From Prism to Glass Interposer: UNESCO Commemorative Year Marking the 200th Anniversary of Joseph von Fraunhofer’s Death and Glass as a Key Material in Integrated Photonics

In 1826, Joseph von Fraunhofer passed away—the namesake of the Fraunhofer-Gesellschaft and a researcher who never separated science from application, but instead consistently viewed them as inextricably linked. UNESCO has designated 2026 as a commemorative year marking the 200th anniversary of his death. Fraunhofer’s work in optics went beyond the pursuit of scientific knowledge for its own sake: he not only gained significant insights into the nature of light but also developed the optical instruments and manufacturing processes that made precise measurements possible in the first place. One material played a pivotal role in Fraunhofer’s life and work: glass.

Glass is one of humanity’s oldest technical materials and a key material for modern photonic technologies. As part of the commemorative year, we are therefore looking not only at Fraunhofer’s historical research, but also at contemporary developments. To this end, RealIZM spoke with Dr. Gunnar Böttger, a physicist who specializes in micro- and nanophotonics and is an industrial cooperation manager, as well as with Kevin Kröhnert, an electrical engineer and research associate at Fraunhofer IZM, about the »PhotonicLEAP« project (concluded in 2025), applied research, and the use of glass in photonic packaging.

From Apprentice to Pioneer of Modern Research

Joseph von Fraunhofer was born on March 6, 1787, in Straubing, Lower Bavaria. He worked with glass throughout his entire life, which was, in a sense, part of his heritage. His grandfather had already been a glazier, and Fraunhofer probably started helping in his father’s workshop in Straubing at a young age. His father was a master glazier. After becoming an orphan at the age of just twelve, his guardian sent him to Munich to begin a six-year apprenticeship with the mirror maker and glass grinder Philipp Anton Weichselberger.

In 1801, another misfortune would ultimately become the turning point that fundamentally changed Fraunhofer’s life. The young apprentice was trapped beneath the rubble when his employer’s house collapsed. Public interest in the rescue efforts and in the apprentice’s survival brought him to the attention of someone who recognized and nurtured his talent: the influential manufacturer, entrepreneur, and politician Joseph von Utzschneider. In addition to providing financial support, Utzschneider gave Fraunhofer access to mathematical and optical textbooks, which he studied with great dedication on his own. Then, in 1806, after completing his apprenticeship, Utzschneider offered him a position as an optician at the Mathematical-Mechanical Institute in Munich and as a glass grinder at the glassworks in Benediktbeuern.

Joseph von Fraunhofer standing beside a precision spectrometer | © Deutsches Museum

Glass: A Material with a History and a Future Between Optics and Photonics

What connects Fraunhofer’s glass workshop of the early 19th century with the photonic technologies of the present day? At first glance, very little. The instruments have changed, and the applications have multiplied. Joseph von Fraunhofer’s name has remained, even though today’s technical complexity hardly seems comparable to that of his time. »I would certainly be interested to know what he would say about what is possible nowadays«, says Gunnar Böttger. »What Fraunhofer accomplished with simple prisms can now be implemented in an integrated form on a chip«, he continues.

Using glass prisms, Joseph decomposed sunlight into its spectrum and, in the process, made visible the dark absorption lines in the solar spectrum, now known as the Fraunhofer lines. His research on the diffraction of light was based on highly precise glass gratings, which enabled him to determine the wavelengths of light with exceptional accuracy. Today, different wavelengths are deliberately utilized for technical applications, such as data transmission, sensing, or optical information processing. »Instead of conventional glass optics, finely structured waveguides and diffraction gratings are used, allowing light of different wavelengths to be separated and processed directly on an integrated chip«.

Glass prisms were used by Joseph von Fraunhofer for the spectral analysis of sunlight. | © Bernd Müller

PhotonicLEAP: Scalable Photonic Packaging with a Glass-Based Interposer

»The goal of the partners in the PhotonicLEAP research project,« Kevin Kröhnert summarizes, »was to develop a disruptive approach to photonic packaging.«

The central contribution of Fraunhofer IZM was the development of a novel glass-based interposer to serve as a carrier platform for the photonic package. Photonic chips, laser sources, and optical coupling elements were integrated onto this interposer. »What is particularly special about this design is that the optical signals within the system are redirected upward and guided through a transparent cover. In contrast, the thermal and electrical contacting is handled via the underside of the interposer. This allows optical and thermo-electrical paths to be deliberately decoupled«, explains Gunnar Böttger.

A key success of the project was the thermal performance of the glass-based interposer. The researchers were able to demonstrate that heat is efficiently dissipated via copper-filled through-glass vias (TGVs). Reliability studies also confirmed that, despite the differing material properties of copper and glass, these vertical interconnects do not introduce critical stress.

CAD rendering of a populated glass interposer (18 × 22 mm) featuring an indium phosphide (InP)-based photonic modulator chip and integrated laser sources. | © Tyndall

Glass as a Functional Material: From Classical Optics to Micro-Integration

Between 1821 and 1823, Fraunhofer laid the foundation for a new approach to the experimental study of light diffraction. Using precise angular measurements and custom-made diffraction gratings, he was able to determine the wavelengths of spectral colors with unprecedented accuracy by utilizing the spectral lines he had previously identified as reference points. His investigations made significant contributions to the wave theory of light while also opening up new possibilities for the development of optical instruments, such as telescopes. Today, Fraunhofer’s research into the separation of light into different wavelengths is implemented in a miniaturized form on photonic chips, where structured glass and grating elements enable the same spectral separation for modern information processing.

Gunnar Böttger views the growing convergence of electronic and optical data transmission as a paradigm shift. The integration of waveguides, lasers, and optical chips creates new opportunities for transmitting large volumes of data. This development also increases the importance of glass as an integration platform.

A wide variety of glass types are available for photonics and micro-integration, and their optical, electrical, and mechanical properties can be precisely tailored. »This allows us to customize characteristics, such as transparency at specific wavelengths, high-frequency performance, or thermal expansion behavior, to meet specific requirements,« explains Kevin Kröhnert.

Looking ahead, Gunnar Böttger sees particular potential in quantum technologies. The transparency of glass enables optical control and readout of quantum states throughout the entire package, even if it is hermetically sealed. Moreover, optical components, such as waveguides, lenses, and deflection mirrors, can already be integrated directly into the glass, providing extensive design flexibility for future photonic and quantum technology systems.

Two Centuries of Technology Transfer: Fraunhofer’s Vision Lives On

More than two centuries of technological development separate Fraunhofer’s work at the Institute for Mathematics and Precision Mechanics from today’s photonic packages. Yet, the interplay between scientific insight and practical application continues to define the Fraunhofer-Gesellschaft’s identity, viewing research as part of a process that spans the entire journey from initial idea to final application. For instance, the »PhotonicLEAP« project not only generated new approaches to photonic packaging but also established capabilities and processes that can now be directly translated into practice.

PhotonicLEAP – Thin-glass Packaging for Photonic Chip Integration