5D Lens Printing

Femtosecond 5D printing on free-form surfaces with submicrometer precision for optical sensors

Project funding

Project partner

Background

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 [1]). 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.

Fig. 1: Shown on the left is a schematic illustration of an image sensor with three different lens arrays printed on to it. This allows, close-up and wide-angle imaging with only one sensor.[2].
Fig. 1: Shown on the left is a schematic illustration of an image sensor with three different lens arrays printed on to it. This allows, close-up and wide-angle imaging with only one sensor.[2].

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.

Fig. 2: Schematic representation of the 2PP printing process: The highly focused and cone-shaped laser (red) cures individual 3D pixels - voxels - (whitish) exactly at the focal point. Thus, components can be created layer by layer in the resin (light blue) [2].
Fig. 2: Schematic representation of the 2PP printing process: The highly focused and cone-shaped laser (red) cures individual 3D pixels - voxels - (whitish) exactly at the focal point. Thus, components can be created layer by layer in the resin (light blue) [2].

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.

Fig. 3: The spherical lens blank (light blue hemisphere) on which a drop of photoresist (light blue) is placed is shown on the left. The cone-shaped laser (red) indicates the aspherical contour to be written (dark blue). The finished printed lens is shown on the right [2].
Fig. 3: The spherical lens blank (light blue hemisphere) on which a drop of photoresist (light blue) is placed is shown on the left. The cone-shaped laser (red) indicates the aspherical contour to be written (dark blue). The finished printed lens is shown on the right [2].

Problem statement

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.

Fig. 4: Shown on the left is how in certain areas, the laser (red) is refracted (yellow) by the blank (light blue) when printing on it. The right side shows how the problem can be circumvented by tilting the blank and the laser relative to each other [2].
Fig. 4: Shown on the left is how in certain areas, the laser (red) is refracted (yellow) by the blank (light blue) when printing on it. The right side shows how the problem can be circumvented by tilting the blank and the laser relative to each other [2].

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.

Goals

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.

Results

Here are the first interim results on the research project:

Fig. 5: interim results: Shown on the left is the first attempt to print an aspherical contour on a lens blank with a diameter of 1 mm. The printing process stopped after about 60 %. For illustration purposes, only one half was printed.
Fig. 5: interim results: Shown on the left is the first attempt to print an aspherical contour on a lens blank with a diameter of 1 mm. The printing process stopped after about 60 %. For illustration purposes, only one half was printed.
Fig. 6: Shown are microscope images of a lens aspherization printed with the 5-axis system without a blank. Here, too, only one half of the aspherization has been printed for illustration purposes.
Fig. 6: Shown are microscope images of a lens aspherization printed with the 5-axis system without a blank. Here, too, only one half of the aspherization has been printed for illustration purposes.

Sources

[1] 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)

[2] 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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This image shows Daniel Kurth

Daniel Kurth

M.Sc.

Research Assistant "Mechatronic Systems and Processes"

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