Power-efficient carrier-depletion SOI Mach-Zehnder modulators for 4x25Gbit/s operation in the O-band

In this paper, we communicate on the design, fabrication, and testing of optical modulators for Silicon-based photonic integrated circuits (Si-PICs) in the O-band (1.31 μm), targeting the 100GBASE-LR4 norm (4 wavelengths at 25 Gbit/s). The modulators have been conceived to be later coupled with hybrid-III-V/Si lasers as well as echelle grating multiplexer, to create a hetero-integrated optical transmitter on a silicon-on-insulator (SOI) platform. The devices are based on a Mach-Zehnder Interferometer (MZI) architecture, where a p-n junction is implanted to provide optical modulation through carrier depletion. A detailed study focusing on the best doping scheme for the junction, aimed at optimizing the overall transmitter performance and power-efficiency is presented. In detail, the trade-off between low optical losses and high modulation efficiency is tackled, with a targeted CMOS-compatible voltage drive of 2.5 V. Process simulations of the junction are realized for the doping profile optimization. Modulators of different lengths are also investigated to study the compromise between extinction ratio, insertion losses and bandwidth. Furthermore, coplanar-strip (SGS) travelling-wave electrodes are designed to maximize the bandwidth, to reach the targeted bit rate of 25 Gbit/s. Measurements show modulation efficiencies up to 19 °/mm (or 2.4 V.cm) for a 2.5 V input voltage, with doping-related losses below 1 dB/mm, in line with theoretical estimates, and well-suited to enhance the Si-PIC transmission and power-efficiency. Finally, an electro-optical (EO) bandwidth at 1.25 V bias is measured above 28 GHz.

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