High-Performance Silicon Optical Modulator Developed For Optoelectronic Integration

Reduced complex signal processing demonstrates 200 Gb/s low-power optical modulation.

Points

• A small silicon optical modulator demonstrates low-power optical modulation at 200 gigabits per second.
• By utilizing slow light and integrating an electro-optical equalizer, the digital signal processing load is reduced.
• Contributing to increased capacity and density of optical connections for CPO in AI data centers.

Overview
A research team led by Assistant Professor Keisuke Kawahara and Professor Nobuhiko Nishiyama of the Department of Electrical and Electronic Engineering, School of Engineering, Tokyo University of Science, and Professor Toshihiko Baba of the Graduate School of Engineering, Yokohama National University, in collaboration with a research team led by Research Team Leader Yuriko Maegami of the National Institute of Advanced Industrial Science and Technology (AIST), has demonstrated high-speed optical modulation at 200 gigabits per second (200 Gb/s) with low power consumption using a compact silicon optical modulator ^{[Term 1]}.

With the proliferation of generative AI, data centers need to process massive amounts of data by coordinating numerous computers. To support this performance, high-speed optical communication is required, along with increased speed, miniaturization, and power efficiency of the components.

This research focused on silicon optical modulators, which can integrate a large number of elements, and attempted to achieve both miniaturization and low power consumption. In particular, a photonic crystal waveguide that generates " slow light " ^{[Term 2]}, which transmits light slowly , was introduced into the optical modulator, enabling miniaturization and low power consumption. Waveform shaping, which was conventionally handled by external electronic circuits, was realized on-chip by incorporating an "electro-optical equalizer," achieving high-speed operation while reducing reliance on digital signal processing (DSP) ^{[Term 3]}. The minimum driving energy per bit is 26 femtojoules, which is approximately 1/100th of the conventional standard value. Such modulators are expected to have applications in copacked optics (CPO) ^{[Term 4]}, which are attracting attention as optical connection technology for AI data centers.

The results of this research were published in the international academic journal " Nature Communications " on October 22, 2026 (local time).

Background
With the proliferation of generative AI, data centers are now processing massive amounts of data by coordinating numerous computers. Supporting this performance is high-speed optical interconnection connecting these computers. To expand communication capacity, it's necessary to increase the amount of information that can be transmitted with a single optical signal and integrate numerous communication circuits into a limited space. To achieve this, there is a demand for faster, smaller, and more power-efficient components.

Silicon optical modulators, which convert electrical signals into optical signals, are expected to support high-speed optical interconnects because their numerous elements can be integrated using semiconductor manufacturing technology. However, conventional structures require lengths of several millimeters to obtain sufficient optical modulation, which not only hinders integration but also makes high-speed operation difficult. As a result, there has been the challenge of complex DSPs that compensate for signal waveform distortion that occurs with increasing speed. Furthermore, the need for high drive voltages has led to increased power consumption in the drive circuits, which has also been a challenge.

In photonic crystal waveguides, where tiny holes are regularly arranged in silicon, slow light transmission can be utilized. When slow light is employed, the interaction between light and the material strengthens, allowing short elements to operate at low voltages, leading to miniaturization and power savings. However, there are limitations to the electrical response speed of the elements, requiring compensation by electronic circuits for higher speeds. To achieve increased communication capacity, it was necessary to overcome this speed limitation while maintaining the advantages of small size and low power consumption.

Research results
The research team introduced an electro-optical equalizer for waveform shaping into an optical modulator using a photonic crystal waveguide with a slow-light effect, employing photoelectric collaborative design technology ^{[Term 5]}. The optical modulation section is divided into multiple sections, with a path that delays light and a structure where two light paths intersect placed along the way. High-frequency components are compensated for by combining modulation with shifted timing and modulation with reversed direction. This mechanism shifts part of the waveform shaping, previously performed by electronic circuits, to the optical modulator's structure itself

This optical modulator was fabricated using a silicon photonics process with semiconductor manufacturing equipment for 300 mm wafers at the National Institute of Advanced Industrial Science and Technology (AIST), resulting in the formation of a fine photonic crystal structure with high precision. The element used in the high-speed modulation experiment was short, measuring only 320 µm (0.32 mm), and its area, including the electrodes, was also compact at 0.4 mm² . The 3 dB bandwidth, an indicator of response speed, was more than double that of conventional silicon photonic crystal optical modulators.

