Sound Waves Used to Enhance Optical Communication

A team of researchers from the University of Illinois have shown that sound waves can be used to create ultraminiature optical diodes that are small enough to fit onto a computer chip. These devices, known as optical isolators, may help solve key data capacity and system size difficulties for photonic integrated circuits (the light-based equivalent of electronic circuits), which are used for communications and computing.

Isolators are nonreciprocal or “one-way” devices, analogous to electronic diodes. They protect laser sources from back reflections and are needed for routing light signals around optical networks. Currently, the dominant technology for making such nonreciprocal devices requires materials that alter their optical properties in response to magnetic fields, the researchers said.

There are several problems with using magnetically responsive materials to achieve the one-way flow of light in a photonic chip. First, industry simply does not have good capability to place compact magnets on a chip. But more importantly, the necessary materials are not yet available in photonics foundries. That is why industry desperately needs a better approach that uses only conventional materials and avoids magnetic fields altogether.

Gaurav Bahl, Co-Author

In a research published in the Nature Photonics journal, the team explains how they apply the minuscule coupling between sound and light to provide a unique solution that enables nonreciprocal devices with virtually any photonic material.

However, the device’s physical size and the availability of materials are not the only issues, the researchers said.

Laboratory attempts at producing compact magnetic optical isolators have always been plagued by large optical loss. The photonics industry cannot afford this material-related loss and also needs a solution that provides enough bandwidth to be comparable to the traditional magnetic technique. Until now, there has been no magnet-less approach that is competitive.

Benjamin Sohn, Lead Author

The new device is just 200 by 100 µm in size – about 10,000 times smaller than a centimeter squared – and made of aluminum nitride; a transparent material that conveys light and is compatible with photonics foundries. “Sound waves are produced in a way similar to a piezoelectric speaker, using tiny electrodes written directly onto the aluminum nitride with an electron beam. It is these sound waves that compel light within the device to travel only in one direction. This is the first time that a magnetless isolator has surpassed gigahertz bandwidth,” Sohn said.

The researchers are seeking ways to raise bandwidth or data capacity of these isolators and are assured that they can overcome this obstacle. Once improved, they visualize transformative applications in photonic communication systems, GPS systems, gyroscopes, atomic timekeeping and data centers.

“Data centers handle enormous amounts of internet data traffic and consume large amounts of power for networking and for keeping the servers cool,” Bahl said. “Light-based communication is desirable because it produces much less heat, meaning that much less energy can be spent on server cooling while transmitting a lot more data per second.”

Apart from the technological potential, the researchers cannot help but be captivated by the vital science behind this progress.

In everyday life, we don’t see the interactions of light with sound. Light can pass through a transparent pane of glass without doing anything strange. Our field of research has found that light and sound do, in fact, interact in a very subtle way. If you apply the right engineering principles, you can shake a transparent material in just the right way to enhance these effects and solve this major scientific challenge. It seems almost magical.

Gaurav Bahl, Co-Author

This research was supported by the United States Defense Advanced Research Projects Agency and the Air Force Research Laboratory.