Kiyotoshi Yasumoto

Coupled-mode analysis of an asymmetric nonlinear directional coupler

A coupled-mode analysis of an asymmetric planar nonlinear directional coupler (NLDC) is presented by using the singular perturbation technique. The NLDC consists of a nonlinear waveguide with the core of Kerr-like medium and a linear waveguide situated parallel to each other. The effects of linear coupling and nonlinear modification of refractive index are treated to as small perturbations, and the modal fields of isolated linear waveguides are employed as the basis of propagation model. The self-consistent coupled-mode equations governing the power transfer are derived in analytically closed form. The representative numerical result for the input/output characteristics demonstrates that the asymmetric NLDC is useful for constructing a band-pass power-filter or a band-reject power-filter.

Coupled-mode formulation of multilayered and multiconductor transmission lines

A novel coupled-mode formulation for multilayered and multiconductor transmission lines is developed. In this formulation, the solutions to the original multiconductor system are approximated by a linear combination of eigenmode solutions associated with the isolated single conductor line located in an appropriate reference dielectric medium. The reciprocity theorem is used to derive the coupled-mode equations. The coupling coefficients are expressed in terms of the simple overlap integrals between the eigenmode fields and currents of the individual conductor lines. As a basic application, the dispersion characteristics of two identical coupled-microstrip lines are analyzed using the proposed coupled-mode theory. It is shown that the results are in very close agreement with those obtained by the direct Galerkins moment method over a broad range of weak to strong coupling.

Coupled-mode analysis of power-transfer characteristics in a three-waveguide nonlinear directional coupler

A planar three-waveguide nonlinear directional coupler (NLDC) is analyzed by the use of a coupled-mode approach based on the singular perturbation technique. The self-consistent first-order coupled-mode equations are derived in an analytically closed form, which demonstrates that the power transfer in three-waveguide NLDC is described by linear-coupling terms and nonlinear self-modulation terms. The optical switching characteristics predicted by the coupled-mode theory are discussed and shown to be in good agreement with those obtained from a numerical analysis with the finite-difference beam-propagation method.

COUPLED-MODE ANALYSIS OF POLARIZATION-SPLITTING DIRECTIONAL COUPLER

The theory of a polarization-splitting directional coupler is developed with a coupled-mode approach for two interacting planar optical waveguides separated by a thin metal film. For TM guided modes and TE guided modes the coupled-mode equations governing the evolution of the modal amplitudes are obtained in an analytically closed form that provides a clear description of how the presence of a metal film brings about a strong polarization selectivity in the coupling between two optical waveguides. Representative numerical results show that, when silver or gold is used as the metal, polarization splitting with extinction ratios ∼ − 30 dB and throughput losses ∼0.2 dB is attainable in three typical wavelengths of 0.85, 1.30, and 1.55 μm.