In high-speed modulation experiments, a clear eye pattern ^{[Term 7]} of 180 Gb/s was obtained without waveform compensation by a DSP using the PAM4 ^{[Term 6]} method, which switches the light intensity in four stages . Furthermore, by applying a simple 11-tap DSP to the receiving side, 200 Gb/s operation was achieved at a drive voltage of 1.5 V. In a method that represents the signal by switching the light on and off, 140 Gb/s was achieved at 0.6 V using the same DSP. At this time, the energy consumption per bit in modulation was 26 femtojoules. This is a small value, 1/100th of the conventional standard value for this type of modulator. As described above, a small, low-power optical modulator was fabricated, and high-speed operation with reduced reliance on complex DSPs was achieved.

Social impact
In data centers with numerous optical communication circuits, reducing the power consumption and integration area of ​​each circuit is crucial for increasing overall communication capacity. This research demonstrates that a portion of the waveform shaping, which supports high-speed optical interconnects, can be implemented within the optical modulator. This technology makes it possible to share processing previously handled by electronic circuits with optical elements, which is expected to lead to overall power savings in the transceiver module.

Furthermore, this device can be fabricated using the versatile Multi-Project Wafer (MPW) silicon photonics process, which allows multiple designs to be integrated on a single wafer, and operates at low voltages suitable for advanced CMOS electronic circuits. It is easy to develop configurations that integrate optical and electronic circuits at high density, and is expected to become a foundational technology for achieving high-capacity inter-computer communication supporting AI with less power and mounting area.

Future developments
This research is expected to have applications in CPO (Cyber-Polished Ovens), which involve densely mounting numerous optical transceivers near electronic circuits. Furthermore, this approach of having optical elements handle signal processing can be extended to designing more efficient systems by optimizing the division of roles between light and electrons. It is expected that advancements in such photoelectric collaborative design technologies will lead to sustained performance improvements in data centers that support the development of AI.

This research was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants JP25K23449 and JP23KJ0988, the New Energy and Industrial Technology Development Organization (NEDO) Grant JPNP16007, and Lumentum Japan Co., Ltd., Keysight Technologies, Ltd., and others.

^{[Term 1]}
Silicon optical modulator : A device that modulates information into light by changing the intensity and phase of light using an electrical signal applied to silicon. In this study, a Mach-Zehnder type modulator was used, which utilizes the interference of light passing through two optical paths.

^{[Term 2]}
Slow light : A state in which the group speed of light propagating through a material is extremely slow, or the technique for achieving this. Because the interaction between light and material is strengthened, light can be modulated with low voltage even with short elements.

^{[Term 3]}
Digital signal processing (DSP) : A technology that treats signals as digital values ​​and processes them through calculations.

^{[Term 4]}
Copackaged Optics (CPO) : A technology that integrates semiconductor chips for data processing and optical communication components by placing them in close proximity.

^{[Term 5]}
Photoelectric collaborative design technology : A method of designing the part that transmits information using light and the part that processes and controls information using electricity as an integrated unit, rather than separately.

^{[Term 6]}
PAM4 : A method that switches the light intensity in four stages, with each switch representing 2 bits of information. It can transmit more information at the same switching speed than methods that represent information in two stages (on/off).

^{[Term 7]}
Eye pattern : A display created by superimposing repeatedly obtained signal waveforms. The eye-like opening that appears in the center allows for visual evaluation of the ease of signal discrimination and waveform distortion.