Oliver Pütsch

Thermo-Optical (TOP) Analysis of Transmissive Elements for Laser- Systems

Increasing laser beam qualities make thermal lensing again a hot topic and demand for a thermo-optical simulation for improving classical ray tracing and enabling optimization possibilities for thermally aberrated optical systems. This paper summarizes the approach for coupling FEM and ray tracing using a weighted least squares approximation algorithm and demonstrates the abilities of the coupled simulation in the case of a CO2 laser system for polishing of glass and plastics. It can be demonstrated that the algorithm can be used for the analysis of higher order aberrations, since the application contains a Gaussian to top-hat conversion lens group which suffers from thermal gradients. Finally, the benefits and further developments of analyzing thermal gradients in optical simulation are being discussed.

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.

Annular beam shaping system for advanced 3D laser brazing

Abstract As laser brazing benefits from advantages such as smooth joints and small heat-affected zones, it has become established as a joining technology that is widely used in the automotive industry. With the processing of complex-shaped geometries, recent developed brazing heads suffer, however, from the need for continuous reorientation of the optical system and/or limited accessibility due to lateral wire feeding. This motivates the development of a laser brazing head with coaxial wire feeding and enhanced functionality. An optical system is designed that allows to generate an annular intensity distribution in the working zone. The utilization of complex optical components avoids obscuration of the optical path by the wire feeding. The new design overcomes the disadvantages of the state-of-the-art brazing heads with lateral wire feeding and benefits from the independence of direction while processing complex geometries. To increase the robustness of the brazing process, the beam path also includes a seam tracking system, leading to a more challenging design of the whole optical train. This paper mainly discusses the concept and the optical design of the coaxial brazing head, and also presents the results obtained with a prototype and selected application results.

Adaptive optical beam shaping for compensating projection-induced focus deformation

Abstract Scanner-based applications are already widely used for the processing of surfaces, as they allow for highly dynamic deflection of the laser beam. Particularly, the processing of three-dimensional surfaces with laser radiation initiates the development of highly innovative manufacturing techniques. Unfortunately, the focused laser beam suffers from deformation caused by the involved projection mechanisms. The degree of deformation is field variant and depends on both the surface geometry and the working position of the laser beam. Depending on the process sensitivity, the deformation affects the process quality, which motivates a method of compensation. Current approaches are based on a local adaption of the laser power to maintain constant intensity within the interaction zone. For advanced manufacturing, this approach is insufficient, as the residual deformation of the initial circular laser spot is not taken into account. In this paper, an alternative approach is discussed. Additional beam-shaping devices are integrated between the laser source and the scanner, and allow for an in situ compensation to ensure a field-invariant circular focus spot within the interaction zone. Beyond the optical design, the approach is challenging with respect to the control theory’s point of view, as both the beam deflection and the compensation have to be synchronized.

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.

Development of an adaptive laser beam shaper

The intensity distribution of a laser beam has a high impact on laser processing applications. In many applications, the emitted light of a laser beam source is transformed from a Gaussian intensity distribution into other intensity distributions with the intent to improve the process quality. Numerous approaches have been pursued for this purpose in the past. The vast majority of these optical systems is static. As a result the laser material processing process is limited to a specific intensity distribution. Different systems like membrane deformable mirrors can be used for shaping multiple intensity distributions. However, the control of such systems is complex and requires a deep understanding of the underlying operating principle of the specific mirror system. In this paper a new approach for active beam shapers made of catalog components, like spherical and cylindrical lenses, is introduced. Two optical systems for active beam shaping are designed which can change between two different intensity distributions by moving an individual spherical/ cylindrical lens along the beam path. One system forms a laser spot with Gaussian like intensity distribution and a TopHat shaped intensity distribution respectively. The second optical system is capable of forming a laser spot with Gaussian intensity distribution and a homogenous line shaped intensity distribution respectively. Also the mechanical housing for these optical systems is presented.

Integrated optical design for highly dynamic laser beam shaping with membrane deformable mirrors

The utilization of membrane deformable mirrors has raised its importance in laser materials processing since they enable the generation of highly spatial and temporal dynamic intensity distributions for a wide field of applications. To take full advantage of these devices for beam shaping, the huge amount of degrees of freedom has to be considered and optimized already within the early stage of the optical design. Since the functionality of commercial available ray-tracing software has been mainly specialized on geometric dependencies and their optimization within constraints, the complex system characteristics of deformable mirrors cannot be sufficiently taken into account yet. The main reasons are the electromechanical interdependencies of electrostatic membrane deformable mirrors, namely saturation and mechanical clamping, that result in non-linear deformation. This motivates the development of an integrative design methodology. The functionality of the ray-tracing program ZEMAX is extended with a model of an electrostatic membrane mirror. This model is based on experimentally determined influence functions. Furthermore, software routines are derived and integrated that allow for the compilation of optimization criteria for the most relevant analytically describable beam shaping problems. In this way, internal optimization routines can be applied for computing the appropriate membrane deflection of the deformable mirror as well as for the parametrization of static optical components. The experimental verification of simulated intensity distributions demonstrates that the beam shaping properties can be predicted with a high degree of reliability and precision.

An improved approach for process monitoring in laser material processing

Process monitoring is used in many different laser material processes due to the demand for reliable and stable processes. Among different methods, on-axis process monitoring offers multiple advantages. To observe a laser material process it is unavoidable to choose a wavelength for observation that is different to the one used for material processing, otherwise the light of the processing laser would outshine the picture of the process. By choosing a different wavelength, lateral chromatic aberration occurs in not chromatically corrected optical systems with optical scanning units and f-Theta lenses. These aberrations lead to a truncated image of the process on the camera or the pyrometer, respectively. This is the reason for adulterated measurements and non-satisfying images of the process. A new approach for solving the problem of field dependent lateral chromatic aberration in process monitoring is presented. Therefore, the scanner-based optical system is reproduced in a simulation environment, to predict the occurring lateral chromatic aberrations. In addition, a second deflecting system is integrated into the system. By using simulation, a predictive control is designed that uses the additional deflecting system to introduce reverse lateral deviations in order to compensate the lateral effect of chromatic aberration. This paper illustrates the concept and the implementation of the predictive control, which is used to eliminate lateral chromatic aberrations in process monitoring, the simulation on which the system is based the optical system as well as the control concept.

Electro-optically actuated liquid-lens zoom

Progressive miniaturization and mass market orientation denote a challenge to the design of dynamic optical systems such as zoom-lenses. Two working principles can be identified: mechanical actuation and application of active optical components. Mechanical actuation changes the focal length of a zoom-lens system by varying the axial positions of optical elements. These systems are limited in speed and often require complex coupled movements. However, well established optical design approaches can be applied. In contrast, active optical components change their optical properties by varying their physical structure by means of applying external electric signals. An example are liquidlenses which vary their curvatures to change the refractive power. Zoom-lenses benefit from active optical components in two ways: first, no moveable structures are required and second, fast response characteristics can be realized. The precommercial development of zoom-lenses demands simplified and cost-effective system designs. However the number of efficient optical designs for electro-optically actuated zoom-lenses is limited. In this paper, the systematic development of an electro-optically actuated zoom-lens will be discussed. The application of aberration polynomials enables a better comprehension of the primary monochromatic aberrations at the lens elements during a change in magnification. This enables an enhanced synthesis of the system behavior and leads to a simplified zoom-lens design with no moving elements. The change of focal length is achieved only by varying curvatures of targeted integrated electro-optically actuated lenses.