Flexible, ultra-broadband, optical wavelength conversion will be one of the key issues in future optical multiband networks that make use of the full low-attenuation bandwidth of Standard Single-Mode Fibers (SSMF). This chapter outlines the authors’ work to realize broadband wavelength conversion between extreme optical wavelength bands using optimized multimodal optical waveguides with high cubic nonlinearity. In a first part, the physical principles of ultra-broadband optical wavelength conversion in a highly nonlinear multimodal Silicon-On-Insulator (SOI) waveguide, and methods to model and optimize its functionality, are explained. Subsequently, examples of optimized SOI waveguide designs are presented that enable ultra-broadband conversion of signals within, or between various low-attenuation wavelength bands of an optical fiber. In a second paragraph, the fabrication of SOI waveguides is delineated in general, and measured key parameters of samples of various generations of optimized waveguides are revealed. In a final section, the performance of the developed optical converters is demonstrated exemplarily in laboratory experiments.