We strive to create ever faster computers, and since light travels at the highest speed in the universe, it seems like the perfect candidate for computation. However, in computing systems, not only the speed of data transmission matters, but also the precise timing of data arrival. While there are methods to slow down light, modern equipment is designed for specific speeds and fixed architectures. This raises a challenge: how can we control the arrival time of optical signals using the same hardware platform?

Programmable Photonic Circuit

Researchers from Seoul National University and the University of Seoul have developed a programmable photonic circuit capable of dynamically adjusting the transit time of light pulses through a chip. This allows for precise control over when signals arrive. Unlike devices with fixed delays set during manufacturing, the new system can be reconfigured to change delay times, bandwidth, and frequency characteristics of the optical signal.

Advantages of Photonics

Traditional processors (CPUs and GPUs) transmit data as electrical signals, with electrons moving through metallic connections. As computational demands and processor sizes grow, especially in artificial intelligence systems, transmitting data via metal requires more energy and creates bandwidth bottlenecks.

Photonics converts data into light signals. Photons can carry larger amounts of information at high speeds with less heat loss. However, the high speed of light is not always desirable in computing, since different signals may travel along different paths and perform various operations before they need to be used together. To synchronize them, signals must be delayed, synchronized, buffered, and filtered—tasks that electronics handle easily, but photonics finds challenging.

CRIT Technology and Its Limitations

One method for delaying light is coupled-resonator-induced transparency (CRIT). This technology uses optical resonators—microscopic structures that allow light of certain frequencies to circulate inside. Under specific conditions, interference creates a narrow window through which light signals pass with significant delay. However, once set, the structure of the resonators is fixed, and changing parameters often requires a new chip.

A New Solution: Programmable Architecture

The researchers proposed making the chip’s behavior programmable by introducing tunable elements that allow the resonator network to be reconfigured after fabrication. The architecture includes two adjustable loop couplers, enabling changes in resonator interactions and providing different optical characteristics on the same circuit. Theoretical models showed that it is possible to alter the width and shape of the transmission band, control the efficiency of light passage, and tune the delay time of optical pulses.

Numerical simulations demonstrated that the delay can be dynamically adjusted during circuit operation. The architecture also allows control over the frequency of transmitted light, potentially eliminating the need for separate frequency conversion components in some systems. This combination of features makes the concept attractive: instead of using separate optical components for delay, filtering, and other tasks, a programmable resonator network can be retuned to perform various functions.

Practical Implementation and Prospects

To test the concept, the team modeled a design on a silicon nitride photonic integrated circuit platform and conducted three-dimensional electromagnetic simulations, accounting for real-world defects: material losses, resonator variations, unwanted scattering, coupling variations, phase errors, and thermal interactions between components. The architecture proved functional despite these adverse effects, indicating that it can be realized using existing photonic manufacturing technologies.

If such programmable delay systems can be implemented in practice, they could be used for synchronizing optical signals, creating adjustable delay lines and buffers, modifying bandwidth, and frequency conversion. These functions are especially valuable in optical communications, photonic processors for artificial intelligence, and other systems where precise timing of large numbers of optical signals is required.

Currently, the work is limited to theory and simulations, but it has been shown that such a circuit can be built. The next step will be fabricating real devices and experimental testing, followed by scaling the concept to larger programmable photonic circuits. The ability to programmatically delay light could become an important step toward more flexible photonic-based computing.