Imroatul Nasiyah
In a significant leap forward for optical computing and communications, researchers at the California Institute of Technology (Caltech) have successfully developed a novel method to guide light across silicon wafers with an astonishingly low level of signal loss. This breakthrough, detailed in a recent publication in the prestigious journal Nature, brings the performance characteristics of high-end optical fiber, renowned for its ultralow loss, directly onto the compact and versatile platform of silicon chips. The implications are far-reaching, promising to unlock a new era of highly coherent, energy-efficient photonic integrated circuits (PICs) with potential applications spanning from advanced optical clocks and gyroscopes to the demanding data communication needs of AI data centers and the intricate requirements of quantum computing.
Optical fiber has long been the backbone of modern telecommunications, enabling the high-speed, long-distance transmission of information. This remarkable capability stems from the exceptional purity of the glass core and the meticulous engineering of its surface, which minimize the absorption, scattering, and other forms of signal degradation that plague less refined materials. This phenomenon, known as ultralow loss, is the holy grail that researchers have strived to replicate within the confined dimensions of silicon microchips.
Kerry Vahala, the Ted and Ginger Jenkins Professor of Information Science and Technology and Applied Physics at Caltech, eloquently captured the essence of this long-standing ambition: "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. 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 pioneering work was spearheaded by Caltech postdoctoral scholar Hao-Jing Chen and graduate student Kellan Colburn (MS ’25), working under Vahala’s guidance. Their innovative approach centers on the creation of nanoscale on-chip pathways for light, known as waveguides, fabricated from germano-silicate. This is the very same material that forms the core of high-performance optical fiber. Crucially, the team has ingeniously adapted this material to a lithography-based manufacturing process, making it compatible with the established wafer fabrication techniques used in the semiconductor industry.
Instead of employing straight, linear waveguides, the researchers have opted for a clever design that arranges these pathways in intricate spirals. This architectural choice allows light to traverse significantly longer optical paths while remaining confined within a remarkably small physical footprint. The analogy of winding optical fiber around a spool is apt, but the precision of nanofabrication enables this extended light path to be integrated into a far more compact device than would be possible with traditional fiber.
Henry Blauvelt (PhD ’83), a visiting associate in applied physics and material science at Caltech and chief technology officer at Emcore, a company specializing in photonic circuits, highlighted the practical advantages of this material choice. "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," he stated, underscoring the potential for significant energy savings in data centers.
The performance gains achieved by the Caltech platform are particularly striking across different wavelengths. At near-infrared wavelengths, the newly developed devices have already demonstrated performance levels comparable to some of the most advanced silicon nitride-based devices. Silicon nitride is a widely adopted material in optical technology due to its relatively low signal loss characteristics.
However, the true transformative power of the germano-silicate platform becomes evident at visible wavelengths. In this spectral range, the new material significantly outperforms silicon nitride, a limitation that has historically hampered the development of visible-light PICs. Hao-Jing Chen elaborated on the key to this advancement: "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. At visible wavelengths, our recent platform exceeds silicon nitride’s record by a factor of 20, and we have more room to improve."
This dramatic reduction in signal loss has profound implications for the performance of optical devices. For instance, lasers fabricated using the new platform exhibit a remarkable improvement of over 100-fold in the coherence length of their light, meaning the light waves maintain their phase relationship for much longer distances, a critical factor for many advanced applications.
"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 added, pointing to the direct impact on fundamental scientific instrumentation and emerging quantum technologies.
The pursuit of ultralow loss over distances measured in meters, or even kilometers, might initially seem counterintuitive for devices that fit on a minuscule chip. Kellan Colburn acknowledged this apparent paradox, admitting that aiming for "losses that can be described by percentages over kilometers" might seem "a little ridiculous" when considering the chip’s mere 2-centimeter diameter. However, he quickly emphasized the profound practical utility: "But, in reality, there are a lot of applications where this would be very powerful."
A prime example illustrating this necessity is the ring resonator, a fundamental optical device employed in both scientific research and data transmission. In a ring resonator, light enters and is guided into a circular path, where it can circulate for extended periods. This repeated circulation amplifies light at specific frequencies. While the physical ring itself may be only a few millimeters in size, the total effective distance traveled by the light is directly determined by how little energy is lost from the waveguide with each pass. "That’s where low loss over meters, or ultimately kilometers, really matters," Colburn explained. "The longer light can circulate, the higher the performance of resulting devices can be."
The benefits are particularly exponential for lasers that rely on these resonators to enhance coherence. Every tenfold decrease in loss translates into a hundredfold improvement in coherence, a cascading effect that dramatically boosts performance.
The ability to engineer ultralow-loss waveguides across the visible spectrum opens doors to a vast array of technological possibilities. Vahala described the versatility of this breakthrough as its "Swiss Army-knife quality—it can be applied in a wide range of settings." To demonstrate this broad applicability, the researchers showcased several devices fabricated with the new material in their Nature paper, including advanced ring resonators, multiple types of lasers, and sophisticated nonlinear resonators capable of generating a wide spectrum of frequencies.
The researchers view these current achievements not as an endpoint, but rather as a significant stepping stone. "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 concluded.
The groundbreaking paper, titled "Towards fibre-like loss for photonic integration from violet to near-infrared," features a comprehensive list of Caltech authors, including 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 contributed as part of the Summer Undergraduate Research Fellowship program. The research also involved international collaborators Dirk Bouwmeester from UC Santa Barbara and Leiden University, and Christopher Holmes and James Gates from the University of Southampton. The project was generously supported 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, underscoring the significant national and institutional investment in this transformative research.
