Researchers Develop Compact Chips That Convert Light Into Microwaves

Leveraging the higher quality of optical electronics, researchers have created a low-noise microwave oscillator using photonics.

In a major step toward integrated microwave-photonic circuits, NIST researchers have partnered with several organizations to shrink a system once tabletop-bound into a chip-scale device for GPS, radar, and wireless communications.

The researchers shrunk a microwave-photonic oscillator from a tabletop-only system to a system that could potentially provide new accuracy capabilities for next-gen applications.

Synchronization can be a major issue among large numbers of connected devices. While free-running microwave oscillators have sufficiently addressed this problem in the past, more robust measurement and communication applications require efficient phase noise reduction and synchronization between devices. According to the researchers, microwave photonics could create cleaner microwave frequencies in such use cases. 

NIST and its research partners—NASA Jet Propulsion Laboratory, Yale University, the California Institute of Technology, the University of California Santa Barbara, the University of Virginia, and the University of Colorado Boulder—have announced a photonic circuit that converts light into microwaves to improve navigation, communication, and radar systems.

Researchers Bring Optical Precision to Microwaves

Each research partner played a crucial role in the photonic circuit prototype. First, NIST, JPL, the University of Virginia, Caltech, and UC Boulder developed a microwave oscillator that generated a microwave signal by leveraging optics' relatively high speeds and precision.

They did this by focusing semiconductor lasers into a reference cavity (essentially a small mirror box) and matching the light frequencies inside to the cavity's size. This entailed perfectly fitting the peaks and valleys of the light waves between the cavity's walls. The light built up power in those frequencies, keeping the laser's frequency stable. A frequency comb then converted that stable, high-frequency light into low-pitched microwave signals. 

The two continuous wave lasers, designed by Caltech, created unique frequencies that “locked” in place with self-injection locking (SIL) microresonators and Fabry-Perot cavities, designed by UC Boulder. These lasers acted as a reference to create two unique beat frequencies with the help of a third laser and a microcomb to create a phase-locked frequency comb. 

The combined microwave-photonic system consists of many components, each of which improves performance over its microwave counterpart and enables a high-quality output signal. 

The microcomb was a key component of the device, generating the 20-GHz-spaced optical frequencies that ultimately created the microwave output. The microcomb used a dual coupled-ring resonator designed by UCSB and Caltech researchers.

The comb's output consisted of many optical frequencies separated by 20 GHz. This was fed into a modified unitravelling carrier (MUTC) photodetector, which created a 20-GHz microwave signal. The optical resonators' high Q-factor then created a microwave signal with extremely low phase noise. These microwaves are essential for maintaining accurate timing and synchronization in technologies like radar, communication networks, and navigation systems.

A Group Effort to Miniaturize

While systems based on the (greatly simplified) operating principle mentioned above have existed for some time, they have primarily been restricted to tabletop systems. As a result, it was previously impractical to use such a system in a real device. With the group effort from NIST and its collaborators, however, designers are one step closer to employing optics in microwave applications.

The microwave-photonic oscillator exhibits a much lower phase noise compared to a free-running microwave oscillator.

Compared to traditional microwave electronics, the optical microwave oscillator exhibited much lower phase noise, with values as low as -102 dBc/Hz at 100-Hz offset and -141 dBc/Hz at 10-kHz offset, representing a 50-dB phase noise reduction near the carrier. As a result, applying microwave photonics could potentially improve high-precision applications.

Democratizing Precise Timing

The reported microwave-photonic oscillator could just be the first step toward integrating lasers, modulators, detectors, and optical amplifiers onto a single chip. If further developed, this research could make it considerably easier to move sensitive, low-noise signals out of the lab and into the hands of radar technicians, astronomers, cell tower operators, and beyond.