Jochen Stollenwerk

Algorithm for irradiance tailoring using multiple freeform optical surfaces

The design of freeform lenses and reflectors allows to achieve non-radially symmetric irradiance distributions whilst keeping the optical system compact. In the case of a point-like source, such as an LED, it is often desired to capture a wide angle of source light in order to increase optical efficiency. This generally results in strongly curved optics, requiring both lens surfaces to contribute to the total ray refraction, and thereby minimising Fresnel losses. In this article, we report on a new design algorithm for multiple freeform optical surfaces based on the theory of optimal mass transport that adresses these requirements and give an example of its application to a problem in general lighting.

High resolution irradiance tailoring using multiple freeform surfaces.

More and more lighting applications require the design of dedicated optics to achieve a given radiant intensity or irradiance distribution. Freeform optics has the advantage of providing such a functionality with a compact design. It was previously demonstrated in [Bäuerle et al., Opt. Exp. 20, 14477-14485 (2012)] that the up-front computation of the light path through the optical system (ray mapping) provides a satisfactory approximation to the problem, and allows the design of multiple freeform surfaces in transmission or in reflection. This article presents one natural extension of this work by introducing an efficient optimization procedure based on the physics of the system. The procedure allows the design of multiple freeform surfaces and can render high resolution irradiance patterns, as demonstrated by several examples, in particular by a lens made of two freeform surfaces projecting a high resolution logo (530 × 160 pixels).

Designing optical free-form surfaces for extended sources

LED lighting has been a strongly growing field for the last decade. The outstanding features of LED, like compactness and low operating temperature take the control of light distributions to a new level. Key for this is the development of sophisticated optical elements that distribute the light as intended. The optics design method known as tailoring relies on the point source assumption. This assumption holds as long as the optical element is large compared to the LED chip. With chip sizes of 1 mm² this is of no concern if each chip is endowed with its own optic. To increase the power of a luminaire, LED chips are arranged to form light engines that reach several cm in diameter. In order to save costs and space it is often desirable to use a single optical element for the light engine. At the same time the scale of the optics must not be increased in order to trivially keep the point source assumption valid. For such design tasks point source algorithms are of limited usefulness. New methods that take into account the extent of the light source have to be developed. We present two such extended source methods. The first method iteratively adapts the target light distribution that is fed into a points source method while the second method employs a full phase space description of the optical system.

Limitations of the ray mapping approach in freeform optics design.

It was previously demonstrated by Bäuerle et al. [Opt. Express20, 14477 (2012)] that the computation of ray paths through the optical system (ray mapping) can be used to design multisurface freeform optical elements creating a prescribed irradiance pattern for a zero-étendue source. The procedure outlined there uses the heuristic step of reducing the ray mappings curl to improve adherence to surface integrability criteria. This Letter formally derives a quantitative estimate for the limitations of this approach in the collimated case and shows the key factors influencing the quality of the final optics.

Laser Carbonization of PAN-Nanofiber Mats with Enhanced Surface Area and Porosity

Here we present a novel laser process to generate carbon nanofiber nonwovens from polyacrylonitrile. We produce carbon nanofabrics via electrospinning followed by infrared laser-induced carbonization, facilitating high surface area and well-controlled hierarchical porosity. The process allows precise control of the carbonization conditions and provides high nanoscale porosity. In comparison with classical thermal carbonization, the laser process produces much higher surface areas and smaller pores. Furthermore, we investigate the carbonization performance and the morphology of polyacrylonitrile nanofibers compounded with graphene nanoplatelet fillers.

Laser-based production of carbon fibers

Carbon fiber reinforced plastics are excellent materials for applications in lightweight constructions in the automobile or aviation sectors due to their 2.5-fold higher specific strength compared to aluminum. However, the high manufacturing costs of carbon fibers are one of the main limiting factors for the exploration of new fields of applications. The precursor fibers mostly consist of polyacrylonitrile which is transformed into carbon fibers in furnace processes. Almost one half of the manufacturing costs can be assigned to the stabilization and carbonization of the carbonaceous precursor fibers. The furnace processes takes up to 2 h and produces high energy costs due to the needed temperatures of about 1500 °C. In this paper, a new laser-based manufacturing process for carbon fibers is presented. The process is developed at the Fraunhofer Institute for Laser Technology (ILT) and shows potential for the implementation of a fabrication process with reduced energy and time costs compared to the conventi...

