The coupling between a silicon nanowire (NW) and a single mode fiber (SMF) is challenging. Design and optimization ofcompact spot-size converters (SSCs) for silicon photonics devices are presented by using numerically efficient and rigorousfull-vectorial finite-element based approaches. The multi-Poly-Silicon layers based SSCs are proposed and optimized forthe quasi-TE and quasi-TM polarizations sequentially. The coupling losses can be reduced to 2.72 and 2.45 dB for thequasi-TE and quasi-TM polarizations, respectively by using an eleven Poly-Si layers based SSC. A polarizationindependentSSC is also proposed based on the phase-matched multi-Poly-Silicon-layer and lower taper waveguide forboth the quasi-TE and quasi-TM polarizations. Coupling to a lensed fiber with the radius of 2 μm, the optimizedpolarization-independent SSC is with the coupling losses of 0.34 and 0.25 dB for the quasi-TE and quasi-TM polarizations,respectively. The on-chip integrated SSC opens up the feasibility of a low cost passive aligned fiber-pigtailed electronicphotonicsintegrated circuits platform.
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