Improved design for SOI based evanescently coupled multilayer spot-size converter

We report an improved version of a spot-size converter (SSC) consisting of a silicon nanowire evanescently coupled to a phase-matched Poly-Si multilayer structure. With wider transversal dimensions the multilayer structure expands the mode significantly thus increasing the coupling efficiency with the conventional single-mode fiber. Detailed optimization process of a 17-layer based SSC is discussed and its coupling efficiency with a high-NA fiber of radius 2 μm is obtained as 98% providing only 0.087 dB loss. Vertical alignment tolerance between the optimized SSC and a high-NA fiber of radius 2 μm is also shown. This novel design does not consist of a taper and can be fabricated by using CMOS compatible process. It has a short device length and more relaxed alignment tolerances with the fiber. Full-vectorial and computationally efficient finite element method and the least squares boundary residual method have been used for the analysis and optimization of the proposed structure