New optical frequency comb chip advances portable atomic clocks and navigation systems

Researchers have demonstrated a chip-based optical frequency comb that matches benchtop systems in key tests while allowing a single device to perform multiple measurement tasks, marking an important step toward portable atomic clocks and GPS-free navigation systems. The device employs a new method called self-aligned parameter-driven cavity solitons (SParCS). Optical frequency combs produce a series of precisely spaced optical frequencies, forming a ruler that scientists use to measure optical frequencies and link them to electronic signals. Researchers from the Joint Quantum Institute (JQI), the National Institute of Standards and Technology (NIST), and international partners tested the chip by linking microwave signals (oscillating billions of times per second) with optical sources running at hundreds of trillions of cycles per second.

In an atomic clock test, the chip converted an extremely stable optical reference into a microwave signal readable by conventional electronics. The raw optical frequency it measured, oscillating at hundreds of trillions of cycles per second, differed from the expected atomic value by about 100,000 oscillations per second.

Innovation of the SParCS method

Traditional chip-scale frequency combs typically inject a single laser into a microscopic ring resonator. The resulting comb extends to frequencies on both sides, but the outer 'teeth' can become weaker and less distinct, requiring additional equipment to amplify and stabilize them. SParCS instead uses two lasers at different frequencies as the comb's edges or 'bookends.' Other frequencies form between them. The researchers combined this approach with a synchronization technique to align the frequency lines into a usable comb. This solved a problem that arose when the team first demonstrated parameter-driven cavity solitons in 2024.

That experiment produced multiple sets of overlapping frequency lines, making the device difficult to use for practical measurements. The two-laser design also enabled the researchers to create a comb spanning an octave, meaning its highest frequency is roughly twice its lowest. This range is critical for determining the comb's zero-frequency offset, a calibration needed for precise frequency measurements.

Chip performance comparison

The researchers directly compared the chip with a traditional benchtop frequency comb using the same microwave and optical sources. Within their test range, the chip-generated frequencies matched the larger system's stability and noise performance. The team also reversed the conversion, transferring the stability of an optical source to a microwave signal. Using a low-noise laser reference, the chip produced low-noise microwaves, a capability that could also benefit radar and other precision measurement systems. Importantly, the researchers used the same chip across different demonstrations, changing only the connected light sources rather than fabricating specialized microcombs for each application.

JQI research scientist Grégory Moille said: "As experimentalists, this new optical frequency comb simplifies much of our work; this system allows a single operator to handle each application."

The researchers noted that previous chip-based frequency comb experiments could take several people weeks or months to yield practical results. Their lab has now moved most microcomb experiments to SParCS and stabilized more than 10 times as many microcombs in the past year as in all previous years combined. The team still plans to expand the frequency range and investigate whether this approach can eventually operate with a single laser. The research has been published in the journal Nature.

Impact of the new chip on frequency measurement

The emergence of the new optical frequency comb chip marks a major advance in frequency measurement technology. Through the SParCS method, researchers have successfully improved the stability and accuracy of frequency combs, making the realization of portable atomic clocks and navigation systems more feasible. This technology not only simplifies the experimental process but also significantly improves experimental efficiency, with potential application value for future precision measurement systems such as radar. As research progresses, if single-laser operation can be achieved, it will further drive the adoption and application of this technology.