A new UV-optic with an integrated autofocus sensor has been developed for submicron laser processing. In general, a distinction can be made between reflective and refractive objectives. Both have different advantages and disadvantages and, therefore, must be adapted to the application. For refractive objectives designed for UV-purposes, only a limited number of materials are available. With a small spectral bandwidth, such as that for some laser sources, an all quartz lens can be used. The frequency quadrupled Nd:YAG laser already has in its basic set-up a spectral bandwidth of less than 100 GHz. In the seeded mode, only one longitudinal mode will be excited. In order to achieve focus diameters below 0.5 micrometers , at least an N.A. of 0.65 is necessary. Such an objective has been designed and built at the Laser Zentrum Hannover (LZH). For this objective, automatic focussing aids are necessary. They have to ensure that the focal plane is within the depth-of-focus (DOF), which is below +/- 0.4 micrometers for the designed objective. Currently, all available sensors in this area show major drawbacks concerning the desired flexibility in being used for different laser processes. Therefore, a sensor, based on the autofocus principle, has been modified and built at the LZH. The results have proven that the resolution and the mean deviation are within the desired range (DOF).
Characterization of the effects of the interaction between short laser pulses and a material has been made in the conditions of surface cleaning and surface micro-machining. Fast heating of the target surface results in a pressure wave that propagates inside the sample and a shock wave in the surrounding gas medium. By monitoring the energy of the shock wave using a microphone and a piezoelectric transducer, it is possible to relate the ablation rate to the observed signals. Experiments have been made using Nd:YAG and KrF lasers. At high fluences, in particular at 1.06 micrometers , gas breakdown drastically alters the process conditions and disperse the recorded data. By using an additional optical beam deflection technique, we have determined the range of validity for monitoring of the process using simple detection devices.
The depth of focus of lenses employed in laser cutting can be extended using aspheric overcorrection. This allows the depth of focus to be enhanced without the need for longer working distance optics. However, as with conventional lenses, the performance of such lenses will depend on the mode quality of the laser machining system. Comparative trials on a range of zinc selenide aspheric lenses at CO2 wavelength, machining stainless steel under inert gas conditions, reveal that the surface finish of the cut edge, is strongly dependent on choice of laser system. The work underlines the urgent need for laser beam measurement techniques which can study the wavefront quality, as well as the beam intensity profile. A measurement technique which samples typically 1 X 10-4 of the main beam is described, which allows non invasive, in-line measurement of the beam intensity, and the wavefront. This provides sufficient information to predict and model the final image quality that is associated with any combination of lens design and laser source.
We present in this work the parameters study in case of laser cladding (stellite 6 onto stainless steel) with a pulsed Nd:YAG laser and sprayed powder. We characterize the clads by geometrical consideration (cross-section) that allows us to obtain different results concerning laser parameters: particularly the pulse energy (fluence) and the pulse duration. The clad starts up above a fluence threshold and the quantity of melted powder reaches a constant value above a pulse duration. This pulse duration value depends directly on the frequency and powder thermal properties. From those results and a study of laser/powder interaction, we develop an analytical model. This model evaluates, from the cross-section, the melted powder quantity.
The heat energy released by chemical reactions was measured in the interaction of 50-ns Nd- laser pulse with titanium, germanium, coke, and sulphur samples placed in different gas media. The samples were exposed to focused laser radiation in the intensity range of 1 - 800 MW/cm2. It was found that the maximum value of the ratio of released chemical energy to the laser pulse energy is approximately 0.2 for Ti irradiated in oxygen as well as for Ge irradiated in air. For interaction of laser pulse with sulphur in air, the chemical energy released in the laser flare is a factor of 2 greater than the laser pulse energy. In 1-atm oxygen gas medium, this factor increases up to the values more than 3 and the continuous burning of the target arises for some shots. The source of chemical energy is the synthesis of sulphur dioxide. Thermal effect of chemical interaction depends on focal spot size at fixed target size.
A vision directed CO2 laser cutting concept for separating a patterned material from a backing net web has been successfully achieved. The concept allows automatic processing of flexible materials at higher feed velocities with improved cut edge quality, when compared with conventional mechanical cutting concepts. This paper presents the aspects of the analysis and test work undertaken to establish suitable parameters for the CO2 laser cutting system of a lace processing machine operating at feed velocities of one meter per second. The issues investigated include analysis of workpiece characteristics and how this determines the laser beam manipulation requirement, power and type of CO2 laser, fundamental cutting performance data, quality of cut ends of the net fibers, optical arrangement for high speed pattern cutting and finally the performance of the integrated vision tracking and laser cutting system.
