Markus Bohrer

Rf-Excited-CO[sub]2[/sub] Laser With Improved Electrode Geometry

A new laser gas flow design for transversely rf-excited CO2-lasers has been presented at the GCL VII Conference. Due to the imperfect electrode shape the electrical input power was limited by the occurrance of filaments in the discharge. The prensent paper deals with improvements of the rf-electrodes. Metals with high electrical conductivity and low permeability i.e. diamagnetic materials as copper, aluminum, and brass are proper electrode materials. Furthermore, the shape of the electrodes has main influence on the distribution of the current of the glow discharge. The use of small contoured electrodes enhances ignition of the discharge, however, the maximum input power for arc free operation is low Electrodes with a larger outer diameter enable a streamer free discharge at higher input powers. Consequently the electrodes were optimized concerning length, diameter, and alignment to ensure the use of the maximum electrical power available from the generator. The electrode temperature is determined by heat conduction from the discharge tube. Electrodes which are separated from the tube need no cooling. Air cooling with a blower improves the performance of the system.

INFLUENCE OF THE FOCUSING ELEMENT FOR CO2 LASER BEAMS ON MONITORED PROCESS SIGNALS

In typical optical sensor setups for CO2 laser material processing scraper mirrors or beam selectors (ZnSe mirrors) are used to allow sensor positions which have a coaxial path for both the laser beam and the monitored visible or near- infrared signal. Calculations and experiments have shown a vital influence of the focusing element for the laser beam on the measured signal. Critical topics are the distance between focusing element and monitored process, as well as the dependence of focal length on wavelength.

Correlative microscopy including CLSM and SEM to improve high-speed, high-resolution laser-engraved print and embossing forms

The industrial market for processing large-scale films has seen dramatic changes since the 1980s and has almost completely been replaced by lasers and digital processes. A commonly used technology for engraving screens, print and embossing forms in the printing industry, well known since then, is the use of RF-excited CO2 lasers with a beam power up to about 1 kW, modulated in accordance to the pattern to be engraved. Future needs for high-security printing (banknotes, security papers, passports, etc.) will require laser engraving of at least half a million or even more structured elements with a depth from some μm up to 500 μm. Industry now wants photorealistic pictures in packaging design, which requires a similar performance. To ensure ’trusted pulses’ from the digital process to the print result the use of correlative microscopy (CLSM and SEM) is demonstrated as a complete chain for a correlative print process in this paper.

Application of wavelets for online laser process observer

The online system, plasmo process observer, enables observation of plasma created by e.g. laser welding in two frequency bands, visible and near infrared. The system offers the feature of online detection of failures like pores and enables the user to store the measured data in databases. This paper deals with both aspects. First the application of wavelets for data preprocessing as a first step of online classification of failures is introduced. This time scale-analysis is compared to standard algorithms in frequency or time domain like DFT or digital filters respectively. The second part of this paper shows the capabilities of wavelets for data compression. This is necessary due to the large amount of data generated by the system. It is shown how important data can be extracted of noisy signals by using wavelets. The process observer has been successfully implemented in welding and drilling applications for automotive and aerospace industry with a very high recognition rate of all defects.

Welding of aluminium, copper, and brass

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

High power CO2 laser development with AOM integration for ultra high-speed pulses

There is a 500 billion USD world market for packaging expected to grow to a trillion in 2030. Austria plays an important role world wide for high speed laser engraving applications — especially when it comes to high end solutions. Such high end solutions are fundamental for the production of print forms for the packaging and decorating industry (e. g. cans). They are additionally used for security applications (e. g. for printing banknotes), for the textile printing industry and for creating embossing forms (e. g. for the production of dashboards in the automotive industry). High speed, high precision laser engraving needs laser resonators with very stable laser beams (400 – 800W) especially in combination with AOMs. Based upon a unique carbon fiber structure – stable within the sub-micrometer range – a new resonator has been developed, accompanied by most recent thermo-mechanical FEM calculations. The resulting beam is evaluated on an automated optical bench using hexapods, allowing to optimize the complete beam path with collimators and AOM. The major steps related to laser engraving of dry offset printing plates during the full workflow from the artists design to the printed result on an aluminum can is presented in this paper as well as laser characteristics, AOM integration and correlative CLSM and SEM investigation of the results.

Calibration of ultra high speed laser engraving processes by correlating influencing variables including correlative evaluation with SEM and CLSM

Laser engraving is used for decades as a well-established process e. g. for the production of print and embossing forms for many goods in daily life, e. g. decorated cans and printed bank notes. Up to now it is more or less a so-called fire-and-forget process. From the original artist’s plan to the digitization, then from the laser source itself (with electronic signals, RF and plasma discharge regarding CO2 lasers) to the behavior of the optical beam delivery — especially if an AOM is used — to the interaction of the laser beam with the material itself is a long process chain. The most recent results using CO2 lasers with AOMs and the research done with scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM) — as a set for correlative microscopy to evaluate the high speed engraving characteristics — are presented in this paper.

Ultra stable carbon fiber high power CO2 laser with high quality laser beam and AOM implementation

High security printing as well as ultra high precision engraving need laser resonators with very stable laser beams (600 - 800W) especially in combination with AOMs. Based upon a unique carbon fiber structure - stable within the sub-micrometer range - a new resonator has been developed, accompanied by most recent thermo-mechanical FEM calculations. The resulting beam is evaluated on an automated optical bench allowing to optimize the complete beam path with collimators and AOM. Synchronous on-line evaluation with PyroCams and thus knowledge about how to minimize distortions within the nonlinear elements is presented in this paper.

AOM optimization with ultra stable high power CO2 lasers for fast laser engraving

A new ultra stable CO2 laser in carbon fibre resonator technology with an average power of more than 600W has been developed especially as basis for the use with AOMs. Stability of linear polarisation and beam pointing stability are important issues as well as appropriate shaping of the incident beam. AOMs are tested close to the laser-induced damage threshold with pulses on demand close to one megahertz. Transversal and rotational optimization of the AOMs benefits from the parallel-kinematic principle of a hexapod used for this research.

Sensors and controls for intelligent machining

Laserwelding is a most important technique in automatic production lines for high numbers of production units. Visualizing the process accelerates the production, enables controlling the process parameters and reduces waste. We present latest results of an online visualization system in this industrial process. Sensor of different sensitivity, combined with optical materials, new in this field, give information to signal processors for mathematical evaluation. Computed result give immediate response to the system. We used the latest digital signal processing technique to cover the need for quick response within a few microseconds. Thus the system can react and for example exceed the laser power or add material to avoid pores.