Strip-slot direct mode coupler.

We present a direct strip-slot waveguide mode coupler without any auxiliary structures. Contrary to popular belief, an apparent mode mismatch between strip and slot waveguide does not deteriorate conversion efficiency. Separated electric and magnetic field distributions in a slot waveguide lead to highly efficient modal coupling in the direct strip-slot coupler and result in high conversion efficiency. Accurate experimental characterization shows that the direct strip-slot waveguide mode coupler is capable of up to 96% conversion efficiency with a broad bandwidth. Being simplest and of high efficiency, the direct strip-slot waveguide mode coupler can encourage potential applications of slot waveguides.