Solder reflow boost for photonic component alignment

Technology News |
By Nick Flaherty

Researchers at Fraunhofer IZM are developing a cost-efficient solder reflow method for the coupling of optical components for photonic encryption systems.

The €15m Silhouette project is working on a modular standardized platform with an electro-optical interposer as the interface turning electronic into optical signals, transmitting it to its intended recipient, validating it, and translating it back into electronic signals.

A team at Fraunhofer IZM is developing and integrating the microelectronic and optoelectronic components of the interposer. Wirking with Osram Opto Semiconductors of Regensburg and the quantum optics developer and distributor qutools of Munich, they chose optical waveguides to channel the light signals, rather than lenses or mirrors, as a route to further integration.

However the challenge of this approach is the precision needed to couple the waveguides for lossless transmission.

 “The waveguides used on the optical ICs are extremely tiny with a diameter of less than one micrometer. When you get to such tiny dimensions, any active alignment of the components would be far too costly and labor intensive. That is why we are developing passive options as an alternative, which means that the photonic circuits could be mass produced at lower cost, which makes them a real choice even for smaller enterprises,” said Dr Hermann Oppermann who is leading the team.

The process uses self-alignment of the components with customised cavities in the interposer. These cavities measure around ten micrometers and provide a type of mechanical stop in the system.

Solder bumps are placed in each cavity are used to align the structures. When the solder is melted, it catches the laser diode of the integrated waveguide and pulls it into place by surface tension. First tests show that this solder-driven self-alignment managed to move several hundred diodes into the target location in one go. Used in industrial production, this parallel mass reflow of laser diodes could lower the production costs for the interposers by five percent.

In a first step, the designs of the individual components are carefully aligned with each other, and the necessary electrical connections developed.

The project runs to 2024 with plans for a demonstrator model assembled, and the coupling efficiency tested and known for the entire system. First indications suggest that the integration of an interposer in the key generators and analog multipliers represents a major leap forward for encrypted communication and, by implication, more reliable data security.

Alongside the Fraunhofer Institute for Photonic Microsystems IPMS (Dresden) as coordinator, the Fraunhofer Institute for Reliability and Microintegration IZM (Berlin) is represented by its IZM-ASSID unit from Moritzburg and the project also includes the Fraunhofer Institute for Telecommunications, Heinrich Hertz Institute HHI (Berlin), the Institute of Electronic Packaging Technology (IAVT) and the Integrated Photonic Devices (IPD) Group of the Technical University of Dresden.

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