UVA-led research team continues next big step

image: Xu Yi, an assistant professor of electrical and computer engineering at the University of Virginia’s School of Engineering and Applied Science, leads the Soliton Photonic Integrated Nitride Continuous Synthesizer (SPhiNCS) project to translate the purity and stability of high frequency optical signals in the microwave regime where defensive capabilities for positioning, navigation and timing typically operate.
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Credit: Tom Cogill

Xu Yiassistant professor of electrical and computer engineering at the University of Virginia, leads a research team pursuing the next big technological leap in radar and global positioning systems – part of a national effort to advance these technologies using photonics.

Yi’s ambition is to translate the purity and stability of high frequency optical signals into the microwave regime where defensive capabilities for positioning, navigation and timing typically operate. To put that into perspective, the system they plan to develop will operate at up to 110 gigahertz, frequencies 20 times faster than WiFi and three times faster than 5G.

Yi formed a team with colleagues from UVA School of Engineering and Applied Science’s Charles L. Brown Department of Electrical and Computer Engineering, the University of California at Santa Barbara, Morton Photonics and Honeywell to achieve this goal. They were awarded a three-year, $2.4 million grant from the Defense Advanced Research Projects Agency’s GRYPHON program, which means RF generation with photonic oscillators for low noise.

The project builds on Yi’s accomplishments in photonics, one of the department’s research strengths. Yi and his teammates Andreas Belingprofessor of electrical and computer engineering, and Steven M. Bowersassociate professor of electrical and computer engineering, have made breakthroughs in high-speed light sensing and ultra-low-noise circuitry to bring photonics technology closer to reality.

“Conventional approaches start at low frequencies and scale up to get to higher frequencies that are useful for sensing and communications,” Beling said. “Our project tackles the problem from the other side, to start high and split, by converting light into radio waves.”

Radars, the global positioning system and space missions rely on microelectronic systems. Yi’s team focuses on the component at the heart of these systems, the microwave oscillator. The oscillator produces a high frequency electromagnetic wave or pulse of energy to coordinate and schedule data streams through high-speed digital systems, synchronize linked systems, and convert signals from high to low frequencies.

Yi specializes in a specific type of photonic device called a microresonator-based frequency comb, or microcomb. The microcomb efficiently converts photons of a single wavelength to multiple wavelengths. Yi’s innovations in optical frequency division provide a way to create a chip-sized, low-noise system that is continuously tunable over a very wide range.

An ideal oscillator provides a perfect signal at a single frequency. Since real-world systems such as military radars and commercial 5G systems operate at varying frequencies, they are much less stable, a limitation commonly referred to as phase noise.

“Phase noise – how much the signal oscillates – is the metric everyone cares about,” Beling said. “Once you have a super stable signal generated in a small integrated package, it opens up new possibilities in applications such as communications, positioning and telemetry.”

“In this specific setting, if we are looking for low noise, photonics has a key advantage over electronics, Yi said. “With this program funding, we have the opportunity to build a really good team and push this to the limit, to see how far this technology can go.”

To work with deployed systems, optical signals must be converted into the electrical domain, like a solar cell that converts light into power. Beling’s group demonstrated this capability in stand-alone devices. Its photodetectors offer proof of concept for output power or signal strength; linearity, which is another way of expressing a clean signal; and bandwidth.

“We have this photodetector technology established at UVA,” Beling said. “But instead of daisy-chaining discrete components, we envision a signal generator or synthesizer fabricated as one integrated device.”

Bowers will focus on systems integration, which involves both photonic and electronic devices and their interaction. Bowers’ main task is to develop an optoelectronic feedback system to achieve continuous tunability with improved phase noise performance.

“A tuning fork is a really resonant device,” Bowers said. “When you strike a tuning fork, it produces a pitch. If you have a tuning fork for the ‘a’ note and you also want to tune the ‘b’ note, you need a brand new tuning fork. The Photonics Teammates can create these truly phenomenal Q-factor tuning forks, these highly resonant devices, but we need to monitor their output and send a feedback signal back to ensure that the signal remains stable.

Bowers leads the Integrated electromagnetics, circuits and systems research group, whose members will design and integrate opto-electrical control circuits and control algorithms to meet this requirement. Bowers is also an affiliate faculty member of the Lab linkUVA’s world-class center of excellence in cyber-physical systems.

“What’s really challenging with this program is getting such a clean signal and being able to tune it over a wide range of frequencies,” Bowers said. The team’s midterm goal is to generate a 1 to 110 gigahertz signal with one gigahertz resolution. Bowers feedback and control circuitry will extend the resolution to the right of the decimal point, to a resolution of one hertz or more.

Bowers’ second task is to develop electronic frequency dividers to extend the advantages of phase noise performance to the one gigahertz level – to optically produce a clean 32 gigahertz signal and divide it by 32 to obtain a signal of a gigahertz that’s just as stable, meaning no extra noise.

In sum, Yi and other teams that have won GRYPHON grants are being asked to deliver at least an order-of-magnitude jump in one of three target metrics: size, phase noise, and frequency range. . This combination of features is unprecedented today and will establish new source technology that is expected to transform the types and capabilities of military and commercial radar and communications systems.

The described project is sponsored by the Defense Advanced Research Projects Agency. The content of this story does not necessarily reflect government position or policy, and no official endorsement should be inferred.

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