Spot-Size Converter Based on Long-Period Grating

We propose a spot-size converter (SSC) by exploiting the high-efficiency mode conversion capability that a long-period grating (LPG) possesses. Our proof-of-concept LPG-based SSC, fabricated with optical polymer material on silicon nitride (Si 3 N 4 ) platform, has a length of ∼700 μm, can improve the coupling efficiency between the LP x 01 mode of an ultra-high numerical aperture fiber and the E x 11 mode of the Si 3 N 4 waveguide from ∼24% to ∼33% at 1548 nm wavelength, corresponding to 1.4-dB reduction in insertion loss. Therein, the LPG achieves a maximum mode conversion efficiency of 90% between the two E x 11 modes of the polymer cladding and the Si 3 N 4 core. Our proposed SSC provides a new path to achieve low-loss fiber-to-chip coupling.