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.