Optical isolators integrated on silicon photonics chips

Optical isolators prevent reflected light from travelling back towards laser sources, helping maintain optical performance. A new technology developed by Kyocera Corporation and the Research Institute of Electrical Communication (RIEC) at Tohoku University enables optical isolators to be integrated directly onto silicon photonics chips using localised laser annealing.

The development addresses a key challenge in integrating optical isolators with silicon photonics. Magneto-optical garnet, a material commonly used in isolators, requires heat treatment at 600°C or higher to achieve the necessary crystalline properties. Heating an entire photonics chip to these temperatures can damage electrodes, wiring and other components.

The new process uses a laser to heat only the areas containing the magneto-optical garnet, limiting thermal exposure to the rest of the chip. This could support more highly integrated silicon photonics and applications such as co-packaged optics (CPO), which combine optical and electronic circuits in the same semiconductor package to reduce signal loss and power consumption.

The technology is particularly relevant to data centre applications, where increasing demand for bandwidth is driving the development of more energy-efficient optical interconnects.

A paper describing these results has been published in IEEE Access.

A near-infrared laser beam is applied only to the optical isolator circuit region, which measures about 700 x 700 μm and contains a magneto-optical garnet film. The localised heating causes the garnet to crystallise, giving it the properties needed to suppress reflected light. Unlike conventional methods that heat the entire chip in a furnace, this method limits the heat exposure of surrounding optical circuits and electrodes.

Using this technology, the research team fabricated a device that uses light interference and experimentally demonstrated its operation as an optical isolator. The team compared the optical output of forward-propagating signal light with that of backward-propagating reflected light. The results confirmed an isolation ratio of 13.6 dB in the optical communication wavelength range, corresponding to an approximately 95% reduction in back-reflected light. Electron microscopy also confirmed that the magneto-optical garnet in the laser-irradiated area had successfully crystallised on the silicon waveguide.

Kyocera and Tohoku University have previously worked together to develop related optical isolator technologies that can be integrated onto optical circuits. To move them towards commercialisation, the two organisations aim to achieve lower optical loss, higher efficiency and improved productivity for mass production.

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