A Three-Dimensional Polarization-Insensitive Grating Coupler Tailored for 3D Nanoprinting

Efficiently coupling light from optical fibers into photonic integrated circuits is a key step toward practical photonic devices. While a notable coupling can be achieved by out-of-plane couplers such as grating couplers, their basic planar geometry typically tends to be sensitive to the polarization of light. This is partly due to the fact that the design spaces of such grating structures—typically fabricated with techniques such as electron-beam lithography—are only two-dimensional with a simple extrusion into the vertical dimension. This makes it challenging to optimize for both polarizations simultaneously, as performance typically degrades when trying to achieve high efficiency in both. As a result, conventional approaches either suffer from increased losses or require additional filtering components to account for different polarizations. In this work, we present a fully three-dimensional, polarization-insensitive grating coupler which has a highly efficient simulated coupling efficiency of over$\bf {80\%}$in both polarizations. This performance matches that of state-of-the-art couplers that are performant for one polarization only. This comes at the cost of a moderately larger size due to the lower refractive index materials typically available in 3D nanoprinting. Our design method uses density-based topology optimization with a multi-objective approach that combines electromagnetic simulations with a fictitious heat-conduction model acting as a soft constraint to promote structural integrity. This ensures that the designed structures are feasible for fabrication. Our work opens new possibilities for robust 3D photonic devices, enabling advanced integration, fabrication, and applications across next-generation photonics and electronics.