DESIGN AND FABRICATION OF BROAD-BAND SILICA-BASED OPTICAL WAVE-GUIDE COUPLERS WITH ASYMMETRIC STRUCTURE

Waveguide-type directional couplers with different guide widths in the interactive region are designed and fabricated with silica-based waveguides on silicon substrates. We demonstrate that these asymmetric couplers have broad-band coupling-ratios of, for example, 50% +/- 5%, 25% +/- 3%, or 5% +/- 2% over a wide wavelength range from 1.3 to 1.55-mu-m, which are in good agreement with the results calculated using the beam propagation method (BPM). The excess loss of these couplers is as low as 0.7 dB, including input and output single-mode fiber coupling losses.