How do you add an optical isolator to a silicon photonics chip without exposing the entire chip to high temperatures? A laser-based process could solve it.
Kyocera and Tohoku University have developed a technology to integrate optical isolators directly onto silicon photonics chips using laser annealing. The method locally heats the material needed to form the isolator instead of exposing the entire chip to the high temperatures normally required.
Optical isolators allow light to travel in one direction while suppressing reflected light travelling back towards the laser source. This helps prevent signal instability in optical communication systems and is becoming increasingly relevant as silicon photonics moves towards faster interconnects and co-packaged optics.
The challenge is that magneto-optical garnet, a material commonly used in optical isolators, needs to be heat-treated at around 600°C or higher to develop the required crystalline properties. Heating an entire silicon photonics chip to this temperature could damage its electrodes, wiring and other components.
The researchers addressed this by using a near-infrared laser to heat only the area where the optical isolator is being formed. The laser was applied to a region measuring approximately 700 × 700 micrometres, where a magneto-optical garnet film had been deposited. This localised heating crystallised the garnet while limiting the thermal impact on the surrounding circuit.
Using the process, the team fabricated a magneto-optical Mach–Zehnder isolator on a silicon waveguide and demonstrated its operation. The device achieved an isolation ratio of 13.6 dB, reducing back-reflected light by approximately 95% in the optical communication wavelength range. Electron microscopy also confirmed crystallisation of the garnet in the laser-irradiated area.
The development could support more compact co-packaged optics, where optical and electronic components are integrated closely to reduce signal paths, losses and power consumption. The researchers now aim to reduce optical losses, improve efficiency and make the process more suitable for mass production.
