Coupled-mode theory for low-crosstalk higher-order modes of optical waveguides

Optical crosstalk between waveguides limits the miniaturization of photonic integrated circuits. Recently, it has been shown that, using higher-order modes, the crosstalk can be significantly reduced not only between neighboring waveguides, but also between other photonic components, such as waveguide couplers and interferometers. This opens the possibility of further miniaturization of photonic chips. Here, we present a theoretical approach to investigate the peculiar properties of these low-crosstalk modes. The approach includes a semianalytical method for computing the long-range coupling coefficients of the modes. It allows us to study the dependence of light propagation on these coefficients in both finite and infinite waveguide arrays. Our findings shed light on the crosstalk suppression mechanism and intriguing discrete diffraction phenomena realizable in waveguide arrays using higher-order modes.