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Recent progress of highly efficient and high-speed electro-optic (EO) modulator technology has received intensive research attention in microwave photonics and fiber-optic networks. Among the different types of materials used in the modulator, the thin-film EO waveguide offers intrinsic advantages such as a large EO coefficient (r33>150 pm/V), traveling wave modulation, and excellent compatibility with other materials and silicon-on-insulator substrates. State-of-the-art efficient EO modulators rely on the phase modulation in various types of waveguide structures. Particularly, highlights of the polymer modulators can be impressed due to the advantages in fabrication allowed by the spin-on-preparation technique, which enables various waveguide structures on the silicon-on-insulator substrates. To date, spin-on polymer waveguide modulators have shown outstanding performance such as high data rate transmission, low-power consumption, and easy integration to other substrates. The progress makes the modulator device as one of the few possible solutions to realize over 120 Gbaud high-speed signalization [1]. It meets the critical demand in the emerging optical interconnects for short-reach and data center networks. Despite being fast and efficient, reliability issues should be investigated, i.e., stabilities against high-temperature exposure, high-intensity optical signal, humidity exposure, and recycling thermal shock. While progress is significantly being made, careful consideration should be continued to fully address the industrial applications [1], [2]. The heterogeneous integration of strong EO coefficient materials such as ferroelectric epitaxial films is the alternative technique. We have recently prepared the heterogeneous thin-film LnNbO3 and silicon waveguide modulator on an insulator and showed the modulation bandwidth of 60 GHz and 200 Gbit/s PAM4 transmission with error-free signal accuracy [3]. Based on the technique, furthermore, we are investigating the high-speed modulators using the ferroelectric films-on-insulator (FFOI), consisting of ferroelectric oxides with perovskite phase such as PZT (Pb[Zi, Ti]O3), PLZT([Pb, La][Zi, Ti]O3) and BTO (BaTiO3). The EFOI modulators provide very strong EO coefficients, thus promising the modulation with lower driving voltage, lower cost, and more compact footprint properties. In this study, we utilize the spin-on technique using precursor sol-gel solutions to obtain FFOI substrates and fabricate waveguide modulators. We measured an effective EO coefficient of around 200 pm/V and half-wave voltage length (Vp×L) product smaller than 1.0 V×cm in a Mach-Zehnder interferometer (MMI) waveguide modulator. This paper discusses state-of-the-art high-speed modulator devices in telecommunications applications and our approach from a view point of materials, fabrications and devices.  

References

1. G-W. Lu, et al., “High-temperature-resistant silicon-polymer hybrid modulator operating at up to 200 Gbit s−1 for energy-efficient data centres and harsh-environment applications,” Nature Communications, 11, 4224 (2020).

2. H. Sato, et al., “A 100 Gbaud on-off-keying silicon-polymer hybrid modulator operating at up to 110°C,” IEEE Photonics Technology Letters, 33, 1507 (2021)

3. J. Mao, et al., “Heterogeneous silicon-on-lithium niobate electro-optic modulator for 100-Gbaud modulation,” 7, 126103, APL Photonics (2022).