Caltech researchers have found a way to move light across silicon wafers with exceptionally little signal loss, reaching levels at visible wavelengths that approach those of optical fiber. The advance could support a new generation of highly coherent and energy-efficient photonic integrated circuits (PICs), with potential uses ranging from optical clocks and gyroscopes to AI data center communications and quantum computing.
Optical fiber already forms much of the hidden infrastructure behind modern communications. It allows information to travel over long distances at high speed because the glass inside the fiber is extremely pure and its surface is engineered to be exceptionally smooth. As a result, most of the light entering one end can reach the other without being absorbed, scattered, or otherwise lost. Researchers refer to this 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 team reports the technique in a recent paper published in Nature. The study’s lead authors are Caltech postdoctoral scholar Hao-Jing Chen and graduate student Kellan Colburn (MS ’25), who carried out the work in Vahala’s lab.
Bringing Fiber Optic Glass Onto Chips
The researchers create waveguides (nanoscale on-chip pathways that channel light) using germano-silicate, the same type of glass used in optical fiber. They adapt the material to a lithography-based manufacturing method suitable for making devices on wafers.
Instead of running in a straight line, the waveguides are arranged in spirals. This allows light to travel a much longer optical path while remaining inside a very small area. The idea resembles winding optical fiber around a spool, but nanofabrication makes it possible to fit the path into a far smaller footprint.
“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.
A 20 Fold Advantage at Visible Wavelengths
At near-infrared wavelengths, devices built with the new Caltech platform have already matched the performance of some of the best previous devices made from silicon nitride. Silicon nitride is widely used in optical technology because it can transmit data with relatively little signal loss.
The advantage becomes much larger at visible wavelengths. The new germano-silicate platform substantially surpasses silicon nitride in this part of the spectrum.
“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.”
Reducing loss can have a dramatic effect on the performance of optical devices. Lasers produced with the new platform, for example, show more than a 100-fold improvement over previous designs in how long their 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.
Why Tiny Chips Need Kilometer-Scale Performance
At first, the effort to measure losses over distances of meters or even kilometers might seem excessive for devices that fit on a small chip.
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.
One example is the ring resonator, a basic optical device used in both scientific research and data transmission. Light enters the device and is directed into a ring, where it can continue circulating for an extended period. This repeated circulation strengthens the light at particular frequencies.
Although the physical ring may measure only a few millimeters, the total effective distance traveled by the light depends on how little energy is lost from the waveguide each time it goes around.
“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 rely on these resonators to increase coherence, the benefit grows rapidly. Every factor of 10 decrease in loss produces a factor of 100 improvement in coherence.
From Optical Clocks to Quantum Technology
The ability to create ultralow-loss waveguides across visible wavelengths could be useful for many different technologies.
“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 demonstrate that versatility, the researchers describe several devices made with the new material in their paper. These include ring resonators, multiple types of lasers, and nonlinear resonators capable of generating a range of frequencies.
The researchers also see the current results as an early stage rather than an endpoint.
“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,” Vahala says.
The paper is titled “Towards fibre-like loss for photonic integration from violet to near-infrared.” 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.
