Lasse Büsing

Design, alignment and applications of optical systems for parallel processing with ultra-short laser pulses

During the last years, the average power of commercial ultra-short pulsed laser sources increased significantly. The efficient utilization of the high average laser power in the field of material processing requires an effective distribution of the laser power onto the work piece. One approach to increase the efficiency is the application of beam splitting devices to enable parallel processing. But the shaping and steering of multiple beams requires particular optical systems which are not state of the art today. Limitations for large spot arrays are evaluated and considered for the design concept of appropriate optical systems. For the purpose of micro structuring with high demands on the spatial accuracy, an optical system based on a diffractive 14×14 beam splitter (DOE) is designed and set up. All partial beams are coupled into a scanner device by using a relay lens system. Furthermore, this relay lens system offers a practicable solution to remove higher diffraction orders of the DOE. Due to the scanner a highly dynamic, simultaneous deflection of all partial laser beams can be achieved. For the alignment and the experimental evaluation of the complex optical system appropriate measurement devices are necessary. The simultaneous determination of several spot positions is realized by a camera system and adapted evaluation software. First experiments of large-area processing metal foils show promising results.

Deformation of ultra-short laser pulses by optical systems for laser scanners

Current experiments of processing glass with ultra-short laser pulses (< 1 ps) lead to scan angle depending processing results. This scan angle depending effect is examined by simulations of a common focusing lens for laser scanners. Due to dispersion, focusing lenses may cause pulse deformations and increase the pulse duration in the focal region. If the field angle of the incoming laser beam is variable, the pulse deformation may also vary as a function of the field angle. By ray tracing as well as wave optical simulations we investigate pulse deformations of optical systems for different scan angles.

High power parallel ultrashort pulse laser processing

The class of ultra-short-pulse (USP) laser sources are used, whenever high precession and high quality material processing is demanded. These laser sources deliver pulse duration in the range of ps to fs and are characterized with high peak intensities leading to a direct vaporization of the material with a minimum thermal damage. With the availability of industrial laser source with an average power of up to 1000W, the main challenge consist of the effective energy distribution and disposition. Using lasers with high repetition rates in the MHz region can cause thermal issues like overheating, melt production and low ablation quality. In this paper, we will discuss different approaches for multibeam processing for utilization of high pulse energies. The combination of diffractive optics and conventional galvometer scanner can be used for high throughput laser ablation, but are limited in the optical qualities. We will show which applications can benefit from this hybrid optic and which improvements in productivity are expected. In addition, the optical limitations of the system will be compiled, in order to evaluate the suitability of this approach for any given application.

Theoretical and experimental analysis of scan angle-depending pulse front tilt in optical systems for laser scanners

Abstract For realising fast and highly dynamical laser-based material processing, scanner systems are already utilised for many different industrial applications. Furthermore, ultra-short pulsed (<1 ps) laser sources provide possibilities of processing most different materials with highest accuracy. Owing to the large spectral bandwidth of ultra-short laser pulses, dispersion in optical components becomes relevant. The dispersion in optical systems for laser scanners may lead to scan angle-depending pulse properties as, for example, pulse front tilt. The investigation of these effects is not state of the art today but absolutely necessary to exploit the full potential of laser scanners for ultra-short pulse applications. By means of an exemplary focusing lens, the simulation and experimental analysis of scan angle-depending pulse front tilt is presented for the first time.

Modeling of optical aberrations due to thermal deformation using finite element analysis and ray-tracing

Thermo-optical simulation is an important extension of classical ray-tracing because many applications, especially in laser technology, have to deal with thermal effects. This paper discusses an approach for modeling thermally induced surface deformations of rotational symmetric optical systems: the discrete deformation data generated by Finite Element Analysis (FEA) are approximated using a global even polynomial which is then transferred to the ray-tracing. The implemented algorithm is validated by comparing approximated data to an analytic deformation function. Finally, the benefit of modeling the temperature dependent refractive index and the thermal deformation is demonstrated using the example of a plastic lens.