Rib waveguides for Kerr nonlinear optics

Journal article, 2025

Optical amplifiers are fundamental to high-throughput optical communication systems, but traditional rare-earth-doped amplifiers with limited optical bandwidth increasingly constrain the scalability of next-generation fiber networks. Integrated optical parametric amplifiers (OPAs), based on Kerr nonlinear optics, are potential candidates to address this challenge by offering broadband gain across arbitrary wavelengths when they are operated in an un-depleted pump regime with available low-noise pump lasers. However, their performance is currently limited by optical losses in meter-scale waveguides, which limits the maximum achievable gain. In this work, we challenge the conventional preference for strip waveguides in Kerr-based systems and demonstrate with numerical studies that at the maximum effective length rib waveguides fabricated on the silicon nitride (Si3N4) platform can offer substantially higher gain, despite a lower effective nonlinear coefficient. This comes at the cost of longer length, which we address using a meander-style spiral concatenation, and we also show how to avoid active stitching error compensation in electron-beam lithography during the fabrication of these meter-long waveguides. We further investigate the fabrication tolerance of group velocity dispersion in both geometries and show that rib waveguides maintain comparable performance. These results pave the way not only for practical OPAs but also for other devices based on χ(3) nonlinearity such as wavelength converters and optical sampling oscilloscopes by simultaneously providing high gain, broad bandwidth, and a low noise figure.