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.