Caltech Researchers Develop Ultralow-Loss Light Channeling on Silicon Wafers

Caltech researchers have achieved a significant breakthrough in moving light across silicon wafers with minimal signal loss, reaching performance levels at visible wavelengths akin to those of optical fibers. This advancement opens the door to a new generation of highly coherent and energy-efficient photonic integrated circuits (PICs) suitable for applications in optical clocks, gyroscopes, AI data center communications, and quantum computing.

Optical fiber currently forms the backbone of modern communication infrastructure, enabling high-speed data transmission over long distances due to the purity and smoothness of the glass used. This ultralow loss performance allows most light entering the fiber to reach its destination without significant absorption, scattering, or loss.

“For years, we have been working to translate the spool-based fabrication of optical fiber onto silicon wafers while trying to preserve the fiber’s hallmark of ultralow loss,” said Kerry Vahala, Ted and Ginger Jenkins Professor of Information Science and Technology and Applied Physics at Caltech. “We have developed a method to print optical circuits made from the same material as optical fiber directly onto 8- and 12-inch wafers used for computer chips. This shift toward fiber-like performance, especially in the visible bands, will enable new technologies that benefit from negligibly low circuit energy loss.”

The research team’s findings are detailed in a paper published in Nature, with lead authors including Caltech postdoctoral scholar Hao-Jing Chen and graduate student Kellan Colburn.

Innovative Use of Germano-Silicate

The researchers created waveguides—nanoscale pathways that channel light—using germano-silicate, the same type of glass found in optical fibers. This material was adapted for a lithography-based manufacturing method conducive to wafer fabrication. Unlike traditional straight-line waveguides, these new pathways are spiraled, allowing light to traverse longer distances within a compact area, similar to winding fiber around a spool but on a much smaller scale.

“Germano-silicate waveguides demonstrate extremely low loss and are also readily adaptable to efficiently transfer light between optical fibers and semiconductor lasers, which is critical for reducing the overall energy cost of server infrastructure,” noted Henry Blauvelt, chief technology officer at Emcore and a co-author of the study.

Significant Advances at Visible Wavelengths

Performance comparisons at near-infrared wavelengths have shown that the new Caltech devices match some of the best existing silicon nitride devices, which are valued for their relatively low signal loss. The advantage markedly increases at visible wavelengths, with the germano-silicate platform outperforming silicon nitride by a factor of 20.

“Due to the comparatively low melting temperature of the material, we can put our devices into a furnace to ‘reflow’ the surface of our waveguides and achieve atom-level smoothness, significantly reducing scattering losses that have hindered conventional visible PICs,” Chen explained. “At visible wavelengths, our recent platform exceeds silicon nitride’s record by a factor of 20, and we have more room to improve.”

The reduction in loss dramatically enhances the performance of optical devices, with the new lasers exhibiting over a 100-fold improvement in light coherence duration compared to previous designs.

Applications and Future Potential

The ultralow-loss capabilities of these waveguides present numerous potential applications, including chip-scale atomic sensors, optical clocks, and ion-trap systems. “One of the reasons this is so compelling is that it has a Swiss Army-knife quality—it can be applied in a wide range of settings,” Vahala stated.

Colburn underscored the importance of aiming for kilometer-scale performance metrics for even small chip devices. “It may seem a little ridiculous that we are targeting losses that can be described by percentages over kilometers. But there are applications where this would be very powerful,” he remarked, highlighting the ring resonator’s role in enhancing light circulation.

The research team views their current findings as a stepping stone rather than a final product. “We haven’t gone as far as we want to go, but we’ve made significant progress over the last five years,” Vahala concluded.

The paper titled “Towards fibre-like loss for photonic integration from violet to near-infrared” features contributions from graduate students Peng Liu, Hongrui Yan, Jinhao Ge, Jin-Yu Liu, and Phineas Lehan, as well as former students and postdoctoral scholars. The work received funding from multiple agencies, including the Defense Advanced Research Projects Agency and the Air Force Research Laboratory.