MiliLas

Production of micro lenses by means of lasers

Aim of the development

The goal of the research project was to develop and qualify flexible, laser-based manufacturing technologies for monolithic microlens arrays made of glass that meet technical and economic requirements, even on a small-series scale. Existing manufacturing methods generally require lithography masks, which are very expensive and only pay off in large quantities. In addition, these methods take a very long time; especially if very deep lens profiles (~50 µm) are to be etched into the glass. In the first manufacturing step, a 2.5D machining process should be developed to create lenticular surface profiles in glass surfaces. The shaping process is characterized by layer-by-layer (= 1 µm), selective material removal using ultrashort pulse lasers. For this purpose, material-adapted laser and process parameters for the ablation of thin glass layers had to be determined and transferred to the generation of lens-shaped surface structures with low roughness. In the second manufacturing step, a polishing process was developed to achieve high optical transparency on the lens surface again after the ablation process. In this process, a micrometer-thin layer close to the surface had to be smoothed by remelting using a CO2 laser. For this purpose, material-adapted temperature regimes for low-stress processing as well as suitable laser and process parameters for laser polishing of profiled glass surfaces had to be determined. Due to the high optical transparency and the good mechanical, chemical and thermal resistance, silicate materials (optical glasses such as borosilicate and quartz) for demanding operating conditions were to be used as substrate material in the project. In addition to the development of the entire process chain from the 3D model to the testing of the manufactured microlens arrays (see Figure 1), other focal points of the project included the manufacture of microlens arrays with different lens arrangements and in different materials as well as the characterization of manufactured microlens arrays and the evaluation of process stability. In addition to microscopic measurement of the lens surfaces, an optical test rig was to be set up for application tests in order to investigate reproducibility and functionality on the basis of small functional sample series.

Advantages and solutions

The overall goal of the research project was to develop and qualify flexible, laser-based manufacturing technologies for the fabrication of microlens arrays made of glass. To achieve this goal, several intermediate goals had to be met. One focus of the development work was the laser ablation of thin glass layers (approx. 1 µm) and the achievement of polishable surfaces of low roughness. Suitable process parameters were determined for all three glass materials (Borofloat® 33, borosilicate glass D263 and quartz glass) and the target parameters of ablation depth and roughness were met. Based on this, the shaping of microlens profiles in glass surfaces was to be realized by means of 2.5D ablation. This goal was also achieved. Another focus in the project was the polishing of the rough, profiled glass surfaces. In order to keep thermally induced stresses in the glass to a minimum, tempering of the glass substrates by means of a temperature-controlled surface heater was successfully implemented. Furthermore, suitable temperature regimes and process parameters for the laser polishing of borofloat and quartz glass could be determined in order to avoid deformation of the lens profiles on the one hand and to achieve the lowest possible roughness and sufficient transparency of the glass surfaces on the other hand. The scatter of the target values is still somewhat too high after the polishing process and requires further optimization. The stresses in the glass substrates after laser polishing are not critical for the application, so that no subsequent heat treatment is required. Through the production and comprehensive characterization and testing of test sample series, the reproducibility as well as the process reliability could be evaluated. Overall, the results are well reproducible and prove that series production using the laser process is feasible. It was also shown that different lens sizes and lens arrangements can be implemented quickly and easily on the basis of 3D CAD data. Testing of the manufactured microlens arrays showed that they can be used for beam shaping for lighting applications. It is not known whether the quality is also sufficient for imaging applications because sufficiently precise measurement technology was not available for this purpose. However, the microlenses all have the same focal length, focus shape, and focus distance, indicating high quality, as shown in Figure 2. Compared to conventional manufacturing technologies, the developed process is interesting for one-off and small batch production due to its high flexibility and speed, but can be further improved in terms of accuracy to approach the quality level of competing processes and products.

Target market

Microlens arrays (MLA) are comparatively complex optics, but they can perform the function of several larger optical elements. In medical technology, MLA can be used in sensors in endoscopes, to homogenize laser light for hair removal/tattoo removal, or to image laser beams for refractive surgery. In communications, MLA are used for LED/OLED signaling, coupling/uncoupling light into fiber optic cables, miniaturizing imaging optics in smartphones, or homogenizing light for lighting purposes and projectors. In addition, MLA are used in complex optics of satellites, drones or research devices to make complex optical functions available with reduced mass. MLA are thus present in many high-technology products, often without users taking notice. Project development is relevant in that it makes it possible to produce small batches and one-offs at low cost. This was previously not possible due to the lithography processes. The project results enable ifw Jena to produce microlens arrays from glass with variable parameters such as pitch, arrangement and firing mode directly and without major preliminary work for interested partners. In addition, the processing technique consisting of ablation and polishing can also be transferred to other free forms such as off-axis parabolas or segmented optics. The project results have been published several times (see, for example, Kahle, Martin, et al. "Direct, Laser-based Production of Optics." Journal of Laser Micro Nanoengineering 17.3 (2022): 156-161.) to reach as many interested parties as possible. Companies with a need for such optics or further development of the technology are welcome to contact ifw Jena.