Researchers establish principles for transmitting light-delivered data via nonreciprocal circuits

The development of fiber optics technology has been indispensable to increasing the speed at which information is delivered over large distances by relying on light to carry information rather than electricity. Currently, incoming light signals are converted into electrical signals, after which the information they carry is processed. Digital communications and sharing of information would be even faster and more energy efficient if light could be used throughout the entire process, but significant additional advances in integrated optical circuits and light-based computing are still required.

In recent years, scientists have been working on ways to develop and use nonreciprocal optical circuitry—which manipulates light waves so that they are allowed to travel only in one direction—to solve these challenges and improve the ability to process large amounts of information. Nonreciprocal optical circuits can be used, for example, to avoid unwanted reflections that interfere with data transmission and can destabilize on-chip light sources. In a new paper published in the journal Optica, the flagship journal of the Optical Society, researchers at the Advanced Science Research Center (ASRC) at The Graduate Center of The City University of New York (CUNY) lay out a rigorous theoretical framework that clarifies the fundamental principles governing resonant nonreciprocal circuits and resolves some outstanding questions on their potentials and limitations.

The science of studying nonreciprocal optical circuits is in many ways still at its infancy, and significant confusion has arisen in scientific literature regarding what is possible or not possible in systems that break reciprocity and allow one-way propagation of light. Recent papers have argued that nonreciprocal resonant optical circuits may be able to indefinitely store multi-frequency light waves without loss of integrity, enabling devices to process data much more effectively. But the new research from ASRC scientists shows that nonreciprocal circuits provide no advantage compared to conventional systems in overcoming the common trade-off between the time delay that can be imparted on an incoming signal and its frequency bandwidth, a central challenge in modern optical computing systems. Their theory clarifies the underlying principles that govern how light interacts with nonreciprocal devices, establishing the ultimate limits in their performance, and the opportunities that they may realistically provide to enhance their interaction with the incoming signals.

"We were intrigued by recent claims on nonreciprocal devices that appeared too good to be true," said Sander Mann, first author of the new paper and a Graduate Center postdoctoral fellow who works in the lab of Andrea Alù, director of the ASRC's Photonics Initiative and professor of physics at The Graduate Center. "Our theory clarifies the fundamental principles that govern light propagation in resonant nonreciprocal devices, and shows realistic opportunities to use them in ways to improve optical signal transmission, storage, processing and computing."

In addition to providing rigorous, structural bounds on the possibilities of nonreciprocal devices, the theory developed by the ASRC researchers points to several interesting properties of nonreciprocal circuits that may prove beneficial in the transport of light signals, and ultimately improve the speed and efficiency in the processing of data.

"Our group has been working on nonreciprocal light propagation for a few years, and we have been discovering many opportunities offered by these one-way devices," said Alù. "While the phenomenon of one-way transport of light is established, the principles governing it are quite counterintuitive and easily lead to confusion. Our newly developed theory clarifies the opportunities and limits of using nonreciprocal devices to slow light, and we now are looking into ways to operate near the newly derived bounds to maximally enhance the interaction of light with nanoscale devices and nonlinearities."