An excimer laser micromachining system based on material ablation by mask imaging is presented. The system comprises beam delivery and mask imaging optics, on-line workpiece inspection, and computer-controlled workpiece positioning. The use of dielectric optical masks offers high power processing capability. Generation of suitable masks can be performed using the system itself. In this case a pinhole instead of a complex mask is imaged onto the blank (high reflection coating on fused silica). The fluence is adjusted to selectively ablate the layer without damaging the substrate. The mask pattern is generated by computer controlled stage moving and laser operation (laser spot writing). Special care has to be taken to avoid accumulation of ablated particles (debris) on the surface which could cause diminished contrast and promote damage. Using commercial Schwarzschild-type reflective objectives (15X or 25X demagnification) mask areas of about 3 mm X 3 mm can be imaged achieving fluences up to 20 J/cm2 on the workpiece. To improve mask illumination a beam homogenizer can be integrated. Micrometer resolution is achieved: Patterns of lines and spaces with periodicities of 2 micrometers (polymers) and 5 micrometers (ceramics) were formed using this system.
Aluminum is a highly reflective and thermally conductive material that is often difficult to process with laser beams. Oxide films on the surface can prevent surface alloying as with the case of Cr alloying into aluminum by in-situ powder feeding. This paper describes a new technique using mixed excimer laser and CO2 laser beam processing to generate a Cr alloyed layer on a 2014 aluminum alloy substrate. It is shown that improvement of alloying homogeneity following in-situ powder feeding can be achieved with mixed beam processing. Conditions under which this can be achieved have been studied and the affect of laser processing parameters on the surface roughness, oxidation and absorption is discussed. Microstructural and surface morphological analysis using optical microscopy and SEM are also discussed.
This paper demonstrates the technical feasibility and basic phenomena of using laser techniques for the non-contact removal of embedded contamination down to depths of 0.1 - 4 mm thick in construction materials such a concrete, brick, plaster/mortar, stones and stainless/mild steels. In this study a high power CO2 laser and a YAG laser were used. The techniques investigated include laser vaporization removal, laser combustion/decomposition removal, laser melt ejection removal, laser thermal fracture removal and laser HAZ delamination removal. The work showed that melt ejection removal can be applied to metal objects with removal depth up to 1.5 mm/pulse while the other four methods are more effective for nonmetallic materials with removal depth up to 3 mm for each pass. Particularly when hydraulic bond materials such as concrete, cement, mortar/plaster, rendering and stones are involved the thermal fracture and HAZ delamination methods were found very effective. Paint, epoxy, and plants such as moss and lichen on the construction bodies can be removed effectively by laser generated combustion/combustion. One of the advantages of laser contamination removal is the energy controllability, remote operation capability (convenient for nuclear decontamination), low waste and high efficiency. In addition amorphous glazing can be generated on the surfaces of construction materials such as bricks and concrete during vaporization removal providing a means of sealing the remaining surface. Optical microscopy, SEM, EDAX, and x-ray diffraction were used to study the affects of laser treatment under various conditions. Mathematical representation of the processes were discussed. Comparison was made between the methods and optimum operating condition provided.
Titanium nitride and titanium carbide films were deposited on silicon substrates by XeCl excimer laser reactive ablation of titanium in nitrogen and methane atmospheres, respectively. A series of 10,000 pulses at the fluence of approximately 5 J/cm2 and repetition rate of 10 Hz were directed to the target. The pressure in the chamber was fixed, during every irradiation series, at a given value within the range 6 X 10-4 - 10 mbar of N2 or CH4. Very flat films with thickness exceeding 1 micrometers were deposited. The structural characteristics of the deposited films were investigated by Rutherford backscattering spectrometry, scanning electron microscopy, and by x-ray diffraction. Under specific experimental conditions very pure nitride films were deposited.
In this paper we describe the results of a complete laserobot characterization. The laser system considered consists of a 5.0 kW CO2 laser source and an anthropomorphic robot with an internal path for the beam. The focused laser beam diameter, the best focus position, the beam quality factor M2 was compared, in regards to two laser output windows and two kinds of mirrors inside the robot. The influence of the beam parameters, obtained in the different conditions, on the laser process has been analyzed considering penetration tests on steel sheets.
New manufacturing technologies such as laser cutting, laser caving and laser marking have highly evolved in recent years. If it comes to the point where digitized data are needed for rapid prototyping, small batches, etc., either time consuming scanning methods or programming `by hand' are needed. New developments relate image processing methods to these tasks. Applied to laser cutting and laser caving applications the features and the usefulness of vision technologies are described. Especially the complex manufacturing process `laser caving' highlights the applicability of image processing technologies.
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.
