Calculation of the Coupling Coefficient of Twin-Core Fiber Based on the Supermode Theory with Finite Element Method

Abstract

Currently, coupled mode theory (CMT) is widely used for calculating the coupling coefficient of twin-core fibers (TCFs) that are used in a broad range of important applications. This approach is highly accurate for scenarios with weak coupling between the cores but shows significant errors in the strong coupling scenarios, necessitating the use of a more accurate method for coupling coefficient calculations. Therefore, in this work, we calculate the coupling coefficients of TCFs using the supermode theory with finite element method (FEM) that has higher accuracy than CMT, particularly for the strong coupling TCF. To investigate the origin of the differences between the results obtained by these two methods, the modal field distributions of the supermodes of TCF are simulated and analyzed in detail.

Dual-core optical fibers for efficient mid-infrared generation via third-order frequency mixing and coupling-length phase matching

“…( 6) and Eqs. (22)(23)(24)(25)(26)(27). In some cases the accuracy of the calculation was checked using COMSOL Multiphysics [27] to find the propagation constants of the supermodes, and then either Eq.…”

“…These CLPM equations replace the simple ∆k = 0 condition of the single-mode case by subtracting (as in Eqs. [22][23][24] or adding (as in Eqs. 25-27) specific linear combinations of the coupling constants.…”

“…i ω i /c are the propagation constants for the even and odd supermodes in the two parallel waveguides, respectively. n Substituting the relationships ( 28) into (22)(23)(24)(25)(26)(27), delivers the CLPM conditions in terms of the supermode propagation constants alone:…”

“…Dual core fibers can be understood in terms of the modes propagating in one of the two cores and how they couple to the modes in the other core, which lead to the CLPM expressions (22)(23)(24)(25)(26)(27), or they can also be understood in terms of the two lowest frequency supermodes of the dual-core structure, which lead to the alternative form of the CLPM expressions, (30)(31)(32)(33)(34)(35). Fig.…”

“…Next, we will discuss the use of dual-core, or twin core, fibers [21][22][23] with large coupling between the two cores to achieve the necessary design flexibility and obtain phase-matched frequency conversion for a wide range of pump wavelengths and generated wavelengths. The effect of the distance between the cores can be understood in terms of coupling-length phase matching (CLPM) [24], or in terms of intermodal coupling between the nearby supermodes of the dual core waveguide.…”