Precision molding of collimating optics used in laser diodes

Violet and blue single mode laser diodes are responsible for im- portant functions in digital data storage, in metrology and in sensors. Many applications need parallel light in order to transfer data securely over longer distances, e.g. via thin optical fibers. High precision collimating lenses made specifically for blue and violet laser diodes align the light beams as needed.

The aim of the internal Fraunhofer project “Tailored Optics”, being carried out by the Fraunhofer IPT and its partners, is to manufacture biaspherical collimating lenses in large lots using precision molding and newly developed tool inserts. An insert with a deep concave geometry together with a further flat concave insert on the other side is to be used to produce the lenses. The challenge in tool design is to overcome the shrinkage of the lens during the molding process. In the past, numerous trials and post-processing operations had to be carried out with the tool in order to approach the required geometry of the optics after cooling. The Fraunhofer IPT is therefore working on FEM simulations that can precisely predict the effects of contraction on different components. Thanks to the resultant data, it has become possi-ble to produce aspherical tools via a precision grinding process that already take this material contraction into account.

Other research and development activities are being carried out in relation to the manufacture of mold inserts, specifically related to the lenses’ steep flank angles and their gen-erally small diameters. One of the most important aspects during forming is the ability to center the optical surfaces with up to 5 µm accuracy and to keep within a maximum profile deviation of 30 nm RMS.

Array condensers for high performance projectors

Double-sided lens arrays can be used in particularly small, high performance projectors. These array condensers consist of a single, round component with a number of separate lenses, and can significantly shorten the optics in the projectors. As a result, the projector can uniformly illuminate a rectangular field more efficiently.

During the manufacture of the optical elements, it is important that both sides of the glass wafer are centered in rela-tion to each other with a deviation of less than 20 µm. Cur-rently, it is possible to pre- cision mold components with up to 1760 separate lenses as small as 0.4 x 0.7 mm² using tools made of binderless tungsten carbide. The shape deviation of the lenses is less than 200 nm with a surface roughness of less than 4 nm. The Fraunhofer researchers are faced with the challenge of developing very accurate NC programs for the mold inserts in order to produce tools with exactly the profile that is required. During the subsequent molding process, one must pre- cisely center the functional surfaces in relation to each other to ensure that the projector image is uniformly illuminated.

“Tailored Optics” – Economics-oriented strategic alliance for precision optics

As part of the “Tailored Optics” economics-oriented strategic alliance (WISA) , the Fraunhofer IPT is working with four other Fraunhofer institutes on the processes involved in the manufacture of high pre- cision, hot-formed glass optical components. The aim is to develop the molding tools for the replication process and the entire manu- facturing process in terms of the highest possible standards of pre- cision and the lowest possible manufacturing costs. This is key to strengthening the position of European optics manufacturers in mass markets such as the medical technology, laser technology, telecommunication and automotive industries in comparison to the strong competition from Asia.

”Tailored Optics“ project partners

• Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Dresden
• Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Jena
• Fraunhofer Institute for Production Technology IPT, Aachen
• Fraunhofer Institute for Mechanics of Materials IWM, Freiburg
• Fraunhofer Institute for Machine Tools and Forming Technology IWU, Chemnitz