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, 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," said Kerry Vahala (BS '80, Ph.D. '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 ( https://www.nature.com/articles/s41586-025-09889-w ). 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. Here is an exclusive Tech Briefs interview, edited for length and clarity, with Chen and Colburn.
Tech Briefs: What was the biggest technical challenge you faced while developing this method?
Colburn: The largest technical challenge was the devices that we developed are effectively like the lowest loss devices ever made. And when you're starting to get into the levels of the lowest possible losses, you need to do every single step in a fabrication flow absolutely perfectly. Any single defect that you could have will lead to an increase in loss or a drop in the quality factor. When you work in the nosebleeds, anything will kill.
Tech Briefs: Can you explain in simple terms how photonic integrated circuits work, please?
Colburn: It's effectively hoarding over two ideas that have existed for a very long time. This also gets at the soul of our paper: total internal reflection — i.e. if you clad a material in a material that is a lower index than it, then it will allow light to effectively be confined and be guided within the core of the material.
The thing that everyone is familiar with in terms of total internal reflection is optical fiber. The way that optical fiber works is that there are two classes of differing index, one with the core that is a higher index than the cladding, and it can guide light for extremely long distances at that point, in part due to its extremely low loss nature.
Then the second principle that is ubiquitous is semiconductor manufacturing. In traditional electronic computer chips, they take a material that can guide electricity, and they effectively etch it to create circuits out of these little semiconductor electronic materials. The way they do that is through nanofabrication.
Going back to the statement I made, these two things, semiconductor manufacturing and total internal reflection — now instead of etching materials that we want to guide electrical circuits, we can instead etch materials that are higher index and then cloud it in the lower index and guide light down the circuit.
Tech Briefs: Do you have any set plans for further research, work, etc.?
Colburn: We've really unlocked the visible band, but if you were to compare us to fiber optic, yes, we had a factor of 20 increase compared to leading-edge platforms. But fiber optic is still about a factor of 100 higher than what we are. We know that we can reach there effectively because the material that we're using is the exact same material as fiber optic. Your material will effectively set your ultimate lowest loss.
Fiber optic was developed in 1972, and the original fiber optic was roughly the same level where we are at right now. It took about 20 years of collective effort to get it down to where it is. We’d like to build and keep advancing our limits.
Tech Briefs: Is there anything else you'd like to add that I didn't touch upon?
Chen: I think if we have a very long waveguide, we can also do some applications like fiber optical gyro on a chip. Also, we can achieve a fiber laser coherence. If we have a long waveguide, we have a large model volume, we can reduce the thermal noise, so we can achieve a very high coherent laser on the chip, especially on the high offset frequency.
Colburn: The other thing to note is that this presents itself as a very promising platform for photonic integrated circuit-based quantum computing, because effectively the loss along the circuit in the classical regime, in the quantum regime, that's the decoherence of your quantum state. So, if you generate a quantum state, what ends up happening is that that state decoheres. The lower the loss you have, classically corresponds to the lower decoherence you have in the quantum regime.
