Edgar Willenborg

Laser polishing and laser shape correction of optical glass

Mechanical polishing of glass is a time consuming process especially for lenses deviating from spherical surface such as aspheres. With laser polishing, the processing time can be significantly reduced and the wear of hard tooling can be avoided. Using laser radiation for polishing, a thin surface layer of the glass is heated up just below evaporation temperature due to the interaction of glass material and laser radiation. With increasing temperature, the reduced viscosity in the surface layer leads to the reduction of the roughness due to the surface tension. Hence, a contactless polishing method can be realized nearly without any loss of material or need of polishing agent. In this paper, results for laser polishing of fused silica, BK7, and S-TIH6 are presented with area rates up to 5 cm2/s. However, the results show that the achieved roughness with laser polishing is strongly influenced by the thermal properties of the type of glass. During laser polishing, the glass material is relocated at the surf...

Surface structuring by remelting of titanium alloy Ti6Al4V

Conventional surface structuring processes often share two crucial disadvantages. First, after the structuring process itself, some kind of surface finishing is often needed, based on another technology, which means a substantial additional expense. Second, all conventional surface structuring processes are based on the removal of material, which is wasted without any further use during processing. A new approach of structuring metallic surface structuring by laser remelting (WaveShape). In this process, no material is removed but reallocated while molten. This structuring process is based on the new active principle of remelting. The surface structure and the microroughness result from a laser-controlled self-organization of the melt pool due to surface tension. Up to now, basic research has been focused on hot work steel 1.2343 (AISI: H11), and promising results have been achieved for this material. Current research and development are now seeking to expand the spectrum of processable materials. Since r...

Development of a laser-based process chain for manufacturing free form optics

This paper presents the development of a laser based process chain for manufacturing fused silica optics. Due to disadvantages of conventional methods concerning costs and time when manufacturing optics with nonspherical shape, this process chain focuses on aspherical and free form surface geometries, but it is also capable of producing spherical optics. It consists of three laser based processing steps, which in combination produce the optics. In a first step, fused silica is ablated with laser radiation to produce the geometry of the optics. A subsequent laser polishing step reduces the surface roughness and a third step uses laser micro ablation to remove the last remaining redundant material. Most of the conducted experiments are carried out using CO2 laser radiation, but it is also possible to ablate material with ultra short pulse laser radiation. Besides describing the experimental setup and the mechanisms of the ablation and polishing step, the paper presents and discusses results achieved to date. Although the process chain is still under development, the single process steps already reach promising results for themselves and moreover, first elements are manufactured using the first two process steps together.

Surface structuring by laser remelting of metals

Surface structuring by remelting with laser radiation is a new approach to shape metallic surfaces (“WaveShape”). In this structuring process, surface material is reallocated in its molten state instead of being removed, because the process is based on the new active principle of remelting. The surface structure and the microroughness result from a laser-controlled melt pool due to surface tension. Basic research has been conducted with promising results, especially for the hot work steel 1.2343. Since remelting is a thermally driven process, significant differences between metallic materials were expected due to their thermophysical properties such as thermal conductivity, absorption coefficient, viscosity, heat capacity, etc. Therefore, the presented research soughed to expand the spectrum of processable materials. Within the framework of our investigation, we compared the achieved structure height as well as melt pool dimensions for six different materials. Furthermore, we successfully tested new struc...

Glass processing with pulsed CO_2 laser radiation

Recent results of processing fused silica using a high-power Q-switched CO2 laser source with a maximum output power of 200 W are presented. Compared to the processing with continuous wave laser radiation, the main advantage of pulsed laser radiation is the influence of the light-matter interaction with high laser peak power at small average laser power. An application for the approach presented in this paper is the flexible manufacturing and form correction of optics. This laser-based process is nearly independent of the surface geometry and can even be enhanced by laser polishing and expanded to other glass materials. Hence, the high-power Q-switched CO2 laser source is used to ablate glass material with an ablation rate up to 2.35  mm3/s and also for ablating glass material locally in a vertical dimension down to 3 nm.

Optical set-up for dynamic superposition of three laser beams for structuring and polishing applications.