Hybrid Production Systems

While virtual product development allows great freedom in terms of design, actual development processes are rather restricted. Those boundary conditions are at best hardly possible to exert influence on. Therefore, future research has to focus both on the realisation of the concept of one-piece-flow while simultaneously increasing flexibility and productivity and on the technological advancement. Hence, hybridisation of manufacturing processes is a promising approach, which often allows tapping potentials in all the aforementioned dimensions.

The development of incremental sheet forming from flexible forming to fully integrated production of sheet metal parts

Incremental Sheet Forming (ISF) was devised as a flexible forming process in the 1990s. The basic principle of ISF is that a generic forming tool moves along a tool path and progressively forms a metal sheet into the desired shape. The tool is either moved using CNC machines or industrial robots. Applying CNC technology or robots to sheet metal forming allows for replacing expensive dedicated tooling and for a fast transfer from the CAD model to the formed part. Since its first applications in the 1990s ISF has undergone tremendous developments. Various process variants such as double-sided ISF and hybrid process combinations such as heat-assisted ISF as well as stretch-forming and ISF have been put forward. The present contribution gives an overview of these developments with a special focus on the outcome of the research accomplished within the cluster of excellence “Integrative Production Technology for High Wage Countries”, where the development of fully integrated sheet metal production facilities is envisioned as the next evolution step of ISF. The development of dedicated equipment for hybrid and fully integrated sheet metal manufacturing and specialized CAX environments as well as applications are described to show the potential of the technology.

Laser-based production of thin wear-protection films

Wear-protection coatings are widely applied in the automotive industry to improve tribo-mechanical properties of highly stressed engine components. Today these coatings are produced by physical vapor deposition (PVD) which is a very time consuming production-step. Wet-chemical processes based on nanoparticulate materials have a high potential to be an energy and resource efficient technique to produce functional coatings, as they are free from any need for expensive vacuum technology and any other elaborate equipment. The major challenge of the process under investigation in this paper is to adapt a laser-based thermal post treatment to turn the applied dried film into a densely-packed layer with the desired properties. Thus the in-line capable process that is introduced is a key step to obtain protection layers on various substrate-materials with low thermal stability. Short interaction times between laser beam and work piece offer the possibility to achieve the necessary high peak-temperatures in connection with short heat penetration depths.In order to select an appropriate coating material different materials provided by Merck KGaA, Darmstadt are characterized regarding their optical properties by fitting a physical model for the layer permittivity to measured transmittance and reflectance spectra in the wavelength range 250 – 2500 nm. A finite element method is used to simulate the induced temperature-time-profiles at the surface of a coated substrate. Based on the simulation laser treatment experiments are carried out to achieve densification of the films.Wear-protection coatings are widely applied in the automotive industry to improve tribo-mechanical properties of highly stressed engine components. Today these coatings are produced by physical vapor deposition (PVD) which is a very time consuming production-step. Wet-chemical processes based on nanoparticulate materials have a high potential to be an energy and resource efficient technique to produce functional coatings, as they are free from any need for expensive vacuum technology and any other elaborate equipment. The major challenge of the process under investigation in this paper is to adapt a laser-based thermal post treatment to turn the applied dried film into a densely-packed layer with the desired properties. Thus the in-line capable process that is introduced is a key step to obtain protection layers on various substrate-materials with low thermal stability. Short interaction times between laser beam and work piece offer the possibility to achieve the necessary high peak-temperatures in connec...

On the interplay of morphology and electronic conductivity of rotationally spun carbon fiber mats

Carbon-based materials are used as electrode materials in a wide range of electrochemical applications, e.g., in batteries, supercapacitors, and fuel cells. For these applications, the electronic conductivity of the materials plays an important role. Currently, porous carbon materials with complex morphologies and hierarchical pore structures are in the focus of research. The complex morphologies influence the electronic transport and may lead to an anisotropic electronic conductivity. In this paper, we unravel the influence of the morphology of rotationally spun carbon fiber mats on their electronic conductivity. By combining experiments with finite-element simulations, we compare and evaluate different electrode setups for conductivity measurements. While the “bar-type method” with two parallel electrodes on the same face of the sample yields information about the intrinsic conductivity of the carbon fibers, the “parallel-plate method” with two electrodes on opposite faces gives information about the el...