Innovative Optical Device with a Breakthrough “Racetracks” Design

Caltech researchers have made significant advancements in frequency microcombs, harnessing ultra-low-loss silicon nitride to overcome dispersion challenges through innovative design. This development holds the potential to incorporate microcombs into compact devices with the advantage of cost-effective manufacturing processes.

A few years ago, Caltech’s Kerry Vahala introduced a groundbreaking optical device known as a turnkey frequency microcomb, with applications ranging from digital communications and precision timekeeping to spectroscopy and astronomy.

Understanding Microcomb Technology

This device, created on a silicon wafer, takes laser light input at one frequency and transforms it into a well-organized series of distinct frequencies, forming a sequence of pulses as short as 100 femtoseconds (quadrillionths of a second). (The term “comb” in its name stems from the frequencies being evenly spaced like the teeth of a comb.)

Advancements in Microcomb Materials

Now, Kerry Vahala, Caltech’s Ted and Ginger Jenkins Professor of Information Science and Technology, Applied Physics, and Executive Officer for Applied Physics and Materials Science, along with colleagues from his research group and John Bowers’ group at UC Santa Barbara, has achieved a significant breakthrough in the way short pulses are generated within a crucial new material known as ultra-low-loss silicon nitride (ULL nitride), composed of silicon and nitrogen. This silicon nitride is meticulously purified and deposited as a thin film.

In theory, microcomb devices producing short-pulse microcombs from this material would require minimal power for operation. Unfortunately, the normal dispersion property of ULL nitride, which causes light or electromagnetic waves to travel at different speeds based on their frequency, prevents the proper generation of short light pulses (known as solitons) in this material. This limitation posed a challenge to the operation of microcombs.

Overcoming Optical Constraints

In a Nature Photonics paper, the researchers detail their development of a new microcomb, addressing the inherent optical limitations of ULL nitride by generating pulses in pairs. This achievement is significant because ULL nitride is manufactured using technology similar to that used for producing computer chips, suggesting that these microcombs could potentially be integrated into a variety of handheld devices with smartphone-like form factors.

Design and Functionality of Microcombs

A key characteristic of a typical microcomb is a small optical loop that resembles a miniature racetrack. During operation, solitons naturally form and circulate within it.

However, when this loop is constructed from ULL nitride, dispersion disrupts the stability of the soliton pulses, as explained by co-author Zhiquan Yuan (MS ’21), a graduate student in applied physics.

Imagine the loop as a racetrack with cars. If some cars travel faster and others slower, they will spread out as they circle the track instead of staying tightly packed. Similarly, ULL nitride’s normal dispersion causes light pulses to spread out in microcomb waveguides, rendering the microcomb ineffective.

To address this, the team devised a solution: creating multiple racetracks and pairing them up in a figure-eight configuration. In the middle of this ‘8,’ the two tracks run parallel with only a tiny gap between them.

Analogously, this is akin to two racetracks sharing a straight section. As the cars from each track converge on this shared portion, they encounter something akin to a traffic jam. Just as merging lanes on a freeway force cars to slow down, the conjoined section of the two microcombs compels paired laser pulses to bunch up. This counteracts the pulses’ tendency to spread out and enables proper microcomb functionality.

Innovative Approaches and Future Prospects

“In effect, this counteracts the normal dispersion and gives the overall composite system the equivalent of anomalous dispersion,” explains graduate student and co-author Maodong Gao (MS ’22).

The concept extends further by adding even more racetracks. The team has demonstrated that three racetracks can operate by generating two sets of pulse pairs. Vahala believes this phenomenon can continue to function with many coupled racetracks (microcombs), offering a means to create extensive photonic circuit arrays for soliton pulses.

Furthermore, these ULL microcombs are manufactured using the same equipment used for producing computer chips through complementary metal–oxide–semiconductor (CMOS) technology. John Bowers, a professor of electrical and computer engineering, notes that “The manufacturing scalability of the CMOS process means that it will now be easier and more economical to manufacture the short-pulse microcombs and integrate them into existing technologies and applications.”

Regarding potential applications, Vahala describes a comb as a versatile tool for optics with various functions, making it a powerful asset.

Reference: “Soliton pulse pairs at multiple colors in normal dispersion microresonators” by Zhiquan Yuan, Maodong Gao, Yan Yu, Heming Wang, Warren Jin, Qing-Xin Ji, Avi Feshali, Mario Paniccia, John Bowers and Kerry Vahala, 3 August 2023, Nature Photonics.
DOI: 10.1038/s41566-023-01257-2

The paper detailing this research, “Soliton pulse pairs at multiple colors in normal dispersion microresonators,” is featured in the November issue of Nature Photonics. In addition to Vahala, Yuan, and Gao, other co-authors include applied physics graduate student Yan Yu; Heming Wang (MS/PhD ’21), formerly of Caltech and now at Stanford University; Warren Jin from UC Santa Barbara and Anello Photonics; Qing-Xin Ji (MS ’22); Avi Feshali and Mario Paniccia from Anello Photonics.

Funding for this research was provided by the Defense Advanced Research Projects Agency, the Defense Threat Reduction Agency Joint Science and Technology Office for Chemical and Biological Defense, and the Air Force Office of Scientific Research.

Frequently Asked Questions (FAQs) about Microcomb Technology

More about Microcomb Technology

• Caltech News Article
• Nature Photonics Research Paper
• Kerry Vahala’s Profile at Caltech