Polymer spot size converter on silicon photonics chip for enabling high coupling to single-mode fiber

Silicon photonics (SiPh) technology has gained considerable attention as a result of the growing demand for high-bit-rate optical interconnections. Low coupling efficiencies, resulting from the difference in spot size between silicon photonic chips and single-mode fibers (SMFs), remains a challenging issue. To solve this problem, we fabricated a novel, to the best of our knowledge, polymer spot size expander (SSE) device on the end face of a silicon chip. The fabrication of SSEs using self-written waveguide technology and a dipping method using UV-curable resin was highly reproducible. The spot size of the original 3.83 µm of the SiPh chip was expanded to approximately 7.82 µm at a wavelength of 1.55 µm, and the maximum coupling efficiency achieved with the SMF was -0.88 dB. In addition, the -3 dB tolerance of the coupling efficiency along the vertical optical axis was ±4.4 µm.