Novel Frequency Comb Could Enable Smartphone Spectroscopy

A microcomb developed by researchers at Stanford University could provide the basis for wide-spread adoption in everyday electronics. The frequency comb device is small, energy-efficient, and highly accurate. With further development, the team envisions applications in handheld medical diagnostic devices and widespread greenhouse gas monitoring sensors.

Since their development, frequency combs have been utilized for high-precision measurement applications, such as timekeeping and spectroscopy. However, the technology has required bulky, expensive, and power-hungry equipment, which has limited their use to laboratory settings.

The researchers found a workaround for these issues by integrating two different approaches for miniaturizing frequency combs into one straightforward, easily producible, microchip-style platform.

“The structure for our frequency comb brings the best elements of emerging microcomb technology together into one device,” said Hubert Stokowski, a postdoctoral scholar in the lab of Amir Safavi-Naeini, and lead author of the study. “We can potentially scale our new frequency microcomb for compact, low-power, and inexpensive devices that can be deployed almost anywhere.”

A microscope image showing a thin-film lithium niobate chip that contains eight of the FM-OPO devices. One device has a footprint around 1×10 mm² (highlighted here with a dashed rectangle). Courtesy of Kevin Multani and Hubert Stokowski/Stanford University.

This new device is called an Integrated Frequency-Modulated Optical Parametric Oscillator, or FM-OPO. As the name suggests, it combines two strategies for creating the range of frequencies that constitute a frequency comb. One strategy, called optical parametric oscillation, involves bouncing beams of laser light within a crystal medium, wherein the generated light organizes itself into pulses of coherent, stable waves. The second strategy centers on sending laser light into a cavity and then modulating the phase of the light — achieved by applying radio-frequency signals to the device — to ultimately produce frequency repetitions that similarly act as light pulses.

The two strategies have not seen wide usage because they both come with drawbacks, such as energy inefficiency, limited adjustments to optical parameters, and suboptimal optical bandwidth, where the comb-like lines fade as the distance from the center of the comb increases. The researchers approached the challenge with a thin film lithium niobate optical circuit platform.

“Lithium niobate has certain properties that silicon doesn’t, and we couldn’t have made our microcomb device without it,” said Safavi-Naeini.

Compared to silicon, lithium niobate holds certain advantages. Among them are nonlinearity, in which light beams of different colors can interact with one another to generate new wavelengths, and compatibility, with a broad range of wavelengths.

With integrated lithium niobate at the heart of the frequency comb, the team brought together elements of both optical parametric amplification and phase modulation strategies. With the new approach, the comb was shown to produce a continuous output rather than light pulses, enabling the researchers to reduce the required input power by close to an order of magnitude.

The device also yielded a conveniently “flat” comb, meaning the comb lines farther in wavelength from the center of the spectrum did not fade in intensity. This offers greater accuracy and broader utility in measurement applications.

“Although we had some intuition that we would get comb-like behaviors, we weren’t really trying to make exactly this type of comb, and it took us a few months to develop the simulations and theory that explained its main properties,” said Safavi-Naeini.

The new microcombs, with further honing, should be readily manufacturable at conventional microchip foundries with many practical applications such as sensing, spectroscopy, medical diagnostics, fiber-optic communications, and wearable health-monitoring devices.

The research was published in Nature (www.doi.org/10.1038/s41586-024-07071-2).