Researchers at Seoul National University and Seoul National University have developed a programmable photonic integrated circuit that can slow down light whenever needed.
The team was led by Professors Namkyoo Park and Seung-gyu Yu from the School of Electrical and Computer Engineering at Seoul National University, and collaborated with Professor Xiangji Piao from the School of Electrical and Computer Engineering at Seoul National University.
Slowing down light could help solve computing bottlenecks
The rapid growth of generative AI and large-scale AI models has sharply increased the amount of computing power required in data centers and servers. Traditional electronic semiconductors struggle to keep up with this pace because they consume large amounts of energy and have limited data transfer speeds.
These challenges have led to increased interest in optical computing, which uses light rather than electrical signals to process information. Optical systems have the potential to move data at lightning speeds while using less power.
However, light also poses major challenges. Because they naturally move at a constant speed, it is difficult to slow down or temporarily hold light signals. These features are essential for creating buffer and memory functions in optical computers.
To address this problem, the researchers designed a programmable optical circuit that can control both the speed and shape of the optical signal. Their approach provides more flexibility for “slow lights” than previously proposed methods.
This study was published in a prestigious international journal cutting edge science.
Why optical signals sometimes need to wait
Photonic integrated circuits are emerging as a promising technology that uses light to process information quickly and efficiently. In data centers, optical communications networks, and future computing systems, moving signals quickly is only part of the challenge.
The system must also ensure that the various signals arrive at the correct times. In some cases, optical signals must be delayed so that they can remain synchronized with other information traveling through the system.
One method to create these delays relies on coupled cavity-induced transparency (CRIT), which uses interference between multiple optical cavities.
CRIT allows light within a selected frequency range to pass through the device while slowing down the propagation speed of the optical signal.
- Coupled Resonator Induced Transparency (CRIT): An optical phenomenon in which light within a specific frequency range is selectively transmitted and delayed due to interference between multiple resonators.
- Optical resonator: An optical device that confines or circulates light at a specific frequency for a certain period of time. Used for signal delay, filtering, and modulation.
Limited flexibility due to fixed optical devices
Traditional CRIT devices typically have permanent operating characteristics once manufactured. This makes it difficult to change functionality after manufacturing.
For example, engineers who want to create longer signal delays or work with different frequency ranges often need to design and manufacture entirely new photonic devices.
This lack of adaptability increases the complexity of optical communications hardware and data center infrastructure. Additionally, each new feature required can increase costs and extend development schedules.
This issue is especially important for AI servers and next-generation data centers that need to process vast amounts of information in real time. Therefore, fixed optical components remain a major obstacle for more practical optical computing systems.
Programmable design to control light
The research team developed a different strategy by treating the two optical states of the CRIT system, known as bright and dark modes, as one unified degree of freedom. The researchers also added two controllable loop couplers.
These changes have led to new design principles for programmable photonic integrated circuits. The resonator placement, which was previously fixed in one configuration after manufacturing, can now be adjusted for different purposes.
The researchers showed that the new CRIT structure can be used to delay and control the movement of light on demand. They also demonstrated that the interference between bright and dark modes can be treated as a single integrated design parameter.
This approach greatly expands the flexibility of optical resonator circuits, which was previously limited by fixed designs.
Control delay, bandwidth, and signal shape
Researchers have theoretically demonstrated that two loop couplers can be used to tune the bandwidth and passband shape. They could also control how long signals were delayed and how efficiently those signals traveled through the circuit.
This means that both the speed and transmission behavior of the optical signal can be reconfigured not only within a single resonator, but also across a system containing multiple resonators.
Numerical simulations also showed that the speed of the light pulses can be dynamically adjusted during circuit operation.
As a result, we found that it was possible to change the signal delay time without reducing processing performance. The system can also convert the frequency of light without the need for additional specialized components.
- Light pulse: A short burst of light used as the basic unit for transmitting information in optical communications and computing systems.
Simulations suggest this chip could be practical
The researchers used three-dimensional electromagnetic simulations to test whether CRIT devices could be built on a silicon nitride (Si3N4) photonic integrated circuit platform.
We also evaluated the range of real-world issues that can affect the device during manufacturing and operation. These include material losses, differences in cavity quality, backscattering, coupling variations, loop coupler phase errors, and thermal crosstalk.
Simulations showed that the proposed structure can continue to operate reliably under realistic conditions.
- Silicon nitride (Si3N4) photonic integrated circuits: A low-loss, highly stable waveguide platform widely used in optical signal processing and integrated photonic devices.
- Thermal crosstalk: A phenomenon in which heat generated in one part of a circuit can affect adjacent components and affect device performance.
One chip can perform multiple optical functions
In this study, we introduce a programmable photonic platform that can control both the timing and frequency characteristics of optical signals in real time.
This design overcomes the limitations of traditional optical delay devices, which typically perform only fixed functions. It also suggests that several important functions may eventually be combined within a single optical circuit.
These features include signal synchronization, adjustable delay lines, optical buffers, and frequency conversion.
The same design principles can be useful beyond CRIT systems. The researchers believe this approach could be applied to a wide range of optical circuits based on resonators, providing the basis for more adaptive optical signal processing techniques.
Potential benefits of AI and data centers
If the technology becomes commercially available, a single programmable optical chip could perform multiple tasks, such as controlling signal speeds or switching between different functions.
In that sense, chips could operate in a manner similar to software-defined systems, adjusting their behavior to changing needs.
This flexibility allows data centers and AI servers to process information more efficiently while reducing energy consumption.
Combining multiple signal processing functions on a single chip has the potential to make optical communication equipment and sensor systems smaller and cheaper.
In the long term, this technology could support industries that rely on extremely fast information processing, such as autonomous driving, next-generation communications, and quantum technology.
Researchers plan large-scale programmable photonic system
Professor Namkyoo Park from Seoul National University, co-corresponding author of the study, said, “This study is significant in that it proposes a new design principle that allows the light flow in photonic integrated circuits to be reconfigured as needed, greatly increasing design flexibility. We plan to extend this technology to large-scale programmable photonic integrated circuits based on silicon photonics and photonic AI technology.”
Co-authors Dr. Seung-kyun Park and Ph.D. Student Beomjoon Chae, who led the theoretical framework and numerical analysis, added, “Through this research, we realized that reinterpreting the physics of traditional optical resonators from a different perspective can be a starting point for discovering new capabilities for optical integrated circuits. We plan to further develop this research toward actual device implementation and experimental validation.”
Dr. Seung-kyun Park is a member of KAIST’s InnoCORE PICORE Center and is currently conducting research on photonic AI and quantum optics at the Photonic Systems Research Institute at Seoul National University.
PhD student Beomjoon Chae conducts research on programmable photonic integrated circuits at the Institute of Intelligent Wave Systems at Seoul National University.
This research was supported by the Ministry of Science, Information and Communications through the Innovative Research Centers (IRC) Program, the Basic Research Laboratories (BRL) Program, and the Young Researchers Program.
Dr. Seung-kyun Park also participated in this study with support from the InnoCORE program (PICORE Center).
