Extending Optical Fiber's Ultralow Loss Performance to Photonic Chips

Caltech scientists have developed a way to guide light on silicon wafers with low signal loss approaching that of optical fiber at visible wavelengths. This accomplishment paves the way for a new generation of ultra-coherent and efficient photonic integrated circuits (PICs), which will have a profound impact in a variety of on-chip applications including precision measurements, such as optical clocks for timing and gyroscopes for rotation as well as AI data-center communications and even quantum computing.

Even if we are largely unaware of it, optical fiber is all around us. It is what connects our digital world, enabling us to communicate and share data nearly instantaneously regardless of distance. Optical fiber can do this, in large part, because it is made from extremely pure glass and is carefully engineered to be ultrasmooth; when light enters at one end of a fiber, nearly the entire signal continues to the other end without being absorbed, scattered, or otherwise lost. This is what researchers describe as ultralow-loss performance.

"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," says Kerry Vahala (BS '80, PhD '85), the 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 the same 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 scientists describe their method in a paper recently published in the journal Nature. The lead authors of the paper are Caltech postdoctoral scholar Hao-Jing Chen and graduate student Kellan Colburn (MS '25), who completed the work in Vahala's lab.

To make its waveguides (nanoscale on-chip pathways that channel light), the team adapts germano-silicate, the same glass material used in optical fiber, through a lithography-based manufacturing process. The waveguides are laid out in a spiral geometry to extend their optical path length, analogous to winding light around a fiber spool but with a significantly smaller footprint enabled by nanofabrication. "Germano-silicate waveguides demonstrate extremely low loss and are also readily adaptable to efficiently transfer light between optical fibers and semiconductor lasers, which is of paramount importance in reducing the overall energy cost of server infrastructure," says Henry Blauvelt (PhD '83), a visiting associate in applied physics and material science at Caltech; chief technology officer at Emcore, a company specializing in photonic circuits; and an author of the recent paper.

Devices made with the Caltech team's new platform have already matched, at near-infrared wavelengths, previous top-performing devices made from silicon nitride, a material widely used in optics because of its low-loss performance for transmitting data. Importantly, the new material significantly outperforms silicon nitride at visible wavelengths. "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 to get their smoothness down to the level of individual atoms, which largely suppresses the severe scattering loss that has limited conventional visible PICs," Chen says. "At visible wavelengths, our recent platform exceeds silicon nitride's record by a factor of 20, and we have more room to improve."

Loss dramatically impacts optical device performance. For example, laser devices fabricated using the new platform exhibit more than a 100-fold improvement relative to predecessors in terms of how long light remains coherent.

"The expanded wavelength coverage our method offers will support many important atomic operations, making chip-scale atomic sensors, optical clocks, and ion-trap systems possible," Chen says.

Colburn acknowledges that it might at first seem "a little ridiculous" that the researchers are aiming for losses that can be described by percentages over kilometers. "After all, our chips are only 2 centimeters across. But, in reality, there are a lot of applications where this would be very powerful," he says. For example, think of the ring resonator, a fundamental optical device widely used in both fundamental science and data transmission. In ring resonators, light enters at one point and gets fed into a ring where it continues to propagate for a long time, a process that resonantly enhances the light at a few frequencies. Even though the ring is just millimeters in scale, the effective path that light traces in such a resonator is determined by the loss of the waveguides. "That's where low loss over meters, or ultimately kilometers, really matters," Colburn says. "The longer light can circulate, the higher the performance of resulting devices can be." For lasers that use these resonators to improve coherence, every factor of 10 reduction in loss translates to a factor of 100 improvement in coherence.

More generally, the ability to engineer ultralow-loss waveguides in the visible bands has many applications. "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 says. To illustrate this point, the Caltech team describes in the paper several optical devices they built with the new material. This includes ring resonators, different types of lasers, and nonlinear resonators that generate a range of frequencies.

And the team is just getting started, Vahala says. "We haven't gone as far as we want to go, but we've made significant progress over the last five years, and that's what we're reporting on here," he says.

The paper is titled "Towards fibre-like loss for photonic integration from violet to near-infrared": https://www.nature.com/articles/s41586-025-09889-w. Additional Caltech authors are graduate students Peng Liu (MS '24), Hongrui Yan, Jinhao Ge (MS '24), Jin-Yu Liu (MS '24), and Phineas Lehan; former graduate student Qing-Xin Ji (PhD '25); former postdoctoral scholar Zhiquan Yuan (PhD '24); and Hanfei Hou, who participated in the research as part of the Summer Undergraduate Research Fellowship program. Dirk Bouwmeester of UC Santa Barbara and Leiden University in the Netherlands and Christopher Holmes and James Gates of the University of Southampton in the United Kingdom are also authors. The work was funded by grants from the Defense Advanced Research Projects Agency, the Air Force Research Laboratory, the Engineering and Physical Sciences Research Council, and the Kavli Nanoscience Institute at Caltech.