Reinhart Poprawe

Laser Machining by short and ultrashort pulses, state of the art and new opportunities in the age of the photons

An overview is given of the applications of short and ultrashort lasers in material processing. Shorter pulses reduce heat-affected damage of the material and opens new ways for nanometer accuracy. Even forty years after the development of the laser there is a lot of effort in developing new and better performing concepts of lasers. The driving force is higher accuracy at reasonable cost, which is realised by compact systems delivering short laser pulses of high beam quality. Another trend is the shift towards shorter wavelengths, which are better absorbed by the material and which allows smaller feature sizes to be produced. Examples of new products, which became possible by this technique, are given. The trends in miniaturization QS predicted by Moore and Tanigychi are expected to continue over the next decade too thanks to short and ultrashort laser machining techniques.

White-light frequency comb generation with a diode-pumped Cr:LiSAF laser

We have created a broad spectrum spanning more than an optical octave by launching femtosecond pulses from a battery operated Cr:LiSAF laser into a photonic crystal fiber. Despite the massive broadening in the fiber, the comb structure of the spectrum is preserved, and this frequency comb is perfectly suited for applications in optical frequency metrology.

Manufacturing with novel high-power diode lasers

Direct applications of high-power diode lasers (HPDLs) like hardening, heat conduction welding of metals, and joining of polymers have already been demonstrated also in the industrial environment. Relatively low intensities in the range of 10/sup 3/ W cm/sup -2/ are sufficient for these applications. While the commercial HPDL systems are built on the basis of diode laser bars with 40 W output power, in the meantime a record continuous-wave-output power of 267 W per bar has been demonstrated. The achievable higher output power per bar will lead to enhanced applicability of HPDLs and thus to a further steep increase of their industrial use. Improved packaging technology, multiplexing the emission of single bars and coherent coupling as well as promising new diode laser structures like Z-shaped broad area emitters, is discussed. In this paper, emphasis is laid on the potential applicability of commercial HPDLs for metal working with elevated intensities up to 10/sup 5/ W cm/sup -2/, like oxygen cutting and the worldwide first deep-penetration HPDL-weld up to a sheet thickness of 6 mm in stainless steel. These results have been predicted by proper theoretical modeling. Strong reduction of phase space dimension takes place in convective-diffusive-type free boundary problems typical for thermal processing. This property makes it possible to construct approximate finite-dimensional dynamical systems being solvable with controlled error. Numerical solutions of the full problem are used to investigate the quality of the approximate model. Observable quantities of the technical processes like signals from monitoring devices are part of the solution and solutions to the inverse problem are given.

Locally selective bonding of silicon and glass with laser

Abstract A novel method for joining silicon and glass wafers with laser radiation is described. In order to characterize the locally selective bonding with laser (SBL) process, variations of laser parameters have been correlated with temperature measurements during bonding and the achieved bonding results. It was found that the temperature load outside the laser irradiated zone only lasted for seconds and remained below 300°C. The result of the investigations was a parameter field producing reproducible and strong silicon glass bonds. Basic knowledge for the thermal process of bonding and a understanding of the recognized bond defects was developed. Finally advantages and disadvantages of SBL with silicon and glass are discussed with respect to the anodic bonding technology putting emphasis on the low temperature and locally selective bonding capability of SBL.

Modeling, Monitoring and Control in High Quality Laser Cutting

Abstract Electrical discharge machining (EDM) and laser cutting are thermal separation processes widely used in shaping and contour cutting applications. However, there are gaps in understanding the dynamics of the process, especially issues related to cut quality. This work describes the advances in fundamental physical modeling and process monitoring of laser cutting, as well as time varying processes such as contour cutting. The onset of evaporation and the increase of capillary forces are the two physical phenomena relevant to the build-up of adherent dross. The dynamic model predicts a modulation frequency for the laser power that leads to almost complete suppression of adherent dross in contour cutting.

