Artist rendering of microcomb

An international team has reportedly created a scalable approach to creating compact, chip-scale frequency combs using ring resonators—without the additional control and feedback components that have made these devices hard to adapt to mass production. [Image: Brian Long]

The effort to shrink optical frequency combs from bulky lab setups to chip scale has seen spectacular progress in recent years, and promises to put the power of these sophisticated light sources at the service of a range of precision handheld applications. But the new chip-based “microcombs” can be finicky beasts, requiring complex modulations of frequency and power to get the comb going. And squeezing chip-based comb systems into packages that can be scaled up to volume production in the current CMOS manufacturing system has been tough.

An international research group has now moved the needle further on chip-based optical combs that are suitable for mass production (Nature, doi: 10.1038/s41586-020-2358-x). The team reports that its “turnkey” integrated microcomb, based on microring resonators, can be operated simply by turning on the pump laser at a specific critical operating point, eliminating the need for photonic and electronic control circuitry. That could greatly simplify large-scale production of these tiny chip-based frequency combs, making them available for new, compact device designs in sensing, metrology and other areas.

Combs from rings

Frequency combs are laser sources that consist of a spectrum of thousands or millions of discrete, closely spaced lines of different frequencies, arrayed like the teeth of a comb. The Nobel Prize–winning technology has proved a boon particularly to applications in spectroscopy, precision metrology and sensing. But setting up these combs has traditionally been a complex, lab-bench-scale affair involving mutually stabilized, femtosecond mode-locked lasers. Getting comb sources to the compact scale of silicon chips would extend their vast application potential, in areas ranging from communications to handheld, field-ready sensing devices.

One solution that has seen remarkable advances in the past few years has been so-called Kerr combs, generated in ring-shaped microresonators. In such combs, a single-frequency, continuous-wave laser pumps a microring waveguide whose dispersion characteristics have been carefully engineered. Nonlinear four-wave mixing processes in the waveguide lead to a train of dissipative Kerr solitons (DKSs)—solitary, self-reinforcing propagating waves—that, in the frequency domain, form an optical frequency comb.

Toward a scalable design

While DKS generation has taken the community much closer to a scalable approach for creating fully chip-based frequency combs, a few hitches remain. One issue, according to the authors of the new study, lies in the need for “complex startup and feedback protocols” to get the combs to work. This requirement, the team says, requires additional optical and electronic components that make it hard to create a compact, integrated design that can be produced at scale.

The research team—led by OSA Fellows John Bowers of the University of California, Santa Barbara, USA, Kerry Vahala of the California Institute of Technology, USA, and Tobias Kippenberg of the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland—sought to get around that problem. The team’s goal was instead to develop a chip-based frequency comb that could be simply “turned on,” eliminating the need for difficult-to-integrate additional optical and electronic components.

“As easy as switching on a room light”

Artist rendering of microcomb

An international team has reportedly created a scalable approach to creating compact, chip-scale frequency combs using ring resonators—without the additional control and feedback components that have made these devices hard to adapt to mass production. [Image: Brian Long]

To get there, the researchers used inverse fiber tapers to butt-couple a commercially available, chip-scale distributed-feedback laser to another chip containing four high-Q-factor microring resonators, engineered to create DKS-based combs. The microrings are built on a silicon nitride (Si3N4) platform, to ensure compatibility with the existing CMOS manufacturing system.

The team was able to show that, because of careful attention to the details of backscatter feedback between the laser and the comb-generating microrings, the driving laser could create the train of solitons at an extremely well defined operating point, without the need for additional control or feedback components. That makes the process of generating a frequency comb from the setup—as Vahala put it in a press release accompanying the work—“as easy as switching on a room light.”

A wristwatch-scale optical clock?

In addition to operating simplicity, the team believes the new design’ CMOS-compatible materials and architecture will have significant advantages in high-volume production. In particular, because the design strips away the need for additional photonic and electronic components to handle startup and feedback control, the entire package can be “fully integrated into an industry standard (butterfly) package.” The latter aspect, the researchers write, “represents a milestone towards mass production of optical frequency combs.”

It’s a milestone that, according to co-team-leader Bowers, could open up applications in communications, sensing and even ultra-compact optical clocks. “Optical clocks used to be large, heavy, and expensive,” he noted in a press release accompanying the work. “With integrated photonics, we can make something that could fit in a wristwatch, and you could afford it.”

The work, Bowers says, constitutes a “key step to transfer the frequency comb technology from the laboratory to the real world.”