AI data centers need to move enormous amounts of data between chips, but making those connections faster without increasing energy consumption remains a challenge.
Researchers in Japan have developed a compact silicon optical modulator that transmits data at up to 200 gigabits per second (Gb/s), using slow-moving light and a built-in electrical equalizer to overcome a speed limitation that has hindered similar devices.
Conventional silicon modulators, which convert electrical signals into light, typically measure several millimeters and face a trade-off between efficient modulation and high-frequency performance.
Slowing light strengthens its interaction with a device, improving efficiency, but electrical resistance can limit how quickly the modulator responds.
The new design addresses both problems, potentially helping data centers accommodate more optical connections while reducing energy consumption.
“We created a silicon photonic crystal waveguide optical modulator that integrates a built-in electro-optic equalizer to break the bandwidth limitation while preserving the benefits of the slow-light effect,” Toshihiko Baba, one of the study authors and a professor at YOKOHAMA National University, said.
The power of slow light and an equalizer
The researchers combined two techniques in a single silicon device. Its photonic crystal waveguide (a tiny structure that guides light through a silicon chip) contains a carefully arranged pattern of tiny holes that controls how light travels through the material, allowing it to propagate more slowly than in an ordinary waveguide.
This slow-light effect strengthens the interaction between light and the device, allowing a small electrical change to produce a larger change in the outgoing light signal. The result is more efficient modulation in a smaller space.
Conventional slow-light modulators, however, face a problem. Electrical resistance limits their high-frequency response, restricting how quickly they can process changing signals.
To overcome this limitation, the researchers added a built-in electro-optic (EO) equalizer that helps the modulator handle faster-changing signals. It adjusts how different parts of the light signal combine, allowing the device to maintain its performance at higher speeds.
As a result, the range of signal frequencies the device could handle increased from 35 to 80 gigahertz (GHz) in one design and from 23 to 55 GHz in another.
Achieving incredible data transmission speed
The researcher fabricated the device using a 300-millimeter silicon-on-insulator manufacturing process.
The complete device measures just 320 micrometers (µm) in length, including its main and equalizer sections, compared with 2–4 millimeters for conventional silicon Mach-Zehnder modulators. The entire device, including its electrodes, occupies approximately 0.4 square millimeters.
With a low-complexity 11-tap equalizer at the receiver, the team achieved 140 Gb/s using on-off keying, in which light represents data through two signal levels. The modulator consumed 26 femtojoules (fJ) per bit, a femtojoule is an extremely small unit of energy which is equal to one quadrillionth of a joule..
Using pulse-amplitude modulation with four signal levels (PAM-4), the researchers achieved 180 Gb/s without digital signal processing (DSP). Adding the 11-tap receiver equalizer increased the rate to 200 Gb/s, with the modulator consuming 63 fJ per bit.
The best part is, “slow light can be fabricated using CMOS-compatible processes, simultaneously satisfying high speed, low power consumption and small footprint requirements, meeting the next-generation optical transmitter needs of AI data centers,” Baba added.
This means for this approach, there is no need for specialized manufacturing steps often required by some alternative materials, such as thin-film lithium niobate.
A smaller modulator still has hurdles to clear
The design could help data centers accommodate denser optical connections while limiting energy use. However, its optical insertion loss — the reduction in light intensity as it passes through a device — remains relatively high at 11.7–15.5 decibels.
The researchers used an optical amplifier to compensate during their experiments, but practical applications would benefit from reducing this loss through design optimization.
They also plan to expand the range of wavelengths the modulator can handle, potentially enabling multiple optical channels to operate together. The researchers estimate that integrating 16–32 channels could eventually support a combined capacity of 3.2–6.4 terabits per second. These figures are projections, not demonstrated results.
“We believe that this architecture offers promise as a building block for future co-packaged optics in high-density data centers,” Baba said.
The study is published in the journal Nature Communications.