A highly tunable electro-optic isolator for signal routing in photonic integrated circuits

Researchers in University of Illinois Urbana-Champaign’s Grainger College of Engineering have developed an electric-optic isolator for unidirectional light propagation. The compact, low-power and highly tunable design makes it ideal for optical signal routing in data centers.

Integrated photonic devices — tiny circuits that use light instead of electrons — are becoming increasingly important for scalable photonics technologies and for high bandwidth communications. They are particularly valuable for managing low power light-based data transfer inside data centers, which are needed for artificial intelligence, cloud computing and high-performance signal processing. A major challenge for the field is to develop approaches that force light to propagate only in one direction within a photonic circuit, since this can improve robustness to manufacturing defects, protect laser sources and impart more stability to optical signals within the system.

Researchers at the University of Illinois Urbana-Champaign’s Grainger College of Engineering have developed a photonic integrated circuit that functions as a linear optical isolator, allowing light to pass through only one way with extremely low loss, while blocking almost all light propagating in the opposite direction. Their design is inspired by an atomic physics phenomenon called Autler-Townes splitting, which they emulate within the photonic circuit using electrooptic modulation.

“Currently, the best commercial optical isolators are built using magneto-optic materials,” said project lead Gaurav Bahl, a professor of mechanical science and engineering in Illinois Grainger Engineering. “This approach is okay for relatively large-scale photonic systems such as optical fiber networks, but it cannot be used for integrated photonics. Magnetic and magneto-optic materials are not suitable for processing in foundry facilities, these materials also tend to have high optical loss, and their useful properties have a strong wavelength dependence. However, as we continue to scale up our data centers, there is an increasing need for compact and low-power optical signal routing. And so, chip scale integrated isolator devices that don’t use magnetic materials are becoming increasingly important.”

These results have been recently published in the journal Nature Communications.

In prior work, Bahl’s research group developed an “acousto-optic” isolator that emulated Autler-Townes splitting with sound waves, resulting in broken symmetry for light propagation based on its directionality. However, their acousto-optic devices face technical challenges: they are very difficult to fine tune after fabrication, not every device works, and the operation wavelength cannot be easily tuned to adapt to the rest of the photonic circuit.

The new “electro-optic” design uses an electrically tunable material called lithium niobate, with no sound waves involved, circumventing the challenges with the earlier designs. The approach is agnostic to the laser wavelength and can be adapted to suit many different applications. For the present study, the device was designed for the telecom band which is a standard wavelength range used in many optical fiber communication systems. Since nothing in this device needs to move mechanically, a protective cladding layer can now also be added to help protect the device from the environment. This option was not available with the acousto-optic design.

A good figure of merit for an optical isolator is the ratio between the propagation contrast and the forward transparency. The new on-chip device exhibited a figure of merit of nearly 2,000 (about 33 decibels) between forward and backward transmission with extremely low forward loss, which brings it on par with commercial off-chip magnetic isolators.

In addition, the researchers showed that the electro-optic isolator can be tuned over many terahertz, that is, it has many thousands of times more tunability than the previous acousto-optic version. As a result, the isolation function can be adjusted on demand to suit the wavelength that the rest of the photonic system is using.

“As a follow up to this work, we are working on a broadband ‘electro-optic’ isolator that will exhibit nice properties over an extremely wide wavelength range, eliminating the need for tuning,” Bahl said. “Technical innovations such as this isolator can play a key role in solving the big challenges facing nationally critical technologies in AI and computing.”

Gwan In Kim, Violet Workman, Oğulcan Örsel and Jieun Yim also contributed to this work.

The study, “An integrated multi-THz tunable linear isolator based on electro-optic non-reciprocal strong coupling,” is available online. DOI: 10.1038/s41467-026-75451-5

Support was provided by the Defense Advanced Research Projects Agency, the Air Force Research Laboratory, the Office of Naval Research and the Army Research Office.

Illinois Grainger Engineering Affiliations

Gaurav Bahl is an Illinois Grainger Engineering professor of mechanical science and engineering in the Department of Mechanical Science and Engineering. He is also affiliated with the Department of Electrical and Computer Engineering, the Department of Physics, the Holonyak Micro and Nanotechnology Laboratory and the Illinois Quantum Information Science and Technology Center. He holds George B. Grim Professor and Kritzer Faculty Scholar appointments.

Above: Photograph of integrated electro-optic isolators on a lithium niobate chip. Image credit: Gwan In Kim.