New Modulator Boosts Compact Photonic Integrated Circuits

Researchers at the Skolkovo Institute of Science and Technology (Skoltech) have developed an ultra-compact electro-optic modulator. This device uses silicon photonics and plasmonics to control optical signals efficiently. It operates within a small physical footprint.

This technology has potential applications in optical communication systems. It could also be used in analog-to-digital conversion. Devices for generating and processing ultra-high-frequency signals based on photonic technologies may also benefit. The work was published in the journal *Light: Advanced Manufacturing*.

The modulator uses a multimode silicon waveguide. This waveguide is approximately seven micrometers wide and 220 nanometers thick. A thin layer of indium tin oxide covers the waveguide. Applying voltage changes the electron concentration in a two-nanometer layer at the interface between the indium tin oxide and silicon dioxide. This alteration changes the material's refractive index and absorption coefficient, allowing light modulation.

The device's key innovation is its multimode operation. Multiple light modes can propagate simultaneously and couple within the wide waveguide. By controlling the excitation parameters of these modes, researchers can achieve either amplitude or phase modulation of light. The device produces two spatially separated signals with a 180-degree phase shift. This balanced output can improve noise performance and enable spatial modulation within a micron-sized device.

The modulator achieved a direct current (DC) extinction ratio of 20.6 decibels for a 1.6-micrometer structure. This corresponds to a record value of 12.8 decibels per micrometer. The alternating current (AC) extinction ratio was 2.48 decibels at ten megahertz. It decreased to 1.25 decibels at one gigahertz for a 3.6-micrometer modulator. The device's characteristics can be tuned after fabrication by adjusting the position of input and output optical fibers.

This development could lead to compact photonic integrated circuits for signal processing systems with balanced detection. Such systems typically use Mach-Zehnder interferometers several millimeters long. The new device performs the same function within a few micrometers, significantly reducing size and simplifying integration. This compact design is advantageous for high-speed signal processing.