Ultrabroadband High Coupling Efficiency Fiber-to-Waveguide Coupler Using Si $_{3}$N$_{4}$ /SiO$_{2}$ Waveguides on Silicon

An easy-to-fabricate but very efficient fiber-to-waveguide coupler is theoretically analyzed and experimentally demonstrated. In this design, light from a single-mode fiber can be butt coupled into a single-mode high-index-contrast Si3N4/SiO2 waveguide with a measured coupling efficiency of 96% at the wavelength of 1550 nm and >90% in the spectral range from 1450 to 1650 nm. Large horizontal and vertical alignment tolerances of 3.8 μm and 3.6 μm, respectively, are obtained between the fiber and the waveguide coupler. All these experimental results agree well with simulations. The waveguide coupler also features ease of end-facet cleaving and can be used in ultrabroadband high coupling efficiency applications.