Structuring by remelting is an innovative approach for structuring metallic surfaces with laser radiation, where no material is removed but reallocated while molten. Based on this remelting principle an innovative structuring technique is investigated, where laser beams are superposed. A melt pool is generated by a cw laser beam with constant feed rate. A pulsed laser is superposed onto the cw laser and evaporates a small amount of molten material and, therefore, generates vapour pressure, which shapes the melt pool surface. The solidification follows this newly shaped surface. For this process a new optical system was designed and built up, which allows the combination of cw and pulsed laser beams.

Laser polishing of lenses of fused silica and BK7

A novel process for polishing lenses made of fused silica and BK7 with CO2-laser radiation is presented. First results on polishing spherical and aspherical lenses of both materials with an adapted processing strategy are presented.

Laser polishing of metallic freeform surfaces

Laser polishing represents a new surface finishing technique for free formed work pieces which offers shorter process times and lower costs in comparison to the predominantly used method of manual polishing. This new technique is based on remelting a thin surface layer by laser radiation. The surface tension of the melt pool induces a reallocation of material, thus creating a polished surface.When freeform surfaces are polished using this method, the angle of incidence is generally not perpendicular to the treated surface. The influence of this angle on the melt pool and the surface roughness thus obtained is investigated. By adjusting the laser power and the tool path geometries depending on the angle of incidence, adequate measures to compensate for the investigated influences are demonstrated.Additionally, this paper discusses the CAM process chain for laser polishing of freeform surfaces. The contour-aligned tool paths are generated on the basis of a CAD-model of the work piece using CAM software for five-axis milling applications. Afterwards, the tool-path data are processed in technology processor software. Comprising an application database, the software calculates all technology specific information according to the developed processing strategies. Data calculated by this software are then transferred to the 3D laser-polishing machine.Laser polishing represents a new surface finishing technique for free formed work pieces which offers shorter process times and lower costs in comparison to the predominantly used method of manual polishing. This new technique is based on remelting a thin surface layer by laser radiation. The surface tension of the melt pool induces a reallocation of material, thus creating a polished surface.When freeform surfaces are polished using this method, the angle of incidence is generally not perpendicular to the treated surface. The influence of this angle on the melt pool and the surface roughness thus obtained is investigated. By adjusting the laser power and the tool path geometries depending on the angle of incidence, adequate measures to compensate for the investigated influences are demonstrated.Additionally, this paper discusses the CAM process chain for laser polishing of freeform surfaces. The contour-aligned tool paths are generated on the basis of a CAD-model of the work piece using CAM software for ...

Active optical system for advanced 3D surface structuring by laser remelting

Structuring by laser remelting enables completely new possibilities for designing surfaces since material is redistributed but not wasted. In addition to technological advantages, cost and time benefits yield from shortened process times, the avoidance of harmful chemicals and the elimination of subsequent finishing steps such as cleaning and polishing. The functional principle requires a completely new optical machine technology that maintains the spatial and temporal superposition and manipulation of three different laser beams emitted from two laser sources of different wavelength. The optical system has already been developed and demonstrated for the processing of flat samples of hot and cold working steel. However, since particularly the structuring of 3D-injection molds represents an application example of high innovation potential, the optical system has to take into account the elliptical beam geometry that occurs when the laser beams irradiate a curved surface. To take full advantage of structuring by remelting for the processing of 3D surfaces, additional optical functionality, called EPS (elliptical pre-shaping) has to be integrated into the existing set-up. The development of the beam shaping devices not only requires the analysis of the mechanisms of the beam projection but also a suitable optical design. Both aspects are discussed in this paper.

Active optical system for laser structuring of 3D surfaces by remelting

The structuring of functional and design metallic surfaces takes full advantage of economic, flexible and fully automated processing techniques. Structuring by laser remelting enables totally new possibilities for structuring with individual textures without any ablation of material or the utilization of harmful chemical etching. The functional principle requires the superposition of three laser beams emitted from two different laser sources. For this process, a new optical system is designed and built up which allows for the combination of cw and pulsed laser beams on a working plain. To maintain a high degree of flexibility and automation the system allows for a high number of degrees of freedom for each individual beam. To take full advantage of structuring by remelting for the processing of 3D surfaces, the optical system needs to be extended. With additional optical capabilities elliptical pre-shaping can be applied to enable robust and reliable processing. The huge amount of degrees of freedom leads to a challenging, complex optical design that is being discussed in this work.