Published: March 6, 2024 By

integrated photonicAlthough unseen to us, microwave and radio-frequency waves form the vast network that has transformed how we live, communicate and travel. This includes the GPS you use on your next road trip, all your cell phone conversations and the WiFi network over which you are likely reading this story. 

For more than 50 years, the techniques to make these invisible electromagnetic waves have largely remained unchanged. 

However, in recent findings published in the journal Nature, University of Colorado Boulder researchers introduce a new approach that leverages light and integrated photonics to generate microwave signals that could enable entirely new capabilities in communications, navigation and sensing.

“Our approach with integrated photonics provides a source of exceptionally pure microwaves in a power-efficient package that can be fabricated on tiny silicon chips,” said Scott Diddams, professor in CU Boulder’s Department of Electrical, Computer and Energy Engineering (ECEE). 

His research group led this effort together with colleagues from several universities and the National Institute of Standards and Technology (NIST). 

“We envision these photonic chips could ultimately enable higher capacity communications and the capability to better locate and map the world around us in a wide range of applications outside the lab,” said Diddams.  

The integration of the approach in a photonic circuit on chip opens the possibility for compact and power-efficient devices that blur the boundary between optics and electronics for impactful engineering applications. 

“If you were to open up your iPhone, there would be a computer chip that has billions of transistors controlling your phone,” said Diddams. “In the future, we’re going to have little chips of similar size, but with light going around them along with the electronic signals.”

Building on the foundation of CU Boulder’s Nobel Prize achievement

This integration of optical and electronic systems to make low-noise microwave signals is an example of a transformation that is enabled by optical frequency combs. 

Frequency combs function as a synthesizer allowing for the seamless translation between the optical and microwave domains of the electromagnetic spectrum. They were first developed for atomic timekeeping — work pioneered by John “Jan” Hall, one of CU Boulder’s four Nobel Prize laureates

Diddams, one of Hall’s former postdoctoral researchers, has been working on frequency combs for more than 25 years. That led to the current breakthrough in integrated photonics. 

“To start in Jan’s lab with benchtop experiments for the very first time,” said Diddams, “and now we see these technologies being miniaturized to fit on centimeter-sized chips is a beautiful story.” 

Advances in microwave technologies for societal impact

diddams grad studentsIntegrated photonics provides new opportunities to advance communications systems, radar and imaging and high-precision navigation. 

“If we want to detect the speed of aircraft flying toward us or even detect its shape, you can take images in the microwave domain,” said Diddams. 

There are related applications in astronomy, said NIST physicist and ECEE adjoint professor Franklyn Quinlan, who is part of this effort. 

“Astronomers observe the cosmos in the microwave and millimeter wave domains to create stunning images of black holes,” said Quinlan. “Creating these images requires tight synchronization of receivers spread across the globe.” 

Microwave generation using integrated optics has particular benefits for space-based astronomical imaging, where compact size and low power are critical.

These same photonically-generated microwaves could also be used to enhance communication systems with higher capacity and new capabilities for cybersecurity. 

Planned extensions of the work will include wide and fast tuning of the microwave signals from 1 to greater than 100 GHz, which will increase the range of applications. 

“Ultimately, with our integrated photonics approach, you’ll get a much cleaner picture or measurement with high-performance radar, imaging and navigational tools in the future,” said Diddams.   

In a world that is rapidly changing, the way we communicate, connect and travel will continue evolving with it. 

Top Photo: The rainbow-colored chip from collaborators at CalTech and UC - Santa Barbara contains several integrated resonators used to generate narrow line-width lasers and frequency combs used in the photonic microwave oscillator. Bottom Photo: Groman and Kudelin working on the integrated photonic microwave oscillator in Diddams’ lab.