SILICA-BASED WAVE-GUIDE-TYPE WAVELENGTH-INSENSITIVE COUPLERS (WINCS) WITH SERIES-TAPERED COUPLING STRUCTURE

We propose new wavelength-insensitive directional couplers (WINC's) 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 (PLC's) on silicon substrates. We report that 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. Consequently, point-symmetric series-tapered couplers can be obtained with excellent wavelength-insensitive responses. Their splitting ratios are 50+/-5% over a wide range from 1.2 to 1.7 mum and they have a low insertion loss of 0.48 dB, which includes input and output fiber coupling losses. The novel wavelength-insensitive characteristics of these couplers are also demonstrated for the first time.