior in metal-working by higher processing velocities. Deviation from roundness and cylindricity are below 5% and the roughness of the hole surface is less than 5 μm for the erosive process. These tolerances are reached in laser material processing within accurate knowledge and control of the processing and laser parameters Intensity I, beam radius rB and processing time tp.
Laser surface cladding with argon blown powder1 is a process whereby a powder stream is blown into a laser-generated melt pool. The mechanisms in this melt pool which determine the opidmum operating conditions for low dilution, zero porosity and a continuous clad track are discussed. A dilution parameter and a porosity parameter are derived and compared to experiment. Some factors affecting their values are identified.
The laser surface cladding technique was used to form in situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10:1:1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kW CO2 laser. This technique produced ultrafine microstructure in the cladded alloy. The microstructure of the laser surface cladded region was investigated by optical, scanning and transmission electron microscopy and x-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase.
Laser cladding tests have been performed using a high power CO2, laser realized by CISE. In this process a high power beam is focussed on the surface of the substrate to be cladded. The beam produces a surface melting while a powder flow, carried by an inert gas, is blown into the melt pool. The advantages of the laser cladding compared to the conventional cladding processes are the following:
- effective metallurgical bonding between the cladded layer and the substrate with a very low heat transferred to the workpiece;
- high hardness of the cladded layer with a low dilution without cracks or porosities;
- less pre and post-process operations. Example of laser cladding tests of cobalt base powder on stainless austenitic and martensitic steels and on construction steels are reported.
The laser as a cutting tool for sheet metal cutting has beenl well accepted in industry for many years. Several hundreds of units are used for contour cutting of small and medium-sized series on plane metal sheets up to 6 mm thick. Within the last three years, cutting systems have been expanded in three ways: thicker material up to 12 mm can now be cut by using higher powered lasers (1500 W); with the introduction of flying optic systems which cover sheet dimensions up to 4 m x 3 m, the cutting of larger sized metal sheets is possible. In addition, the use of five or six axis systems allows cutting of three-dimensional plastic and metal material. Besides laser cutting, the acceptance of systems for laser welding applications is increa sing. Several systems have been running in production for a couple of years and laser wel ding will probably become the fastest growing market in laser material processing within the next five years. The laser technology is regarded as a beneficial tool for welding, whenever low heat input and, consequently, low heat distortion is requested. To day's main welding application areas are: components of car engines and transmissions, window spacer and stainless steel tube welding, and also car body welding with laser robots or five axis gantry type systems. The output power of CO2-lasers for welding applications is between 1 and 5 kw in most cases.
The versatility of lasers in manufacturing operations is enhanced through robotic manipulation of the laser beam, the workpiece, or a combination of both. This paper presents an evaluation of the current state-of-the-art in laser/robot integration. Commencing with the first simple load/unload systems, through conventional machine tool motion systems, to today's complex flying optics systems, this paper presents advantages and disadvantages of the various manipulations. Current techniques for laser/robot integration are evaluated and examples of installed systems are described. Future developments and prospects for increased system utilization are discussed.
Researches on laser-metal interactions for industrial applications are currently underway at the "Etablissement Technique Central de l'Armement" (ETCA). Our goal is to design a multilaser flexible workshop for industrial machining tasks such as welding, various surface treatments, cutting. Because of its wide range of possible applications the laser tool provides an excellent frame for robotic studies. Some difficulties arise as soon as it becomes obvious that, due to the complexity of the phenomena to control, the environment conditions... no practical model can be found out. This is why we chose a progressive approach which should lead to design a general purpose laser robot with the help of human controlled iterations onto the system knowledge.
With regard to quality inspection during laser cutting, facilities for the application of different sensors are discussed. Especially the use of an endoscope and the registration of the shower of sparks as well as of the temperature in the cutting kerf are introduced, and the correlation to the quality achieved is pointed out.
After a period of research and development lasertechnique now is regarded as an important instrument for flexible, economic and fully automatic manufacturing. Especially cutting of flat metal sheets with high power C02-lasers and CNC controlled two or three axes handling systems is a wide spread. application. Three dimensional laser cutting, laser-welding and -heat treatment are just at the be ginning of industrial use in production lines. The main. advantages of laser technology. are - high. accuracy - high, processing velocity - law thermal distortion. - no tool abrasion. The market for laser material processing systems had 1985 a volume of 300 Mio S with growth rates between, 20 % and 30 %. The topic of this lecture are hiTrh. power CO2-lasers. Besides this systems two others are used as machining tools, Nd-YAG- and Eximer lasers. All applications of high. power CO2-lasers to industrial material processing show that high processing velocity and quality are only guaranteed in case of a stable intensity. profile on the workpiece. This is only achieved by laser systems without any power and mode fluctuations and by handling systems of high accuracy. Two applications in the automotive industry are described, below as examples for laser cutting and laser welding of special cylindrical motor parts.
Besides a review of laser materials processing guidelines some aspects as the statistic of molten spheres in the debris of laser cutting, influence of polarization, contour accuracy limits and the introduction of a critical cutting length with respect to heat -transfer problems during the cutting process are discussed.
Investigations into the use of high-power laser welding in the manufacture of food mixer whisks show that this can be a cost-effective and reliable technique for such simple, cheap products. Comparisons are made with resistance welding, soldering, TIG-welding, EB-welding and Nd:YAG laser welding. Redesign of components leads to greater automation in production.
Laser and electron beams are used as tools for welding, cutting, drilling, melting, tempering or vaporizing in many machining tasks. These beams have fundamentally different physical natures. They are produced differently, and behave differently when penetrating material. The differences between electron and laser devices that occur in machining, the advantages and disadvantages of these technologies in comparison with on another, will be discussed in this article.
Single-pass autogeneous welding by 10kW laser of API 5LX X60 pipeline samples, of 760mm diameter and with wall thickness up to 19mm, in a configuration appropriate 4to J-lay operations, is described. The results of mechanical testing, including Charpy impact properties and their dependence on process conditions, are presented and discussed.
Following several research programs in the 1960's aimed at studying the adverse biological effects of lasers and other optical radiation sources, laser occupational exposure limits were set and general safety standards were developed. Today, the experience from laser accidents and the development of new lasers and new applications have altered the format of the exposure limits and the safety procedures. It is critically important to distinguish between different biological injury mechanisms. The biological effects of ultraviolet radiation upon the skin and eye are additive over a period of at least one workday, and require different safety procedures. The scattered UV irradiance from excimer lasers may be quite hazardous, depending upon wavelength and action spectra. Since laser technology is young, the exposure of an individual in natural sunlight must be studied to evaluate the potential for chronic effects. The safety measures necessary in the use of lasers depend upon a hazard evaluation. The appropriate control measures and alternate means of enclosure, baffling, and operational control measures are presented. Present laser safety standards are explained briefly. Eye protective techniques and eyewear are considered for a variety of sources. The optical properties of enclosure materials are also discussed.
The Austrian legislation about laser safety is discussed. Appropriate control measures for laser use and alternate means of enclosure and baffling are presented. Present laser safety standards are explained briefly. Eye protective techniques and eyewear are considered for a variety of sources. The optical properties of enclosure materials are also discussed.
General laser safety precautions are reviewed. Laser protective filters are discussed in more detail. It is demonstrated that a sandwich consisting of absorbing glasses and dielectric band-rejection filters is the most effective design, especially for multiwavelength filters. Several examples of such filters are described.