Skoltech Team Develops Modulator for Compact Photonic Integrated Circuits

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Researchers at Skoltech have developed an ultra-compact electro-optic modulator based on silicon photonics and plasmonics that enables high-efficiency optical signal control within a small device footprint. The development could find applications in optical communication systems, analog-to-digital conversion, as well as in devices for generating and processing ultra-high-frequency signals based on photonic technologies. The work, supported by a grant from the Russian Science Foundation, was published in the journal Light: Advanced Manufacturing, which ranks in the top 2-5 percent of the SCImago Journal & Country Rank.

The proposed device uses a multimode silicon waveguide of about 7 micrometers in width and 220 nanometers in thickness, with a thin layer of indium tin oxide on top. When voltage is applied, the electron concentration changes in an ultra-thin layer approximately 2 nanometers at the interface between the indium tin oxide and the insulating silicon dioxide layer. This alters the refractive index and absorption coefficient of the material, making it possible to modulate the transmitted light.

The key difference of this new development from existing analogs is the use of the waveguide’s multimode operation. In a wide waveguide, multiple modes can propagate simultaneously and couple to each other. By controlling the excitation parameters of these modes, researchers can achieve either amplitude or phase modulation of light. At the device output, two spatially separated signals are formed having a 180-degree phase shift relative to each other.

“This publication represents an important achievement in understanding the properties of plasmonic integrated modulators based on transparent conductive oxides. All stages of modulator fabrication and testing were carried out at the Plasmonics Laboratory of the Skoltech Engineering Physics Center. The publication demonstrates for the first time the modulator’s DC extinction ratio and its high-frequency response, with a significant difference in extinction ratio caused by fundamentally distinct mechanisms. The unique multimode design enabled a DC extinction ratio of 20.6 dB for a 1.6 µm long structure over a voltage range of –2 to 1.5 V, corresponding to a record value of 12.8 dB/µm. The directly measured AC extinction ratio is 2.48 dB at a modulation frequency of 10 MHz, decreasing to 1.25 dB at 1 GHz over a voltage range of –2 to 2 V for a 3.6 µm long modulator,” shared Junior Research Scientist Anastasia Zemtsova from the Skoltech Engineering Physics Center and first author of the paper.

“The unique feature of the structure lies in its flexible tunability. The same modulator can be amplitude-modulating, phase-modulating, or even combine these regimes. Moreover, balanced output, in addition to noise performance advantages, enables spatial modulation at the scale of a photonic integrated circuit — all within a micron-sized device. In 2023, we demonstrated a single-mode modulator operating at over 40 GHz. The multimode design has taken the device to a new level,” said Daniil Zemtsov, a research scientist at the Skoltech Engineering Physics Center and co-author of the paper.

“The practical significance of this work lies in the ability to create compact photonic integrated circuits for signal processing systems with balanced detection. Such systems require feeding two opposite-phase signals to a photodetector, which suppresses noise and amplifies the useful signal. This is typically achieved using a Mach-Zehnder interferometer several millimeters in length. Our proposed device, just a few micrometers long, performs the same function within a single waveguide, significantly reducing size and simplifying integration. Importantly, the moderate extinction ratio does not limit its application in high-speed signal processing due to its ultra-compact dimensions. The so-called push-pull architecture combining four modulators provides high overall extinction with a moderate single-modulator level (for example, 20 dB vs. 1 dB), which is commonly used in coherent communication systems for advanced 4D modulation formats,” added Professor Vladimir Drachev, the director of the Skoltech Engineering Physics Center and scientific supervisor of the study.

An additional advantage is the ability to tune the modulator’s characteristics after fabrication. By adjusting the position of the input and output optical fibers relative to the chip surface, the contribution of different modes — and consequently the modulator’s performance characteristics — can be modified. This simplifies calibration and enhances practical interest in the development.