Hybrid silicon and lithium niobate electro-optical ring modulator

TLDR

Lithium niobate is the gold‑standard electro‑optic material, yet bulk LiNbO₃ modulators are large; silicon‑on‑insulator ring modulators offer chip‑scale size but lack a linear EO effect, relying on plasma dispersion. The authors aim to demonstrate a hybrid silicon‑on‑insulator and lithium‑niobate ring modulator that achieves high‑speed, low‑tuning‑power operation at gigahertz frequencies. The device is a 15 µm radius silicon microring bonded to an ion‑sliced LiNbO₃ thin film using benzocyclobutene, forming a hybrid electro‑optic ring modulator. Fabricated devices show a loaded Q of 14,000, 3.3 pm/V resonance tuning, a 5 GHz 3 dB bandwidth, and >3 dB extinction digital modulation up to 9 Gb/s.

Abstract

Of all oxides, lithium niobate (LiNbO3) is the gold standard electro-optical material in fiber-optic transmission systems. Modulators based on diffused waveguides in bulk LiNbO3 substrates are, however, relatively large. In contrast, ring modulators based on silicon-on-insulator are of interest for chip-scale electro-optical modulation, but unstrained crystalline silicon does not exhibit a linear electro-optic effect, so modulation is based on alternative mechanisms such as the plasma dispersion effect. Here, we present a hybrid silicon and LiNbO3 electro-optical ring modulator operating at gigahertz frequencies. The modulator consists of a 15 μm radius silicon microring and an ion-sliced LiNbO3 thin film bonded together via benzocyclobutene. Fabricated devices operating in the TE optical mode exhibit an optical loaded quality factor of 14,000 and a resonance tuning of 3.3  pm/V. The small-signal electrical-to-optical 3 dB bandwidth is measured to be 5 GHz. Digital modulation with an extinction ratio greater than 3 dB is demonstrated up to 9  Gb/s. High-speed and low-tuning-power chip-scale modulators that exploit the high-index contrast of silicon with the second-order susceptibility of LiNbO3 are envisioned.