New programmable photonic chip can control how fast light moves

The team was led by Professors Namkyoo Park and Sunkyu Yu of the Department of Electrical and Computer Engineering at Seoul National University, working with Professor Xianji Piao of the School of Electrical and Computer Engineering at the University of Seoul.

Slowing Light Could Help Solve a Computing Bottleneck

The rapid growth of generative AI and large scale AI models has sharply increased the amount of computing power required by data centers and servers. Conventional electronic semiconductors are struggling to keep pace because they consume large amounts of energy and face limits in how quickly they can transmit data.

These challenges have intensified interest in optical computing, which uses light rather than electrical signals to process information. Optical systems could potentially move data at extremely high speeds while using less power.

However, light also presents a major challenge. Because it naturally moves at a fixed speed, it is difficult to delay optical signals or temporarily hold them in place. Those capabilities are essential for creating buffers and memory functions in optical computers.

To address this problem, the researchers designed a programmable photonic circuit that can control both the speed and shape of optical signals. Their approach provides more flexibility over "slow light" than previously proposed methods.

The study was published in the renowned international journal Advanced Science.

Why Optical Signals Sometimes Need To Wait

Photonic integrated circuits are emerging as a promising technology for processing information quickly and efficiently with light. In data centers, optical communication networks, and future computing systems, moving signals rapidly is only part of the challenge.

Systems must also ensure that different signals arrive at the correct time. In some cases, a light signal must be delayed so it can remain synchronized with other information moving through the system.

One method for creating these delays relies on coupled-resonator-induced transparency (CRIT), which uses interference among several optical resonators.

CRIT allows light within a selected frequency range to pass through a device while also reducing the speed at which the optical signal travels.

• Coupled-resonator-induced transparency (CRIT): An optical phenomenon that selectively transmits and delays light within a specific frequency range through interference among multiple resonators.
• Optical resonator: A photonic device that confines or circulates light of a specific frequency for a certain period; used in signal delay, filtering, and modulation.

Fixed Optical Devices Limit Flexibility

Traditional CRIT devices usually have operating characteristics that become permanent once they are manufactured. This makes it difficult to change how they function after fabrication.

For example, engineers who want to create a longer signal delay or work with a different frequency range often need to design and manufacture an entirely new photonic device.

That lack of adaptability increases the complexity of optical communication hardware and data center infrastructure. It can also raise costs and extend development schedules whenever new capabilities are needed.

The problem is especially important for AI servers and next-generation data centers, where enormous amounts of information must be processed in real time. Fixed optical components have therefore remained a major obstacle to more practical optical computing systems.

A Programmable Design for Controlling Light

The research team developed a different strategy by treating two optical states in CRIT systems, known as the bright mode and dark mode, as one unified degree of freedom. The researchers also added two controllable loop couplers.

Together, these changes created a new design principle for programmable photonic integrated circuits. Resonator arrangements that were previously locked into one configuration after fabrication could instead be adjusted for different purposes.

Using the new CRIT structure, the researchers showed that the movement of light could be delayed and controlled as needed. They also demonstrated that interference between the bright and dark modes could be handled as a single integrated design parameter.

This approach greatly expanded the flexibility of photonic resonator circuits that had previously been limited by fixed designs.

Controlling Delay, Bandwidth, and Signal Shape

The researchers theoretically demonstrated that the two loop couplers could be used to adjust the bandwidth and shape of the passband. 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 optical signals could be reconfigured across entire systems containing multiple resonators, rather than only within a single resonator.

Numerical simulations also showed that the speed of optical pulses could be adjusted dynamically while the circuit was operating.

The results indicated that signal delay times could be changed without reducing processing performance. The system could also convert the frequency of light without requiring additional specialized components.

• Optical pulse: A short burst of light used as a basic unit for transmitting information in optical communication and computing systems.

Simulations Suggest the Chip Could Be Practical

The researchers used three-dimensional electromagnetic simulations to test whether the CRIT device could be built on a silicon nitride (Si₃N₄) photonic integrated circuit platform.

They also evaluated a range of real-world issues that could affect the device during manufacturing and operation. These included material losses, differences in resonator quality, backscattering, coupling fluctuations, phase errors in the loop couplers, and thermal crosstalk.

The simulations indicated that the proposed structure could continue to operate reliably under realistic conditions.

• Silicon nitride (Si₃N₄) photonic integrated circuit: A low-loss and highly stable waveguide platform widely used for optical signal processing and integrated photonic devices.
• Thermal crosstalk: A phenomenon in which heat generated in one part of a circuit affects neighboring components, potentially altering device performance.

One Chip Could Perform Several Optical Functions

The study introduces a programmable photonic platform that can control both the timing and frequency properties of light signals in real time.

The design could overcome the limitations of conventional optical delay devices, which typically perform only fixed functions. It also suggests that several important capabilities could eventually be combined within one photonic circuit.

These functions include signal synchronization, adjustable delay lines, optical buffers, and frequency conversion.

The same design principles may also be useful beyond CRIT systems. The researchers believe the approach could be applied to a broad range of photonic circuits based on resonators, potentially providing a foundation for more adaptable optical signal processing technologies.

Potential Benefits for AI and Data Centers

If the technology is commercialized, a single programmable optical chip could perform several tasks, including controlling signal speed and switching between different functions.

In that sense, the chip could operate in a way similar to a software-defined system, with its behavior adjusted according to changing needs.

This flexibility could help data centers and AI servers process information more efficiently while reducing energy consumption.

Combining several signal processing functions on one chip could also make optical communication equipment and sensor systems smaller and less expensive.

Over the longer term, the technology could support industries that depend on extremely fast information processing, including autonomous driving, next-generation communications, and quantum technologies.

Researchers Plan Larger Programmable Photonic Systems

Professor Namkyoo Park, co-corresponding author of the study from Seoul National University, stated, "This research is significant in that it proposes a new design principle that allows the flow of light within photonic integrated circuits to be reconfigured as needed, greatly enhancing design flexibility. We plan to expand this technology toward large-scale programmable photonic integrated circuits based on silicon photonics and photonic AI technologies."

Co-first authors Dr. Seungkyun Park and Ph.D. student Beomjoon Chae, who led the theoretical framework and numerical analysis, added, "Through this study, we realized that reinterpreting conventional photonic resonator physics from a different perspective can serve as a starting point for discovering new functionalities in photonic integrated circuits. We plan to further develop this research toward practical device implementation and experimental validation."

Dr. Seungkyun Park is affiliated with the InnoCORE PICORE Center at KAIST and is currently conducting research on photonic AI and quantum optics at the Photonic Systems Laboratory, Seoul National University.

Ph.D. student Beomjoon Chae is conducting research on programmable photonic integrated circuits at the Intelligent Wave Systems Laboratory, SNU.

The research received support from the Ministry of Science and ICT through the Innovative Research Center (IRC) program, the Basic Research Laboratory (BRL) program, and the Young Researcher Program.

Dr. Seungkyun Park also participated in the study with support from the InnoCORE program (PICORE Center).