Many technologies — from communications, to radar and sensing, to positioning and navigation — rely on low-noise microwave signals for precise timing and synchronization. Advances in these technologies intensify the demand for stable, low-phase noise microwave sources. Although photonic lightwave systems provide advantages over conventional electronic approaches for generating low-noise microwaves, the large size and power consumption of photonic systems restrict their use to laboratory environments. Researchers from the University of Colorado Boulder (CU Boulder), the National Institute of Standards and Technology (NIST), the NASA Jet Propulsion Laboratory, California Institute of Technology, the University of California Santa Barbara, the University of Virginia, and Yale University came together to address this challenge. The interdisciplinary team developed a low-noise microwave generator with a compact, portable form factor by using two-point optical frequency division (2P-OFD), a technique for developing high-performance signal sources, with integrated photonic components. The resulting technology shrinks much of what has been a tabletop system into a chip-sized format, reducing power usage and making the technology more practical for use in a range of devices. “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,” CU Boulder professor Scott Diddams said. The microwave oscillator combines low-noise integrated lasers, an efficient dark soliton frequency comb, and a miniature Fabry-Pérot optical cavity. The researchers stabilized the narrow-linewidth, self injection-locked lasers to the Fabry-Pérot cavity and divided the frequency gap between the lasers with the dark soliton frequency comb. A photodetector is used to detect the stabilized output of the microcomb and produce a microwave signal. According to the team, the system generates signals at values that are unprecedented for an integrated photonic system. NIST researchers test a chip for converting light into microwave signals. The chip (pictured) is the fluorescent panel that looks like two tiny vinyl records. The gold box to the left of the chip is the semiconductor laser that emits light to the chip. Courtesy of K. Palubicki/NIST. Specifically, the integrated photonic approach to optical frequency division (OFD) generates 20 gigahertz (GHz) microwave signals with a phase noise of -135 decibels relative to the carrier (dBc) hertz frequency (Hz?¹) at a 10 kilohertz (kHz) offset — a value that is typically found in much larger commercial systems. The system reduces the small, random changes in the timing of microwave signals, known as timing jitter, to 15 femtoseconds (fs), making the signals more stable and precise. The reduction in the timing jitter also decreases power consumption. The goal of the team is to integrate all the components of the technology, including the lasers, modulators, detectors, and optical amplifiers, onto a single chip, so the system can be integrated into small, energy-efficient devices. Several components remain outside of the chip while they undergo testing. “The current technology takes several labs and many PhDs to make microwave signals happen,” NIST scientist Frank Quinlan said. “A lot of what this research is about is how we utilize the advantages of optical signals by shrinking the size of components and making everything more accessible." “The goal is to make all these parts work together effectively on a single platform, which would greatly reduce the loss of signals and remove the need for extra technology,” Quinlan said. “Phase one of this project was to show that all these individual pieces work together. Phase two is putting them together on the chip.” The approach taken by the team is intended to lead to full integration on a single chip with the volume of the photonic components on the order of one cubic centimeter (1 cm3). The photonics chip-based microwave oscillator could provide compact, portable, low-cost microwave synthesis for several demanding applications, including navigation systems such as GPS, communication networks, radar, atomic clocks, and astronomical imaging. “There are all sorts of applications for this technology,” Quinlan said. “For instance, astronomers who are imaging distant astronomical objects, like black holes, need really low-noise signals and clock synchronization." “And this project helps get those low noise signals out of the lab, and into the hands of radar technicians, of astronomers, of environmental scientists, of all these different fields, to increase their sensitivity and ability to measure new things,” Quinlan said. Diddams noted that the team envisions that the photonic chips will enable higher capacity communications and the capability to better identify and map geographical locations. The researchers plan to extend the work through wide and fast tuning of the microwave signals from one GHz to more 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,” Diddams said. Quinlan noted that the results of the collaborative effort underscore the importance of interdisciplinary research in driving technological progress. The research was published in Nature (www.doi.org/10.1038/s41586-024-07058-z).
