Vertically Curved Si Waveguide Coupler with Low Loss and Flat Wavelength Window

Silicon photonics enabling ultra-small photonic integrated circuits induces a necessity to develop efficient optical couplers that connect silicon waveguides with optical fibers or offchip other optical components. Vertical couplers are promising for both wafer-level testing and the integration of optical components on the chip surface, but so far grating couplers have been the only solution virtually. Very recently, we have developed a few-micron-scale vertically-curved silicon waveguide coupler “elephant coupler” formed by ion implantation method. This paper demonstrates that the coupler has high coupling efficiency, weak wavelength dependence, and weak incident angle dependence of wavelength window, which are not obtained by the grating couplers. The paper also demonstrates that the thermal annealing treatment can reduce optical propagation loss induced by the ion implantation.