A three-dimensional polarization-insensitive grating coupler tailored for 3D nanoprinting

Abstract

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 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.

Figures (11)

• Figure 1: a) The design setup. Our design region has a size of $\qtyproduct{18 x 18 x 4.5}{\upmu m}$ and is illuminated from the top at an angle of $10^\circ$. A waveguide is placed at the edge of the design region. Both the design region and the waveguide have a refractive index of $n_\text{structure}=1.53$. They both sit on top of a substrate with a refractive index of $n_\text{substrate}=1.444$ and are embedded in air $n_\text{air}=1$. b) An optimized, structurally integral grating coupler design. This is a 3D render of the coupler, sitting on top of the substrate with the waveguide.
• Figure 2: a) The coupling efficiency of both polarizations depends on the size of the design region. We look at four different sizes, which are given by $\qtyproduct{14 x 14 x 3.5}{\upmu m}$, $\qtyproduct{16 x 16 x 4}{\upmu m}$, $\qtyproduct{18 x 18 x 4.5}{\upmu m}$, and $\qtyproduct{20 x 20 x 5}{\upmu m}$, respectively. We note that while the design with size $\qtyproduct{20 x 20 x 5}{\upmu m}$ is the optically best performing one, free-floating artifacts start to appear. b) The coupling efficiency and loss for both polarization directions of the $\qtyproduct{18 x 18 x 4.5}{\upmu m}$ grating coupler as a function of the wavelength. We optimized for a single wavelength at $1.55\upmu m$. Note that both figures share their y-axis.
• Figure 3: Various cross-sections of the optimized design. a) Cross-section of the design in the $x-y-$plane at the middle of the waveguide. b) Cross-section of the design in the $x-z-$plane at the middle of the design. c), d) The normalized $|E_x|$ field in frequency domain at a wavelength of $1.55\upmu m$. e), f) The normalized $|E_y|$ field in frequency domain at a wavelength of $1.55\upmu m$.
• Figure 4: a) Optimized design without the heat solver to enforce connectivity, the inset shows the disconnected, floating part. b) Heat distribution of the heat generated by the material in a). The non-connected parts are clearly visible as hotspots. c) Optimized design with the heat solver. d) Heat distribution of the heat generated by the material in c). Both heat plots are normalized with respect to each other.
• Figure 5: Coupling efficiency of the $\qtyproduct{18 x 18 x 4.5}{\upmu m}$ device with regard to lateral misalignment a) and angular misalignment b) of the fiber. Each isoline marks a subsequent loss of the coupling efficiency by $1dB$. Note that the data shown here is interpolated.