A multi-kilohertz pinch plasma radiation source for extreme ultraviolet lithography

A pinch plasma source in the extreme ultraviolet is presented where the special design of the electrodes leads to advantages concerning low erosive operation and an effective coupling of the electrically stored energy to the electrode system. Most promising results of the source parameters with respect to the demands for extreme ultraviolet lithography are achieved when operating with xenon. Intense emission around 11 nm and 13 nm is observed. The plasma column has a diameter of less than 500 μm when viewed from the axial direction. The electrode design allows for an accessible solid angle of around π sr. The shot-to-shot stability is better than 4% (rms). A maximum output of 0.8 mJ/(sr 2% bw) at 13.5 nm has been observed with an input pulse energy of 2 J. Operation at a repetition rate of 1 kHz and an electrical input power of 2 kW has been demonstrated with an average emitted power of around 0.3 W/(sr 2% bw). Approaches of power scaling into the range which is desired for EUVL will be discussed.

Coaxial process control during laser beam welding of tailored blanks

Production of laser beam welded tailored blanks requires both, high quality processing as well as a quality assurance by reliable monitoring systems for each welded part. Actual quality monitoring systems for tailored blank applications make use of different sensors for seam tracking, seam shape detection and process control. The suitable process diagnostic device is a plasma sensor which detects the optical emission of the weld plasma. Experimental evidence show that the reliability of the seam quality prediction can significantly be improved by using a camera system with a two-dimensional spatial resolution instead of an integrating plasma detector. The improvement is achieved by exploiting the information provided by the spatially distributed intensity of the plasma emission. In particular, by coaxial arrangement of the camera with respect to the laser beam axis, the direction of observation allows to detect significant process characteristics. Based upon these results a coaxial process control system was developed that can be adapted for different laser materials processing applications like welding, cutting and surface treatment. The system consists of a high speed camera mounted directly at the welding head. The optical path of the camera goes coaxial with the laser beam path through the focusing optics. The camera images taken from the process are analyzed using image processing algorithms. The algorithms are chosen depending on the type of application to be monitored. In the case of welding tailored blanks the system can monitor a full penetration of the workpiece, deviations from the desired welding path, seam width, stability of the capillary shape and defects of the seam caused by spatter and ejection of molten material. The camera system offers the ability to perform simultaneously different quality monitoring tasks like determination of seam and capillary shape, seam tracking and process control. Thus the number of sensors required for quality monitoring is reduced to one single system.

Simulation of micro-channel heat sinks for optoelectronic microsystems

Water-cooled heat sinks are investigated both experimentally and theoretically as model systems to simulate the energy and mass transport in devices used for cooling of optoelectronic microsystems. The design of the micro-channel heat sinks results in a decrease of their thermal resistance and of the pressure drop of the coolant allowing an increased heat load of an optoelectronic microsystem such as diode laser.

Microholes in zirconia-coated Ni-superalloys for transpiration cooling of turbine blades

Drillings in zirconia coated Ni-superalloys is done by melt extraction with pulsed laser radiation provided by a Nd:YAG slab laser with microsecond pulse duration. This laser system distinguishes itself by a high beam quality and offers the possibility to investigate drilling of holes with a diameter of 200 micrometer by percussion drilling and trepanning. The quality of drilled holes, e.g. the heat affected zone (HAZ), the recast layer and the conicality, are presented. During drilling different process gases are used. The results in drilling velocities, melt thickness and chemical composition of the melting zone are shown for oxygen, argon and nitrogen by SEM and EDX. A numerical simulation of the trepanning process will be presented. The different time scales of the contributing physical processes related, for example, to the small melt film layer during trepanning are described. A coating is distributed on the multilayer system to protect the blade from recast. Aim of the investigation is the production of holes in a multilayer system, consisting of CMSX-4, VPS-MCrAlY and EB-PVD-zirconia. With this used laser system inclined holes up to 60 degrees in this layer system can be drilled. No recast layer and no spalling of the zirconia-layer are observed.

Reliability and degradation mechanisms of InGa(Al)As/GaAs DQW high-power diode lasers

Defects and degradation of InGa(Al)As/GaAs DQW diode laser bars mounted on copper micro-channel heat sinks were investigated. The analytical techniques used are optical microscopy and scanning electron microscopy. The high-power diode lasers were investigatively accompanied through the different phases of their setup process (i.e. mounting, characterization and burn-in). After the setup a long-term lifetest was performed. Changes in surface morphology in the facet area as well as changes in the threshold current, slope efficiency and emission spectrum are observed. Due to the degradation the threshold current increases and the slope efficiency decreases while the emission wavelengths are shifted to higher values showing a broadening of the spectrum. Formation of micro-cracks and alocations of crystalline areas within the facet area are observed. The influence of degradation on performance and life time of the high-power diode laser bars is discussed.