Microcombs enhance silicon photonic chips

Developed in the early 2000s, silicon photonic (SiPh) optical systems enable high-data-volume transmissions over the fiber-optic cables that have replaced many copper wires. But a lack of CMOS SiPh systems with on-chip parallel lasers stands in the way of scalability. Microcombs present a commercially viable solution. Also known as “light rulers,” microcombs take light from a single laser and use microresonators to produce light at hundreds of wavelengths. Each frequency line transmits data, so a single chip-scale microcomb could replace the tens of lasers that are currently needed for multiwavelength optical-communication systems.

Despite significant advances in microcomb integration, existing approaches still rely on bulky and expensive technologies, with passive comb parts that filter, guide, and combine light as the only components integrated on chip. Recently, an entire microcomb has been integrated into a SiPh system by John Bowers from the University of California, Santa Barbara; Xingjun Wang from Peking University in China; and their respective teams. Their energy-efficient, low-cost approach uses CMOS-compatible manufacturing that they say should enable straightforward implementation in practical commercial applications.

The scientists assembled an aluminum gallium arsenide on insulator (AlGaAsOI) microresonator that turned continuous-spectrum light from a distributed feedback (DFB) laser into a coherent microcomb, as illustrated above. Bright microcombs, whose teeth are peaks above the continuous-wave background, are prone to degradation by thermal dissipation in the microresonator. To counteract that effect, the researchers instead generated a so-called dark-pulse microcomb by making notches in the continuous background. The dark-pulse generation mechanism is less sensitive to dissipation, and the pulse shape enables an efficient transfer of light from the pump laser to the comb.

The microresonator also has a record-low oscillation threshold, so it can be pumped by a low-power on-chip laser. And the resulting comb provides turnkey operation—it’s self-stabilizing, meaning that it doesn’t need electromagnetic feedback loops to function. Those capabilities enable the device to shed previously required bulky equipment.

Chips with integrated microcombs can be customized for a wide range of applications. To demonstrate their SiPh chip, the researchers built an optical data link that interconverts electrical and optical signals. The link uses the comb lines as parallel channels and transmits at an aggregate rate of up to 2 terabits per second, which represents significant improvement over the roughly 10 gigabits per second achieved by traditional optical-communication systems. Broadening the range of wavelengths could drive up the rate to 10 terabits per second or more, the researchers say.

The scientists also demonstrated a microwave filter that controls photonic signals’ bandwidth and shape. The filter can be reconfigured in just tens of microseconds for many real-world applications, including 5G, radar, and on-chip signal processing. Signals with tunable bandwidths and flexible center frequencies could support low-cost, easy-to-install infrastructure for future communication, navigation, and data-processing systems. (H. Shu et al., Nature 605, 457, 2022 .)