Towards ultracompact photonic chips using higher-order modes in closely spaced waveguides

Photonic integrated circuits are gaining traction in the field of telecommunications and information processing for their low-loss and high-throughput data transmission in comparison to electronic integrated circuits. However, they are still not used as widely as their electronic counterparts due to a relatively large footprint of photonic chips. One limiting factor to their size is the need to separate optical components by distances on the order of the working wavelength or larger to minimize optical crosstalk between them. In this work, we consider the fundamental and higher-order modes in closely spaced straight and bent waveguides with relatively small cross sections and find that higher-order modes allow one to substantially reduce the crosstalk in both cases. This can be used to considerably reduce the dimensions of photonic chips. We also propose on-chip components that allow selective excitation of higher-order modes. In addition, we design a directional coupler, a 3-dB splitter, and a Mach-Zehnder interferometer capable of operating on higher-order modes. Other ultracompact photonic-chip components, such as optical interconnects, switches, transceivers, and phased waveguide arrays for on-chip LiDAR scanners, can be designed as well based on similar principles.