FAC lenses for laser systems are a key component in many innovative technologies. They are already used in the aerospace industry, medical technology, photovoltaics, display manufacturing, and materials processing, but batch sizes are expected to see another major increase thanks to the use of the lenses in additive manufacturing (3D printing) and LIDAR systems (laser-based Light Detection and Ranging). The LIDAR systems for autonomous driving are a component that is particularly important to safety, and are also used in harsh ambient atmospheres. Aside from high quality, they also need to deliver reliable operation with long-term stability. The only way to meet these requirements is with laser systems that utilize high-end glass micro-optics.
The component most critical to the quality of these systems is the fast-axis collimator lens (FAC lens), which is manufactured from high-grade glass and has an acylindrical surface. However, currently available production methods have yet to demonstrate that they are capable of producing quantities on the order of several tens or hundreds of millions of pieces at a consistently high optical quality, and at attractive prices. Now, LIMO says it has succeeded in making the process for manufacturing FAC lenses that provide the demanded quality suitable for high-volume production on 140x140mm wafers. The company already has larger a wafer generation on its roadmap.
In the glass molding process that has typically been used up to now, glass lenses are processed at high temperatures of approximately 600 to 800°C. However, in order to produce lenses of high optical quality, the glass must be heated and cooled slowly. Considerable precision must be used in configuring the cooling process, in order to prevent undesired stress in the material. In addition, where larger glass areas are used, temperature fluctuations occur during the heating process, and these fluctuations can only be minimized by implementing very costly process-control measures. Because of these limitations, this “hot” production method has a number of drawbacks where large quantities are involved, and is not fully scalable.
The unique feature of the LIMO method is that the glass lenses are produced at room temperature. The company was able to increase the wafer size while also reducing the cycle time per wafer to under 60 minutes. The advantage of this “cold” method is that, since the glass is processed at room temperature, a higher production rate has no adverse impact on the quality of the lenses. LIMO is the world’s only manufacturer to have mastered this technique in high-volume production.
“We have the ability to process 140-mm and larger glass wafer discs without stress, in large quantities. To do so, we make all of the tools and nearly all of the machines used for the fabrication process in-house. This gives us maximum flexibility when it comes to the geometries and the design of the lenses. To ensure extremely high lens quality in high volume production, we have developed an automated high-precision grinding process. There is nothing else like it in the world,” says Dirk Hauschild, Chief Marketing Officer (CMO) at LIMO GmbH.
Using the cold grinding and polishing method developed by LIMO for cylindrical lens arrays, up to 20 thousand lenses can be processed simultaneously in a single step with a wafer edge length of 140mm. The method makes it possible to freely choose from refractive acylindrical lens shapes, which can be symmetrical and asymmetrical as well as wavelike or non-periodic optical elements.
In a single grinding process, up to several millimeters must be removed from the glass surface with an accuracy of just a few nanometers. The lenses must also undergo final polishing as a last process step. The high-grade surface quality is achieved by areal polishing, which further increases the durability and stability of the glass surfaces. LIMO’s goal for 2019 is to manufacture over 5 million FAC lenses (up from a 3 million FAC lenses yearly capacity today). The FAC lenses such as the FAC 300 can be designed from a high-index materials (> 1.8) in a standard focal length ranging from 160 µm to 3000 µm with a low-absorption coating (VIS to FIR). They have a transmission greater than 99% across a broad spectrum (e.g., 770–1070 nm).
LIMO – www.limo.de