Numerical Evaluation of the Coupling Between Several Directional Coupler Designs

Abstract

Background Power division, switching, modulation, and wavelength multiplexing in integrated photonics are made possible via directional couplers. Nonlinear effects, material characteristics, and waveguide geometry all affect how they couple. Because of their microstructure cladding, photonic crystal fibre (PCF) couplers provide stronger field confinement and possibly better coupling than traditional two-core waveguides. A numerical comparison of linear and nonlinear coupling in waveguide and PCF couplers is presented in this paper. Methods Coupled Mode Theory was used to simulate the neighbouring-core interaction, and COMSOL's FEM was used to obtain even and odd supermodes. For both coupler types, effective indices, coupling coefficients, and coupling lengths were retrieved. Evaluation of nonlinear behaviour, such as power-dependent decoupling and critical power thresholds, was made possible by incorporating self-phase modulation into the CMT equations. Results The PCF coupler provided substantially stronger coupling than the standard waveguide. At a wavelength of 1.55 μm, the PCF attained a coupling length of 1107 μm and a coupling coefficient of 0.001418 μm −1 , compared to 3875 μm and 0.000405 μm −1 for the waveguide. Improved field localization and intercore interactions increase coupling in PCFs. Nonlinear calculations revealed that the PCF requires less critical power (29 kW) to accomplish decoupling than the waveguide (83 kW). Conclusion Both architectures showed reduced intercore transfer at high powers due to nonlinear phase mismatch, consistent with Jensen’s hypothesis. PCF couplers outperform the traditional waveguides in both linear and nonlinear regimes. This work provides a direct, side-by-side comparison of a conventional integrated waveguide coupler and a microstructured Photonic Crystal Fiber (PCF) coupler. We map out exactly how structural geometry fundamentally dictates the scaling of the nonlinear coefficient ( γ ) and alters the critical switching threshold ( Pc ), which gives the potential of nonlinear PCF couplers for use in high-speed optical communication, switching, modulation, multiplexing, and wavelength division multiplexing (WDM) applications.