New Lightwave-Isolation Method

Schematic of the realized isolator, formed by two coupled nonlinear resonators connected through a delay line. (Source: A. Alu)

Operation of modern-day technology requires an ever-increasing use of broad­band frequency signals. This, in turn, has grown the demand for reliable, efficient methods of signal trans­mission that prevent inter­ference and are more effi­cient in their use of the scarcely available frequency spectrum. These require­ments are con­strained, however, by reci­procity, that forces the trans­mission of light to be iden­tical in opposite direc­tions.

In past decades, scientists and engineers have addressed these challenges with the creation of isolators: devices that use an external magnetic field to force light waves to travel in a single direc­tion. But this form of wave isolation is costly, and it requires the use of large, heavy magnets that demand a lot of device real estate. An addi­onal drawback is that they cannot be inte­grated into silicon-based circuits and systems. Now researchers at the Ad­vanced Science Research Center (ASRC) at the Graduate Center of the City Univer­sity of New York (CUNY) and at the University of Texas at Austin developed a new lightwave-iso­lation method that may overcome these challenges. The inno­vative approach does not require magnets or any other form of external bias for reliable wave trans­mission, yet it ensures highly efficient broad band­width iso­lation.

“We have been working on over­coming reci­procity without magnets for a few years,” said Andrea Alù, director of the ASRC’s Photonics Ini­tiative. “In the past we have explored using devices with moving or time-changing elements, but these approaches pose other techno­logical challenges. Now we show that a non-magnetic device free of an external power source – thanks to suitably tailored non­linearities – can drama­tically break trans­mission symmetry and realize efficient broadband iso­lation.”

The researchers explain why previous attempts to use non­linearities to induce iso­lation faced poor per­formance. Alù and his team show that any system based on a single non­linear resonator to isolate waves is inhe­rently limited by a quality trade-off between level of isolation, bandwidth, and insertion loss, making any such device poorly per­forming and imprac­tical. In their most recent experiments, the team has been able to overcome and address this problem using two judi­ciously designed non­linear resonators connected through a delay line, showing that this is the minimal confi­guration for enabling low-loss one-way trans­mission over a broad band­width. The combined compo­nents, which were printed on a circuit board, formed a highly effec­tive, fully passive isolator that provides excellent signal inte­grity.

“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,” said Dimi­trios Sounas, research scientist at the Univer­sity of Texas. “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 anti­cipates the findings may find use in a variety of techno­logies, including consumer elec­tronics, surgical lasers, auto­motive radar and lidar systems and nano­photonic circuits and systems. The next stage of research will inves­tigate a variety of approaches to fine-tune the func­tionality of the isolator, including poten­tially adding addi­tional types of nonlinear resonators to realize circu­lators and other multi-port devices. (Source: CUNY)

Reference: D. L. Sounas et al.: Broadband passive isolators based on coupled nonlinear resonances, Nat. Elec., online 8 february 2018; DOI: 10.1038/s41928-018-0025-0

Link: Photonics Initiative, Advanced Science Research Center ASRC, City University of New York, New York, USA

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