Smallest Integrated Frequency Comb Generator

Illustration showing an array of microring resonators on a chip converting laser light into frequency combs. (Source: B. Stern, Columbia Eng.)

Optical frequency combs can enable ultrafast processes in physics, biology, and chemistry, as well as improve communi­cation and navi­gation, medical testing, and security. A major challenge has been how to make such comb sources smaller and more robust and portable. In the past 10 years, major advances have been made in the use of monolithic, chip-based micro­resonators to produce such combs. While the micro­resonators generating the frequency combs are tiny, they have always relied on external lasers that are often much larger, expensive, and power-hungry.

Researchers at Columbia Engi­neering have built a Kerr frequency comb generator that, for the first time, integrates the laser together with the micro­resonator, significantly shrinking the system’s size and power require­ments. They designed the laser so that half of the laser cavity is based on a semi­conductor waveguide section with high optical gain, while the other half is based on wave­guides, made of silicon nitride, a very low-loss material. Their results showed that they no longer need to connect separate devices in the lab using fiber. They can now inte­grate it all on photonic chips that are compact and energy efficient.

The team knew that the lower the optical loss in the silicon nitride wave­guides, the lower the laser power needed to generate a frequency comb. “Figuring out how to elimi­nate most of the loss in silicon nitride took years of work from many students in our group,” says Michal Lipson, Eugene Higgins Professor of Elec­trical Engi­neering and co-leader of the team. “Last year we demon­strated that we could repro­ducibly achieve very transparent low-loss wave­guides. This work was key to reducing the power needed to generate a frequency comb on-chip.”

Micro­resonators are typically small, round disks or rings made of silicon, glass, or silicon nitride. Bending a wave­guide into the shape of a ring creates an optical cavity in which light circulates many times, leading to a large buildup of power. If the ring is properly designed, a single-frequency pump laser input can generate an entire frequency comb in the ring. The team made another key inno­vation: in micro­resonators with extremely low loss like theirs, light circulates and builds up so much inten­sity that they could see a strong reflection coming back from the ring.

“We actually placed the micro­resonator directly at the edge of the laser cavity so that this reflection made the ring act just like one of the laser’s mirrors. The reflection helped to keep the laser perfectly aligned,” says Brian Stern, who conducted the work as a doctoral student in Lipson’s group. “So, rather than using a standard external laser to pump the frequency comb in a separate micro­resonator, we now have the freedom to design the laser so that we can make the laser and resonator interact in new ways.”

All of the optics fit in a milli­meter-scale area and the researchers say that their novel device is so efficient that even a common AAA battery can power it. “Its compact size and low power require­ments open the door to deve­loping portable frequency comb devices,” says Alexander Gaeta, Rickey Professor of Applied Physics and of Materials Science and team co-leader. “They could be used for ultra-precise optical clocks, for laser radar /LIDAR in auto­nomous cars, or for spectro­scopy to sense biological or environ­mental markers. We are bringing frequency combs from table-top lab experi­ments closer to portable, or even wearable, devices.”

The researchers plan to apply such devices in various confi­gurations for high precision measure­ments and sensing. In addition, they will extend these designs for operation in other wave­length ranges, such as the mid-infrared where sensing of chemical and biological agents is highly effective. In coopera­tion with Columbia Tech­nology Ventures, the team has a provi­sional patent appli­cation and is exploring commercia­lization of this device. (Source: CUSEAS)

Reference: B. Stern et al.: Battery-operated integrated frequency comb generator, Nature, online 8 October 2018; DOI: 10.1038/s41586-018-0598-9

Link: Lipson Nanophotonics Group, Dept. of Electrical Engineering, Columbia University, New York, USA

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