Professors Sunkyu Yu And Namkyoo Park's Research Team Develops Programmable Photonic Integrated Circuit That Can Slow Down The Speed Of Light

Photonic integrated circuits are gaining attention as a next-generation technology capable of processing information quickly and efficiently using light. In particular, in the fields of data centers, optical communications, and optical computing, technologies that go beyond simply transmitting optical signals at high speed are becoming increasingly important. These include synchronizing the arrival times of multiple signals and delaying signals when needed.

To realize these functions, researchers have studied coupled-resonator-induced transparency (CRIT) structures, which use interference between multiple optical resonators. CRIT is an optical phenomenon that selectively transmits light within a specific frequency band and, in the process, can slow down the propagation speed of optical signals.

However, conventional CRIT structures are largely fixed once fabricated, making it difficult to reconfigure the same circuit for different functions when application requirements change. For example, to delay optical signals for a longer period or shift them to a certain frequency band, a new optical device tailored to that specific function had to be designed.

As such, optical communication equipment and data center systems have become more complex, while adding new functions has required significant time and cost. In environments such as AI servers and next-generation data centers, where massive amounts of data must be processed in real time, this lack of flexibility has been regarded as a major obstacle to the advancement of optical computing technologies.

Research Achievements
To overcome this limitation, the joint research team proposed a new approach that treats the two optical states that constitute a CRIT system—the bright mode and the dark mode—as a unified system. The team also introduced two controllable loop couplers and established a new design principle for a programmable photonic integrated circuit that can reconfigure optical resonator structures, which were previously difficult to modify after fabrication, depending on specific requirements.

The researchers came up with a new CRIT structure capable of delaying and controlling the flow of light and demonstrated that optical interference between the bright and dark modes can be treated as a single design variable. This significantly expands the design freedom of optical resonator circuits, whose structures had previously been fixed.

In particular, the team proved that two loop couplers can be used to control the width and shape of the frequency band through which optical signals pass, as well as the delay and transmission characteristics of optical signals traveling through the circuit. This means that the propagation speed and transmission characteristics of optical signals can be freely reconfigured not only in a single optical resonator, but also across an entire structure composed of multiple connected resonators.

The researchers also numerically demonstrated how the propagation speed of optical pulses changes in real time while the circuit is being actively controlled. As a result, they confirmed that the delay time of optical signals can be freely adjusted while largely maintaining signal-processing performance. They also verified that the frequency components of light can be converted without adding any special devices.

Furthermore, through three-dimensional electromagnetic field simulations, the research team verified that the proposed CRIT device can be implemented on a silicon nitride (Si₃N₄) photonic integrated circuit platform. They also analyzed various factors that may arise during actual fabrication and operation, including material loss, variations in resonator quality, backscattering, changes in coupling characteristics, phase errors in loop couplers, and thermal crosstalk. The results confirmed that the proposed structure can operate stably even in realistic photonic integrated circuit environments.

Expected Impact
This research is significant because it presents a new programmable photonic integrated circuit platform that goes beyond the limitations of conventional fixed optical signal delay structures and enables the temporal and frequency characteristics of optical signals to be controlled even while the circuit is operating. In particular, the study demonstrates the possibility of implementing key functions required for next-generation optical interconnects—such as optical signal synchronization, variable delay lines, optical buffers, and light-frequency conversion—within a single photonic integrated circuit structure.

The photonic integrated circuit design method proposed by the research team can also be extended beyond CRIT to the dynamic control of various resonator-based optical circuits. This suggests that the design principle could serve as a foundation for next-generation optical signal processing technologies that design and control the flow of light according to specific needs.

If the proposed photonic integrated circuit is commercialized in the future, it is expected to allow the speed of optical signals to be adjusted as needed while enabling a single optical chip to switch among various functions like software. As such, the technology is expected to reduce power consumption and improve data processing efficiency in data centers and AI servers.

Additionally, because various signal-processing functions can be integrated into a single optical chip, the technology could contribute to the miniaturization and cost reduction of optical communication equipment and sensor systems. In the long term, it is expected to serve as a core enabling technology in a wide range of industries that require ultrafast information processing, including autonomous driving, next-generation communications, and quantum technologies.