7/15/2024 Jenny Applequist 4 min read
Written by Jenny Applequist
The approach for making 3D structures inside a wafer previously suffered from too much signal loss.
Four years ago, the groups of Lynford L. Goddard and Paul V. Braun unveiled a new technology called SCRIBE—an innovative lithographic approach for making three-dimensional optical and photonic structures inside a wafer. While SCRIBE was groundbreaking, it hasn’t been usable in practice, as fabricated devices showed severe loss of signal power.
Now, in a new paper published in the Optica journal, the team has announced improvements that have eliminated nearly all the loss and made SCRIBE—and the myriad new functionalities it can enable—ready for prime time.
SCRIBE enables photonic chips that are not constrained by the traditional two-dimensional, single-layer nature of standard chips, and also are not confined to three-dimensional structures that are merely a stack of planes.
“We can make volumetric optical interconnects, which are a system of lenses and arbitrarily shaped 3D waveguides that route light from multiple input ports to corresponding output ports,” explained Goddard. “We can essentially realize an optical table inside a chip. We gain the benefits of bulk optical systems, but miniaturized and aligned to extremely high precision through our microscale 3D printing method.”
Braun added that “SCRIBE has considerable materials and process flexibility. SCRIBE can be performed in both silicon and silica, can be integrated with conventional semiconductor processing steps, and can be used with both commercial and custom photoresists.”
Previously, there was a big problem with SCRIBE: when the optical signal went from fiber to chip, and then back to fiber, only 1 photon of light came out for every 100,000 photons that went in. “So, it was too inefficient for any practical application,” said Goddard. “Therefore, the main goal of the new work was to enable a very low loss connection between a fiber and an optical processing chip that can someday go inside a computer.”
More specifically, the main contributions of the paper pertain to understanding and mitigating three sources of loss: those due to the way the fabrication tool prints, those due to how samples and devices are designed, and those due to the way testing is done.
By making improvements in all three areas, the team dramatically reduced the loss: now, for every 100,000 photons sent in, about 60,000 come out. “And that’s enough that you can do practical applications,” said Goddard.
“We have the ability now to create complicated 3D routing structures inside a chip, and also do some signal processing,” he continued. “We can split signals according to their wavelength... or according to their polarization. We can combine signals... So there are many operations that we can do on these optical signals because we have low loss plus integrated bulk optics and waveguide structures.”
The breakthrough should enable new broadband packaging solutions, as it will allow fiber-to-chip coupling independent of the wavelength or polarization. Further, the solution allows for a structure that is both compact and customizable, so it can be used for interconnecting single- and multi-core fibers, lasers, and photonic integrated circuits.
To achieve their goals, the team had to improve direct laser write print quality in multiple respects. Those improvements are generalizable to other 3D printing systems and may attract widespread adoption.
Goddard is the Associate Dean for Diversity, Equity, and Inclusion in the Grainger College of Engineering and the Edward C. Jordan Professor of Electrical & Computer Engineering. He is also in the Holonyak Micro and Nanotechnology Lab.
Braun is the Director of the Materials Research Laboratory, a Grainger Distinguished Chair in Engineering, and a Professor of Materials Science & Engineering, Mechanical Science & Engineering, Chemistry, and Chemical & Biomolecular Engineering.
Goddard and Braun have multiple pending and issued patents related to this work, and are actively working with industry to commercialize the technology.
The new paper, “Low loss fiber-coupled volumetric interconnects fabricated via direct laser writing,” is available on Optica’s website.