Researchers at Stanford University have introduced a new type of frequency comb, a high-precision measuring device that is innovatively small, ultra-power efficient, and exceptionally accurate. As developments continue, this revolutionary “micro-honeycomb” was detailed in a study published in the journal Naturecan become the basis for the mass introduction of devices in everyday electronics.

Frequency combs are special lasers that produce evenly distributed lines of light that resemble the teeth of a comb, or more accurately, marks on a ruler. Over nearly a quarter-century of development, these “lines of light” have revolutionized many types of high-precision measurements, from timekeeping to molecular detection with spectroscopy. However, since frequency combs require bulky, expensive and energy-intensive equipment, their use is mostly limited to laboratory conditions.

Researchers have found a workaround to these problems by combining two different approaches to miniaturizing frequency combs into a simple, easy-to-fabricate microchip-style platform. Among the many applications researchers envision for their versatile technology are powerful portable medical diagnostic devices and widespread greenhouse gas monitoring sensors.

“The structure of our frequency comb combines the best elements of the new microcomb technology in a single device,” said Hubert Stokowski, a postdoctoral researcher in Amir Safavi-Naini’s laboratory and lead author of the study. “We can scale our new frequency microcombination into compact, low-power, low-cost devices that can be placed almost anywhere.”

“We are very excited about this new microcomb technology that we have demonstrated for a new type of sensitive sensors that are both small and efficient enough to one day be in someone’s phone,” said Safavi-Naini, an assistant professor in the Department of Applied Physics. . at Stanford School of Humanities and Sciences and senior author of the study.

clashing light

This new device is called an integrated frequency modulated optical parametric oscillator, or FM-OPO.

The tool’s complex name suggests that it combines two strategies to create a series of different frequencies, or colors of light, that form a frequency comb. A strategy called optical parametric oscillation involves reflecting beams of laser light in a crystalline medium where the light produced is arranged into coherent, stable wave pulses. The second strategy focuses on sending laser light into the cavity and then modulating the phase of the resulting light by applying radio frequency signals to the device to create repetitions of frequencies that act similarly to pulses of light.

These two strategies for microcombs have not been widely used as both have disadvantages. These challenges include power efficiency, limited optical controllability, and the non-ideal “optical bandwidth” of the comb; here the comb lines disappear as they move away from the center of the comb.

The researchers approached this task in a new way, working on a promising optical circuit platform based on a material called thin-film lithium niobate. This material has advantages over the industry standard material, silicone. Two of these useful properties are “nonlinearity” (allowing light rays of different colors to interact with each other to create new colors or wavelengths) and being able to pass through a wide range of light wavelengths.

Using lithium niobate integrated photonics, the researchers created the components that form the basis of the new frequency comb. These light processing technologies build on advances in the related, more established field of silicon photonics, which involves the fabrication of optical and electronic integrated circuits on silicon chips. In this way, lithium niobate and silicon photonics transformed semiconductors into conventional computer chips, dating back to the 1950s.

“Lithium niobate has some properties that silicon does not have, and we couldn’t make our microcomb without it,” Safavi-Naini said. said.

Surprisingly excellent performance

Later researchers combined elements of optical parametric amplification and phase modulation strategies. The team expected certain performance characteristics from the new frequency comb system on lithium niobate chips, but what they saw turned out to be much better than they expected.

Overall, the comb produced a continuous output rather than pulses of light, allowing the researchers to reduce the required input power by about an amount. The device also received a useful “flat” comb; This means that the intensity of comb lines located further from the center of the spectrum in terms of wavelength is not lost; This ensures greater accuracy and wider application in measurement applications.

“This scallop really surprised us,” Safavi-Naini said. “While we had an inkling that we would have comb-like behavior, we had not actually attempted to create exactly this type of comb, and it took us several months to develop simulations and a theory that explained its basic properties.”

To better understand their high-performance device, the researchers turned to Martin Feuer, the JG Jackson and CJ Wood Professor of Physics and Professor of Applied Physics at Stanford University. Along with other colleagues at Stanford University, Feuer helped develop modern thin-film lithium niobate photonics and understand the material’s crystalline properties.

Feuer, who co-authored the study, made an important connection between the physical principles behind the microcomb and ideas discussed in the scientific literature of the 1970s; these included concepts first proposed by Stephen Harris, professor of applied physics and electrical engineering at Stanford University. .

With further development, the new microchips should be easily produced in conventional microchip foundries, with many practical applications such as sensing, spectroscopy, medical diagnostics, fiber optic communications, and wearable health monitoring devices.

“Our microhoneycomb chip can be placed in anything, and the size of the entire device depends on the size of the battery,” Stokowski said. “The technology we demonstrated could be used in a phone-sized or smaller, low-power personal device and serve a variety of useful purposes.”