Design and Fabrication of Silica-Based Waveguide-Type (3×3) Directional Couplers with Two Identical Outer Waveguides

Abstract

We analytically and experimentally demonstrate the design and fabrication of silica-based waveguide-type (3×3) directional couplers with emphasis on the wavelength-flattened responses of the couplers when the center guide is excited. Complete optical power transfer from the center guide to the two outer guides, which corresponds to a 0.5:0:0.5 splitting ratio (3 dB coupler), is attained with slightly wider outer guides. Wavelength-flattened splitting ratios of 0.33:0.34:0.33 (5 dB coupler) or 0.1:0.8:0.1 (10 dB coupler) over a wide range from 1.3 µm to 1.55 µm are obtained with larger differences between the widths of the center and two outer guides. When the light is launched into one of the outer guides, complete power transfer is always obtained for the outer-to-outer path but not for the outer-to-center path, reflecting the waveguide symmetry of (3×3) structures. These experimental results are in good agreement with results calculated using the beam propagation method (BPM).