Researchers at Fraunhofer Institute for Reliability and Microintegration IZM (Fraunhofer IZM) have developed a laser welding process to connect photonic integrated circuits (PICs) to optical fibers without adhesives. Uniquely, the technology can be used at cryogenic temperatures down to 4 Kelvin. The direct quartz-to-quartz connections created by the technology promise more reliable, faster, and cheaper fiber-PIC connections that will enhance quantum technology applications. Extremely low temperatures are necessary to observe quantum effects in action and a great deal of attention and effort is going into the development of cryogenic quantum computing systems. Quantum technology systems implemented with PIC-based modules promise a more compact solution for safe communication and connections in quantum computing, but photonic systems such as these need reliable glass fiber connections. As part of the QWeld project, researchers at Fraunhofer IZM developed a novel laser welding technology for vertical optical connections. It uses the prototype “PIC-Weld” processing unit assembled and installed at the Institute during a previous research project. The facilities made it possible for to form a direct laser-welded connection between glass fiber and silica-glass PICs, which is more durable and thermally robust than conventional adhesive connections. For the QWeld project, researchers created a stable, adhesive-free, optical bonding solution for PICs that could be used at temperatures as low as 4 Kelvin. The project’s focus was on vertical connections. Courtesy of Fraunhofer IZM. The QWeld project uses PICs that were made via a standard CMOS process, but designed with one special feature: a silicon dioxide (SiO2) top layer that is needed for glass-on-glass laser welding. The fiber is connected vertically to the PIC, usually at a specific inclination. During welding, the laser hits the contact point between the PIC and the glass fiber on both sides and creates a material bond between both in only a few seconds, promising a major acceleration of the manufacturing process. Making this possible under the new conditions set for the project — primarily the combination of SiO2 materials and the exacting standards in terms of high-precision alignment — meant that the entire process and the production system had to be refined and redeveloped, including the addition of a local preheater, added alignment features, and sensors. The welded bond is durable, and the process is reproducible and can be automated. “The novel laser welding process uses a CO2 laser to heat up a specific part of the PIC’s SiO2 layer to minimize the temperature difference between the fiber and the PIC during the actual welding,” said Alethea Vanessa Zamora Gómez, researcher at Fraunhofer IZM and coordinator of the project. “The innovative preheater technology is likely to solve all of the current challenge we encounter when bonding fibers to PICs at cryogenic temperatures.” The researchers expect the technology to have applications beyond enabling cooperation opportunities in cryotechnology and quantum PICs, such as in biophotonics, sensor technology, and high-performance lasers.