Kyocera and Tohoku University integrate optical isolators into silicon photonics chips

Kyocera Corporation and the Research Institute of Electrical Communication (RIEC) at Tohoku University have developed a new technique for integrating optical isolators directly onto silicon photonics chips using a method called laser annealing, a localized, laser-based heat treatment process. An optical isolator is a component that stabilizes optical communication by letting light signals travel in only one direction, reducing unwanted signal reflection. The results were published on September 2, 2026, in IEEE Access, a journal published by the Institute of Electrical and Electronics Engineers.

The development responds to the growing demands generative AI and related technologies are placing on global data networks. Silicon photonics, technology that builds optical circuits on silicon and uses light rather than electrical signals to transmit and process information, offers a path toward faster, more energy-efficient data centers. Growing adoption of silicon photonics is fueling demand for co-packaged optics (CPO), which integrate optical and electronic circuits within the same chip package to minimize signal paths, reduce signal loss, and cut power consumption.

In optical circuits, light that reflects back toward its source laser degrades performance. Optical isolators solve this problem by suppressing reflected light, but building the next generation of compact, integrated optical circuits with CPO technology requires isolators that can be fabricated directly onto the silicon photonics chip itself.

Optical isolators typically rely on a crystalline material called magneto-optical garnet, which needs to be heat-treated at roughly 600°C or higher to function properly. However, heating an entire silicon photonics chip to that temperature risks damaging its electrodes, wiring, and other on-chip components. Finding a way to selectively heat only the magneto-optical garnet material, without overheating the rest of the chip, has remained a significant technical challenge.

To solve this, Kyocera and Tohoku University developed a monolithic integration technique, meaning multiple functional components are built together directly on the same substrate, that fabricates optical isolators directly onto silicon photonics chips through laser annealing, applying a laser beam locally only to the specific areas that need heat treatment. The two organizations demonstrated that the resulting device actually works as intended.

The technology offers two key features. First, localized laser heating allows the magneto-optical garnet to be heat-treated without damaging the rest of the silicon photonics chip. A near-infrared laser beam is applied only to a small optical isolator region, roughly 700 by 700 microns, where the magneto-optical garnet film has been deposited, causing it to crystallize with the properties needed to suppress reflected light. Unlike conventional methods that heat an entire chip inside a furnace, this localized approach largely eliminates thermal impact on the surrounding optical circuits and electrodes.

Second, the team tested the resulting optical isolator directly on a silicon photonics chip. Using this technique, researchers built a device that relies on light interference and experimentally confirmed it functions as an optical isolator. They compared the optical output of forward-traveling signal light against backward-traveling, reflected light, and confirmed an isolation ratio of 13.6 dB within the optical communication wavelength range, corresponding to roughly a 95% reduction in back-reflected light. Electron microscopy also confirmed that the magneto-optical garnet in the laser-treated area had crystallized properly on the silicon waveguide.

Kyocera and Tohoku University have previously collaborated on related optical isolator technologies designed for integration onto optical circuits. To bring this latest technology to commercial use, the two organizations aim to further reduce signal loss, improve efficiency, and enhance manufacturability for mass production. They plan to continue their collaboration going forward, contributing to the development of a more efficient and sustainable information society.