Diffractive optical elements for high-power lasers offer interesting applications in addition to conventional refractive and reflective optics like lenses and mirrors. Their function is based on diffraction at small surface structures in the scale of the wavelength. This paper tries to assess realistically the application potential of diffractive optical elements for industrial high-power lasers. These are excimer lasers ((lambda) equals 193 ... 308 nm), Nd:YAG solid-state lasers ((lambda) equals 1.06 micrometers ) and CO2 lasers ((lambda) equals 10.6 micrometers ). After a short description of both the physical fundamentals of diffractive optical elements and their production processes this paper mainly deals with new methods of beam forming and guiding by diffractive optical elements as well as the integration of these optics into laser systems. Hereby the problems arising from high-power densities as well as the possibility of increasing the efficiency of the beam guiding system are stressed.
An optical surface monitoring system is presented which allows us to detect in an on-line modus the actual surface shape of a coolant-driven adaptive mirror. The system consists of a laser diode and a position sensitive diode (PSD) which detects the deflection of a probe beam depending on the actual surface slope of the controlled adaptive optic. Thus, a direct relation between sensor signal and active element (surface) is realized. The differences to indirect sensor signals (coolant pressure, strain gauge) are shown. In particular, the effects of laser induced surface deformation and of mechanical tolerances on the resulting surface shape are discussed. This surface control will be integrated in a system of flexible beam expanders which allows a complete control of the focal diameter and of the focal position of the spot, independent of the distance between laser and work station. Basically two beam expanders have to be used, each of them containing one adaptive copper mirror. The first expander, located close to the laser, allows us to control the beam diameter on the focussing optic. In the focussing optic itself the second adaptive mirror controls the focal position. The interaction of these independent adaptive elements allows a variation of the focus radius up to 1:5 while the focal position can be changed independently in a range up to 10 mm around the geometrical focus position.
The cutting of patterned edges of lace fabric using a machine based purely on mechanical concepts has fundamental problems. This process imparts forces into the workpiece causing local deformation of the fabric which reduces the cut edge quality and leads to bunching up of the fabric around the pattern following guide and the cutting blade. The process requires constant operator involvement and reduction in processing speeds when cutting complex patterns. It is difficult to apply to elastic fabrics. A non-contact cutting concept for separating the lace pattern from the backing net using a vision directed CO2 laser is described. This approach removes the problems associated with the mechanical lace cutting machines and allows automatic processing at higher feed velocities with improved cut edge quality. The high fabric transport speeds of one meter per second place significant demands on the vision system for sensing the real time position of the material pattern. Also, there is a high speed, high accuracy requirement for the laser beam manipulation system. The design approach for the integrated high speed laser cutting system and bulk lace material handling system is described. A proof of concept demonstrator is presented and results of initial tests are discussed.
The seam-following system SCOUT precisely positions a robot or gantry ready for laser welding -- at an accuracy of +/- 0.05 mm on three Cartesian axes (X, Y, Z) -- and controls the three cardanic angles of orientation (A, B, C) in space. SCOUT automates high-speed bonding and sealing processes. Image processing at 50 Hz provides for feedrates of more than 20 meters per minute. Due to the `online' programming feature the conventional time- consuming preprogramming to define the robot's track prior to processing and for each modification of the workpiece has now become superfluous.
This paper describes a system based upon an optical stereo sensor for measurement of the gap position and the joint gap width in laser welding. The measured width is used to control the filler wire speed. By means of sensor controlled filling of the missing material into the joint gap, manufacturing tolerances can be balanced and at an even seam quality, weld defects can be avoided. The sensor system is integrated into the work table control system thus allowing seam tracking based on the measured joint path. The stereo sensor consists of two lenses each featuring a plane deviating mirror by means of which the joining point is imaged upon each half of a CCD-line sensor. The image plane of the CCD-sensor and the principal plane of the lens system are running parallel and are positioned vertically towards the mirror surface thus avoiding distortions. In order to suppress plasma radiation which occurs during laser welding, the optics are shielded in the nearby infrared zone by means of a band-pass filter and the joining point is illuminated with a laser diode featuring a respective wavelength. A transputer net, coupled to the stereo camera and the wire feed system through a high-speed serial interface, serves the purpose of image interpretation and it shares a memory location with the work table control system for data exchange.
The use of adaptive optics in the beam delivery system of CO2 laser machines is known as a powerful method to optimize the processing results. Focus radius and position can be controlled independently of one another by the appropriate choice of the adaptive optics focal lengths. The improvement of the processing results by the compensation of varying beam path lengths is discussed for CO2 laser beam cutting. A new method for the automatic adjustment of focus parameters to the demands of the process is shown for the examples of cutting mild steels and stainless steels.