In many areas, such as the automotive industry, optical sensor technology is becoming increasingly important. With the accompanying increase in demand for optical sensor technology, the requirements for corresponding optical systems are also increasing, e.g., to enable driving assistance systems. For this reason, the 4th Physics Institute of the University of Stuttgart (PI4) has been researching the manufacture of complex micro-optics for almost a decade (see Fig. 1 and ). For example, it has succeeded in creating close-up and wide-angle images with just one sensor by fabricating different optics directly onto a CMOS sensor.
The 2-photon polymerization (2PP) 3D printing process can be used to produce such complex lenses. In this process, a highly focused laser beam cures individual 3D pixels (so-called voxels) in a special resin. This allows for the creation of structures in layer by layer, 2.5D procedure (see Fig. 2). A very "big" advantage here is the achievable micro- to nanometer resolution. Although the 2PP process makes it possible to flexibly and individually produce micro-optics with high demands towards surface quality and optical design, the printing process of "larger" optics (e.g., 1 mm diameter) often takes several hours.
This is precisely where the research project entitled "Femtosecond 5D printing on free-form surfaces with submicrometer precision for optical sensors" (short name: 5D Lens Printing) comes into play. The project, funded by the "Innovations Campus Mobility of the Future" (ICM), is investigating an approach that should make the fabrication of micro-optics more productive using 2PP. Specifically, inexpensive glass hemispherical lenses (so-called spherical lenses) are to be used as the starting point for the printing process. These then serve as a blank onto which the complex lens geometry (the so-called asphere) is printed (see Fig. 3). This compensates for the non-ideal imaging properties of the spherical lens, saves printing time and material, and retains existing advantages of the manufacturing process.
However, the project is associated with a number of problems:
1. When writing on the blank, the highly focused and therefore conically shaped laser beam is refracted in some areas by the blank itself (see Fig. 4). At present, it is not clear how this refraction of the laser beam by the blank affects the printing process and thus the optical properties of the printed micro-optics. One approach to address this problem is to continuously tilt the blank during the printing process.
2. The continuous tilting of the blank in the printing process requires that the conventional 3-axis printing process (2.5D), is extended to 5 axes ("5D") by adding two rotary axes. Since there are no 2PP printing systems with 5 axes available on the market, one must be created.
3. In addition to the development of a 5-axis printing system, a tool for the 5-axis path planning of the 2PP printing process must also be created. In the first step, this shall be tuned to the specific application of printing lens blanks.
Problems 1-3 fall within the scope of expertise of the Institute of Control Engineering, which has many years of experience with different additive manufacturing processes and their multi-axis application. The wealth of experience covers machine engineering, machine commissioning as well as the development of the associated software components.
4. After the development of the 5-axis printing process, it must be validated by means of print trials. This is done in cooperation between the ISW and the PI4.
5. In parallel to steps 1-4, a sensor-based application for aspherically printed lenses is to be developed. The goal here is to develop the complete application from sensor design to data processing. The development is carried out by the Institute for Information Processing Technology (ITIV) of the Karlsruhe Institute of Technology (KIT). Here, they can apply their extensive knowledge in the field of optical sensor technology, design and simulation of micro-optical systems and methods of artificial intelligence.
1. Development of a 5-axis 2PP printing system.
2. Development of a path planning tool for 5-axis printing of micro-optics using spherical blanks.
3. Validation of the 5-axis printing process.
4. Development of a sensor-based application for aspherically printed lenses.
 Ksenia Weber, Zhen Wang, Simon Thiele, Alois Herkommer, and Harald Giessen, "Distortion-free multi-element Hypergon wide-angle micro-objective obtained by femtosecond 3D printing," Opt. Lett. 45, 2784-2787 (2020)
 D. Kurth, S. Ristok, S. Ruehle, A. Verl, H. Giessen, Multi-axis two photon polymerization machine and software concept for the manufacturing of aspheric lenses on non-planar substrates, 16th CIRP Conference on Intelligent Computation in Manufacturing Engineering - CIRP ICME '22 Virtual Conference, 13-15 July 2022, Procedia CIRP, lsevier, ISSN: 2212-8271, (in print).
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