Theoretical investigation of nanowaveguide‐based optical coupler using mode expansion transfer matrix

We report a mode expansion transfer matrix method to describe the nanowaveguide-based strongly coupling waveguide directional couplers, including slab nanowaveguide coupler, silicon-on-insulator (SOI) nanowires coupler, and silica nanofibers coupler. We propose a general mode expansion description of coupler without the assumption of weakly guiding. For the slab nanowaveguide case, all the coefficients inside those matrices could be identified theoretically using mode expansion method. The analytical results of coupler show excellent agreement with the simulation results obtained by the finite-difference time-domain method. For the nanofiber coupler, with the mode profiles obtained numerically using finite element method, similar matrix equation could be used for the guideline of nanofiber coupler design, which is the key issue of coupler-based nanofiber devices and SOI nanowires devices. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1123–1129, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25093