Silica-based waveguide-type wavelength-insensitive couplers (WINC's) with series-tapered coupling structure

The authors propose new wavelength-insensitive directional couplers (WINCs) with series-tapered waveguides in the coupling region. The wavelength-insensitive characteristics are investigated with respect to the geometric symmetry of the series-tapered coupling structure. An approximate analysis of three types of WINC is made using simple transmission matrix multiplication. These new WINC's were fabricated based on silica-based planar lightwave circuits (PLCs) on silicon substrates. The experimental results for the wavelength-insensitive responses of the series-tapered coupler, which was designed in detail with the beam propagation method (BPM), are in good agreement with the calculated results. The novel wavelength-insensitive characteristics of these couplers are also demonstrated for the first time.>