Multiple Resonators Can Improve Signal Transmission

Scientists have developed a light wave isolation method for efficient signal transmission that is less costly and more versatile than using a single resonator to induce light waves to travel in just one direction. The new approach does not require an external magnetic field or any other form of external bias for reliable wave transmission, yet it provides highly efficient broad bandwidth isolation.

Schematic of the realized isolator, formed by two coupled nonlinear resonators connected through a delay line. Courtesy of Andrea Alù.

Researchers from the City University of New York and the University of Texas at Austin first showed that any isolator formed from a single nonlinear resonator was limited by insertion loss, isolation, bandwidth and isolation intensity range. They then showed that these limitations could be overcome by combining multiple nonlinear resonators with suitable intensity dispersion.

Using a microwave circuit, the team demonstrated that the combination of one Fano and one Lorentzian nonlinear resonator, and a suitable delay line between them, could provide unitary transmission, infinite isolation, broad bandwidth and broad isolation intensity range. It also showed that a larger number of resonators could be used to further increase the isolation intensity range without diminishing the other metrics of the device.

“We have been working on overcoming reciprocity without magnets for a few years. In the past we have explored using devices with moving or time-changing elements, but these approaches pose other technological challenges,” said professor Andrea Alù.

“In this paper, we show that a nonmagnetic device free of an external power source — thanks to suitably tailored nonlinearities — can dramatically break transmission symmetry and realize efficient broadband isolation.”

Researcher Dimitrios Sounas said, “Our breakthrough was in realizing that the poor performance of all past attempts to build nonlinear isolators resided in a limitation stemming from time-reversal symmetry, and that we need to find a way around this challenge. Surprisingly, when two nonlinear resonators are carefully designed and coupled together, one can achieve the best of both worlds: full transmission and infinite isolation.”

The team’s findings could be used in several technologies, including consumer electronics, surgical lasers, automotive radar and lidar systems, and nanophotonic circuits and systems. Next, the team will investigate a variety of approaches to fine-tune the functionality of the isolator, including potentially adding other types of nonlinear resonators to realize circulators and other multiport devices.

The research was published in Nature Electronics (doi:10.1038/s41928-018-0025-0).